• Genetics: There is strong evidence for genetic responses to climate change, although most evidence refers to small organisms with short generation times. There is little documented evidence of evolutionary change from species with longer generation times such as birds, mammals, and trees (95).
• Physiology: Physiology is the functioning of organisms. Physiological processes of organisms may adapt to higher temperatures and other climate-mediated changes, or fail to do so. These responses can occur within a generation or between generations (96). In marine systems, physiological responses to both climate warming and changing ocean conditions are widespread (97).
• Morphology: In general, decreasing body size with warming is expected, thus increasing surface-to-volume ratios, which is generally favored under warmer conditions (98).
• Phenology: For most species, migrations and life-history processes (such as budding and flowering in plants, hatching and fledging in birds, and hibernation in mammals) are closely tied to seasonal and interannual variation in climate, and there is now overwhelming evidence that both have been affected by climate change (99). Across marine, freshwater, and terrestrial ecosystems, spring phenologies have advanced by 2.3 to 5.1 days per decade (100). Changes in the timing of migration events have been extensively documented, including advances in spring arrival dates of long-distance migratory bird species in Europe, North America, and Australia (101). In marine and freshwater systems, advances in the timing of annual phytoplankton blooms, the basis for many aquatic food webs, have occurred more rapidly than temporal shifts in terrestrial plants (102). Such changes in plankton phenology have been attributed to increases in water temperatures, reduction in the duration of ice cover, and the alteration of the seasonal duration of thermal stability or stratification of the water column. Phenological changes due to milder winters are influencing crop and fruit production (103). Climate change has reduced winter chill events in temperate agricultural areas (104), which can desynchronize male and female flowers and trigger delayed pollination, delayed foliation, and reduced fruit yield and quality.
• Abundance and population dynamics: ~80% of communities across terrestrial, freshwater, and marine ecosystems exhibited a response in abundance that was in accordance with climate change predictions (105). 52% of warm-adapted marine species, for instance, increased in abundance, whereas 52% of marine cold-adapted species decreased (106). Excessive heat causes bleaching and mass mortality of corals in the tropics (107). The brown trout has lost habitat in the Swiss Alps (108) due to increased water temperatures. Some of the best evidence for climate-change impacts on the abundance of terrestrial species comes from analyses of bird population trends derived from systematic monitoring schemes in Europe, with warm-adapted species having increased in abundance on average since the 1980s and cold-adapted species having declined (109). Climate change can increase the abundance of temperature-sensitive disease vectors, with subsequent effects on disease outbreaks.
• Distribution: One of the most rapid responses observed for marine, freshwater, and terrestrial species is a shift in their distributions to track optimal habitat conditions (110). There has been a consistent overall trend for tropical, warm-adapted species to expand their ranges into environments previously dominated by temperate cold-tolerant species (“tropicalization”) (111). A similar phenomenon has been documented in the Arctic, where boreal fish communities have responded to warming in the Barents Sea by shifting northward, resulting in a high turnover in Arctic fish communities (“borealization”) (112). Shifts in species distributions have also occurred across elevation gradients. In France, for instance, upslope shift was observed in recent decades in mountainous stream-dwelling fish (113).
• Interspecific relationships: As a by-product of the redistribution of species in response to changing climate, existing interactions among species are being disrupted, and new interactions are emerging (114). These novel biotic interactions can exacerbate the impacts of abiotic climate change (115). This may lead to predator-prey mismatches, for instance due to an earlier bloom or geographical shift of prey compared with the predator. Pollination is a key process linked to yields for a large number of crops. The short-lived, highly mobile insect species that provide pollination services to numerous crops have responded rapidly to changing climates by shifting their ranges throughout North America and Europe (116). Additionally, over the past 120 years, many plant-pollinator networks have been lost with overall decline in pollination services, which is attributed to a combination of habitat loss, pollution, and climate warming (117).
• Productivity: Changes in productivity are one of the most critical impacts of climate change across aquatic and terrestrial ecosystems (118). Both increases and decreases of productivity are possible.
• Ecosystem state shifts: Diminished resilience may lead to ecological regime shifts, in which one ecosystem state shifts to an alternative and potentially undesirable stable state. In lakes, for instance, climate change has increased the risk of regime shifts from clear water to turbid states and increased the occurrence of cyanobacteria blooms (119).

Consequences for us

Disruptions have documented consequences for people, including unpredictable fisheries and crop yields, loss of genetic diversity in wild crop varieties, and increasing impacts of pests and diseases. In addition to reducing greenhouse gases, climate action and policy must also focus on strategies that safeguard biodiversity and ecosystems.

• Fishery: The impacts of climate change on marine fisheries have major consequences for human societies because these currently provide ~17% of the global protein for people (120). The effect of warmer water on fishery may be both positive and negative. In the Arctic, for instance, commercially important fish such as Atlantic cod have increased in biomass primarily because of increases in plankton production from reduced sea ice (121). In Switzerland, on the other hand, trout catches have been halved over two decades because of rising temperatures in Alpine streams (122). Changes in total marine productivity are not just attributed to abundance shifts but also morphological shifts. Eight commercial fish species in the North Sea, for instance, underwent simultaneous reductions in body size over a 40-year period because of ocean warming, resulting in 23% lower yields (123). Reduced body size in fish is also being recorded in lakes and rivers throughout Europe and has been linked to increased temperature and climate-induced shifts in nutrient inputs (124).
• Agriculture: Impacts on plant genetics and physiology are influencing human agricultural systems. For example, yields in rice, maize, and coffee have declined in response to the combined effects of rising temperatures and increasing precipitation variability over past decades (125).
• Human health: Pest and pathogen outbreaks may increase in the future because hundreds of plant pest and pathogen species have shifted their distributions 2 to 3.5 km per year poleward since the 1960s (126). Emerging threats to human health under climate change are vector-borne (mosquitos, ticks) diseases (127).
• Risk reduction climate-related hazards: Changes in ecological processes might compromise the functionality of ecosystems. This is an important consideration because healthy systems (both terrestrial and marine) sequester substantial amounts of carbon (128) and reduce risks associated with climate-related hazards such as floods, sea-level rise, and cyclones (129). In island and coastal communities, coral reefs can reduce wave energy by an average of 97% (130), and coastal ecosystems such as mangroves and tidal marshes buffer storms (131), while on land intact native forests are important in reducing the frequency and severity of floods (132). In many cases, maintaining functioning systems offers more sustainable, cost-effective, and ecologically sound alternatives than conventional engineering solutions (133). 

• Remaining species: According to this analysis, the fraction of remaining species at a location will reduce to 86% (95% confidence interval: 79–93%) when global warming reaches 2 °C above the pre-industrial level (approximately 1880). This implies that under a global mean temperature increase of up to 2 °C, terrestrial ecosystems could lose on average 14% of their current local species. A further global warming up to 3 °C and 4 °C will lead to a reduction of remaining species up to 83% (95% confidence interval: 78–88%) and 78% (95% confidence interval: 72–86%), respectively (177).
• Remaining suitable area: Likewise, the fraction of remaining area with a suitable climate for a certain species will reduce to 65% (95% confidence interval: 56–75%) at 2 °C global warming, and to 50% (95% confidence interval: 39–64%) and 46% (95% confidence interval: 34–63%) at 3 °C and 4 °C global warming, respectively (177).

The study indicates that deserts, temperate forests, shrublands, and tropical and boreal forests are the most sensitive biomes to increasing global temperatures. The results indicate that many species will disappear from areas where they now occur. However, this does not mean that total species richness will necessary decrease as new species can potentially expand their ranges and establish, depending on their ability to disperse. In this analysis, it was assumed that species are unable to disperse or adapt. This is a conservative assumption, but probably not that conservative: on average, no dispersal is close to reality for most species (179). Thus, according to the authors, the metrics used in their analysis are a good indication of projected decreases of terrestrial biodiversity.

Climate change will affect the suitability of habitats for species. If an area becomes unsuitable, decades might elapse between habitat loss and species extinction (36). Information on these time lags is not currently available. Therefore, proportions of 1,103 animal and plant species have been estimated that are ‘committed to future extinction’ as a consequence of climate change over the next 50 years, not the number of species that will become extinct during this period (37). The estimates for 2050, representing different continents, vary from 18% ‘committed extinctions’ for scenarios of minimum expected climate-change, to 24% and 35% for mid-range and maximum climate change projections, respectively.

Species extinctions, although still uncommon, are increasingly attributed to climate change (73). However, consistent, global estimates of species extinctions attributable to future climate change are still lacking (71). Current predictions about extinction risks vary widely, suggesting that anywhere from 0 to 54% of species could become extinct from climate change (72). From a meta-analysis of 131 published predictions, extinction risks from climate change have been estimated for a number of global warming rates. It was concluded that climate change threatens one in six species (16%) globally if we follow our current, business-as-usual trajectory (scenario RCP 8.5, resulting in a global warming of 4.3°C). Lower percentages of species at risk due to global warming gave been calculated for lower global warming (5.2% for 2°C post-industrial global warming, 8.5% for 3°C) (71). These results must be interpreted cautiously, however. In studies, important biological mechanisms that may increase or decrease predicted risks, such as species interactions, evolution, landscape dispersal barriers, habitat degradation, and intraspecific trait variation, are generally omitted (76).

Predicted extinction risks are lowest for North America and Europe, and highest for South America, Australia and New Zealand (71). The relative high risks for the latter regions are partly due to the fact that these regions harbor diverse assemblages of endemic species with small ranges, and are characterized by small land masses that limit shifts to new habitat (74). Endemic species with smaller ranges and certain taxonomic groups such as amphibians and reptiles are predicted to face greater extinction risks (75).

Mass extinction of amphibians has already been associated with global warming (38). In the mountains of Costa Rica, the Monteverde harlequin frog (Atelopus sp.) vanished along with the golden toad (Bufo periglenes). An estimated 67% of the 110 or so species of Atelopus, which are endemic to the American tropics, have met the same fate, and a pathogenic chytrid fungus (Batrachochytrium dendrobatidis) is implicated. Analysing the timing of losses in relation to changes in sea surface and air temperatures, it was concluded with ‘very high confidence’ (>99%, following the Intergovernmental Panel on Climate Change, IPCC) that large-scale warming is a key factor in the disappearances. It was proposed that temperatures at many highland localities are shifting towards the growth optimum of Batrachochytrium, thus encouraging outbreaks (38).

It is very difficult, however, to establish causative relationships between warming and population declines or extinction, due to the interaction with other anthropogenic factors such as habitat loss or previously unseen pathogens (57). According to the IPCC, a large fraction of terrestrial and freshwater species face increased extinction risk under projected climate change during and beyond the 21st century, especially as climate change interacts with other pressures, such as habitat modification, over-exploitation, pollution and invasive species (high confidence) (65). On the other hand, local conservation efforts focused on reducing the impact of other stressors can help in maintaining and enhancing ecosystem resilience and in limiting the impact of climate-related effects (70).

In the oceans, a high-end scenario of climate change (the so-called RCP 8.5 scenario) would culminate in a mass extinction rivaling those in the fossil record of the Earth’s past. Polar species are at highest risk of extinction, but local biological richness declines more in the tropics. By contrast, limiting warming to 2°C would avoid a marine mass extinction and preserve marine biodiversity accumulated over the past ~50 million years of evolutionary history (222). 

The Boreal region could, in principle, gain many species from further south, leading to a high species turnover. The Pannonian region could also theoretically gain eastern Mediterranean species. Thus, these regions stand to lose a substantial part of their plant species diversity, and (in time) to show a major change in floristic composition (9,65). Projected species turnover peaks at the transition between the Mediterranean and continental regions with extirpation of Euro-Siberian species and expansion for Mediterranean or Atlantic species. Southern Fennoscandia is also an area of high potential turnover with the loss of boreal species and gain of Euro-Siberian species. These results cannot be taken as precise forecasts, however, given the uncertainties in climate change scenarios, the coarse spatial resolution of the analysis, and uncertainties in the modeling techniques used (9).

Major future changes in biodiversity have also been concluded from other studies. By 2050, for instance, in 44% of the modeled European area on average 32% of the European plant species might disappear (10). Another modeling exercise projects that by 2050, 80% of the 2,000 (1,350 plants, 157 mammals, 108 reptiles and 383 breeding birds) surveyed current species across Europe would be lost under high greenhouse gas emissions and climate change scenarios (18). The detailed examination of the projections for plants in 2050 under the full range of scenarios suggests that 5% of these species would lose all available habitats (18).

The predicted responses to climate change demonstrate that the distribution of many species in Europe may be affected by climate change, but that the effects are likely to differ between species. The general pattern is of a south-west to north-east shift in suitable climate space, with gains balancing losses for many species. This disparity in species’ response has important implications for EU biodiversity policy as the significance of different countries changes in terms of their future contribution to the conservation of habitats and species (12). Projections indicate that, by the late 21st century, the potential range of many European plant species may shift several hundred kilometres in a northerly direction. This is several times faster than past rates as estimated from the Quaternary record or from historic data (21).

It has also been stated, however, that caution may be required in interpreting results from these models, not least because their coarse spatial scales fail to capture topography or "microclimatic buffering" and they often do not consider the full acclimation capacity of plants and animals. Several recent studies indicate that taking these factors into consideration can seriously alter the model predictions (11).

Many ecosystems in the world, including Europe, suffer already from land-use changes, air pollution, landscape fragmentation and habitat destruction (19,65). These pressures have led to the degraded functioning and species extinction that is at a rate 100–1,000 times greater than is considered normal over history (20).

According to the IPCC, many species will be unable to move fast enough during the 21st century to track suitable climates under mid-and high-range rates of climate change (i.e., RCP 4.5, RCP 6.0 and RCP 8.5 scenarios) (medium confidence). Barriers to dispersal, such as habitat fragmentation, prior occupation of habitat by competing species and human-made impediments such dams on rivers and urbanized areas on land, reduce the ability of species to migrate to more suitable climates (high confidence) (65). Species that cannot move fast enough to keep pace with the rate of climate change will lose favourable climate space and experience large range contractions (66).

Mountains

Species have on average shifted their ranges 16.9 km to higher latitudes and 11 m up in altitude per decade, more than estimates from earlier studies. The average shifts have been larger in those areas that have experienced the strongest warming and have, on average, been sufficient to track temperature changes, but with large variation between species (82). 

An excess of species loss is shown for mountain regions (mid-altitude Alps, midaltitude Pyrenees, central Spain, French Cevennes, Balkans, and Carpathians). The southern Mediterranean and part of the Pannonian regions are characterized by hot and dry summers and are occupied by species that tolerate strong heat and drought. Under the scenarios used here, these species are likely to continue to be well adapted to future conditions. The northern Mediterranean, Lusitanian and Mediterranean mountain regions are the most sensitive regions; the Boreal, northern Alpine and Atlantic regions are consistently less sensitive. Species loss does not necessarily imply the immediate loss of a species from a site; rather it may imply a potential lack of reproductive success and recruitment that will tend to extinction on a longer time scale (9).

For Europe, approximately 2,500 vascular plant species (or approximately 20% of the continent's native vascular flora) were estimated to be centred in the alpine zone from the tree line ecotone to the highest mountain summits (34). This zone comprises only 3% of the terrestrial area of Europe and, hence, limited space would be available for future alpine habitats in warmer climates (33).

A comparison of vegetation samples of 2008 and 2001 above the tree line from 60 summit sites in all major European mountain systems showed that ongoing climate change gradually transforms mountain plant communities; the more cold-adapted species decline and the more warm-adapted species increase, a process described as thermophilization. These results indicate a progressive shrinking of the low-temperature, high-elevation habitats, including parts of the Alps and Mediterranean mountains (33). In fact, declines of extreme high-altitude species at their lower range margins have recently been observed in the Alps (35).

Climate change is considered a large threat to especially montane species. These species often inhabit narrow elevational ranges and it is widely expected that they will be forced to move to higher elevations due to global warming. Generally it is considered that the area decreases with altitude and upslope shifts could leave species with less habitable area as they approach mountain peaks. This would make them more vulnerable to extinction. This line of reasoning, however, seems to be too simple. Scientists have shown that 68% of the world’s mountain ranges do not conform to the dominant assumption in ecology and conservation that area decreases monotonically with elevation from a mountain range’s base (which would be the case if all mountains were shaped like pyramids). Only in a few regions, such as Europe, coastal North America, Southeast Asia and eastern Australia, are pyramid mountains the norm rather than the exception (77). Complex topographies result in landscapes in which available area can actually increase with elevation. Thus, some species responding to climate change by shifting upslope may actually benefit through increases in available area (77). 

An assessment of the impacts of climate change on 2,632 plant species across all major European mountain ranges under four future climate scenarios projected that habitat loss by 2070-2100 will be greater for species distributed
 at higher elevations (152). Depending
 on the climate scenario, up to 36-55 % of Alpine plant species, 31-51 % of sub-Alpine plant species and
 19-46 % of montane plant species are projected to lose more than 80 % of their suitable habitat. Nevertheless, at the finer scale, microclimate heterogeneity may enable species to persist under climate change in so-called micro-climatic refugia (153). 

The analysis of 254 mean national time series undoubtedly demonstrated that species’ phenology is responsive to temperature of the preceding months (mean advance of spring/summer by 2.5 days/⁰C, delay of leaf colouring and fall by 1.0 day/⁰C). The pattern of observed change in spring efficiently matches measured national warming across 19 European countries. Owing to the enormous number of records included, the researchers consider their results representative for Europe (6).

Similar results are shown in other studies (see also the overview of the IPCC in (65)). It was shown, for instance, that a nearly Europe-wide warming in the early spring (February–April) over the last 30 years (1969–1998) led to an earlier beginning of growing season by 8 days (7). The observed trends in the onset of spring corresponded well with changes in air temperature and circulation (North Atlantic Oscillation Index (NAO-index)) across Europe. In late winter and early spring, the positive phase of NAO increased clearly, leading to prevailing westerly winds and thus to higher temperatures in the period February–April. Satellite-based estimates showed an advancement of the start of the growing season in northern Europe by 0.30 day/year over the period 2000-2016 (175), although the spatial heterogeneity was large (spanning from an advancement of 1.5 day/year to a delay of 1.0 day/year.

Since the end of the 1980s the changes in circulation, air temperature and the beginning of spring time were striking. The investigation showed that a warming in the early spring (February–April) by 1°C causes an advance in the beginning of growing season of 7 days. The observed extension of growing season was mainly the result of an earlier onset of spring. An increase of mean annual air temperature by 1°C led to an extension of 5 days (7). According to most other studies advancement of the growing season per °C is less, varying from 2.21 day/⁰C to 2.5 day/⁰C (176). 

Geographical variation in advancement of spring flowering

Several studies have shown that one of the responses of plants to global warming is to advance their onset of flowering in spring. The advancement varies for different regions, partly because the rate of global warming is not the same. In the last decades, global warming in Northern Europe and in alpine regions, for instance, was stronger than in other parts of Europe.

The geographical variation in the advancement of spring flowering has been assessed along a north-south trajectory in Europe, covering 12 countries (Finland, Estonia, Latvia, Lithuania, Poland, Slovakia, Hungary, Slovenia, Croatia, Bosnia and Herzegovina, Montenegro, Macedonia).  This was done for the first flowering date of six plant species in the period 1970-2010. The trajectory covers 5 different regions: Boreal (cool climate), Continental (especially eastern and central Europe), Alpine (mountain slopes), Pannonian (lowland areas of the Carpathian Basin) and Mediterranean (141).

Advancement in plant flowering onsets was largest in the Boreal regions: 2.2 to 9.6 days per decade. For the Continental and Alpine regions spring advancement was a little smaller: 3 to 8.3 days, and 2 to 3.8 days, respectively. Results were least pronounced for the Pannonian and Mediterranean regions. Generally, spring advancement was more pronounced in 1991-2010 than in 1970-1990 (141). 

Projected climate impacts

The CO2 fertilization effect may increase as CO2 concentration increases, but low soil water availability, heat stress and disturbances associated with droughts could reduce these benefits. Droughts have a negative impact on gross primary production, and this impact may deteriorate in future decades in two ways: either because droughts become more intense, or because they occur more often. Global projections have been made for 2075-2099 relative to the historical period of 1850-1999. This was done for a moderate and high-end scenario of climate change, based on 13 global models (203).

A higher intensity of droughts will not further reduce gross primary production, the results indicate. Apparently, the negative impact of less soil water and more heat is compensated by the positive impact of a higher CO2 fertilization. Still, droughts negatively affect primary production, comparable to current conditions. Thus, the projected increase of drought frequency will reduce gross primary production (203).

The stronger negative impact of future droughts on gross primary production will mainly result from the increased frequency of these events. Especially the frequency of extreme droughts are projected to increase. The frequency of moderate and mild droughts will not change that much. As a result, the magnitude of gross primary production reductions associated with extreme droughts will increase much more in future decades than that associated with moderate and mild droughts (203).

In 2075-2099, compared with 1850-1999, the number of extreme droughts per year is projected to increase by a factor of about 3.1 to 3.8 under the moderate and high-end scenario of climate change, respectively. These projected droughts are widely distributed with particularly high risks for the Amazon, Southern Africa, Mediterranean Basin, Australia, and Southwestern United States. As a result, the magnitude of globally averaged reductions in gross primary production associated with extreme droughts is projected to be nearly tripled by the last quarter of this century relative to that of the historical period under both scenarios of climate change (203).

On an annual basis, the positive impacts on gross primary production of CO2 fertilization globally, and higher temperature for high latitudes are much stronger than the negative impacts of more extreme droughts. Thus, globally averaged mean gross primary production is projected to be about 31% to 50% higher for 2075-2099 compared with the historical period under the moderate and high-end scenario, respectively (203).

Plants adapt to higher CO2 concentrations by closing their stomata. As a result transpiration by the plants decreases and surface warming increases, in particular, during temperature extremes. These plant physiological responses to elevated CO2 concentrations may lead to significantly higher projected extreme temperatures in Europe: annual maximum temperatures rise additionally by about 0.6 K (0.1 K in southern, 1.2 K in northern Europe) by 2070 - 2099 because of this effect. Regional climate models that do not include this effect may strongly underestimate future temperature trends (204).

For migratory birds, the timing of arrival on breeding territories and over-wintering grounds is a key determinant of reproductive success, survivorship, and fitness. Over last 30 years of the previous century in Oxfordshire, U.K., the average arrival and departure dates of 20 migrant bird species have both advanced by 8 days; consequently, the overall residence time in Oxfordshire has remained unchanged. The timing of arrival has advanced in relation to increasing winter temperatures in sub-Saharan Africa, whereas the timing of departure has advanced after elevated summer temperatures in Oxfordshire. This finding demonstrates that migratory phenology is quite likely to be affected by global climate change and links events in tropical winter quarters with those in temperate breeding areas (1).

Earlier arrival dates have also been observed for bird trapping data since 1909 on the island of Helgoland (southeastern North Sea). A relation was found not only with local warmer temperatures, but also with the North Atlantic Oscillation (NAO), during the last four decades (2). In late winter and early spring, the positive phase of NAO increased clearly, leading to prevailing westerly winds and thus to higher temperatures in the period February–April (7). For the Lake Constance region (Central Europe) the proportion of long-distance migrants decreased and the number and proportion of short-distance migrants and residents increased between 1980-1981 and 1990-1992, and this was related to increasingly warmer winters (3). According to the researchers, warmer winters lead to declines in numbers of long-distance migrants if resident birds benefit from warmer winters and impose increasing competitive pressure on migrants.

Climatic Envelope Models (CEMs) use the current range where species live to infer climatic requirements (the ‘climatic envelope’) and predict future distribution by establishing envelope movement under climate change scenarios. For all 431 bird species breeding regularly in Europe, future range sizes have been predicted to be 80% of current range sizes, with an average overlap of 39%. Future ranges were predicted to increase for 23.3% of the species, to decrease for 73.3%, and to stay the same for 1.1% of the species. The remaining 2.3% of the species were projected to become extirpated or almost so. According to these results, coastal, wetland and upland birds will be significantly worse off under CEM scenarios than birds associated with woodland, farmland and heathland, while urban birds and those using multiple habitats are doing best (56). CEM as a tool for predicting biotic change is necessarily a simplification, however. There are numerous other, often biotic, factors that influence actual range. Food sources and nest-site availability, as well as competitors, predators and parasites, are likely important factors determining range, but these may be only loosely associated with climate. CEMs also assume that species and the niches will not adapt and that species can move freely to regions with favorable climatic conditions, whereas, in reality, dispersal power might restrict movements (56).

Insect biodiversity declines because of intensive agricultural land use, the loss of natural habitats and climate change. As a result, the provision of ecosystem services essential to agriculture, such as pollination and pest control (223), also declines. In fact, the resilience of natural and agricultural ecosystems to future shocks such as those from extreme climatic events may deteriorate (224). The impacts of agriculture and climate change are related: observations from around the globe suggest that high-intensity agriculture makes insects more vulnerable to climate change. Insect biodiversity would probably benefit from mitigating climate change, preserving natural habitat within landscapes and reducing the intensity of agriculture.

The impact of the combination of these drivers – agricultural land use, the loss of natural habitats and climate change – has been studied by analyzing more than 750,000 records for almost 18,000 insect species at the global scale. This database includes snapshot samples of terrestrial biodiversity from different locations across the globe, enabling spatial comparisons of sites under different land-use and climatic conditions (225).

Higher intensity agriculture = stronger impact climate change: The observations in the database indicate that the combination of intensive agriculture and climate change has led to large reductions in insect biodiversity. Intensively used agricultural systems that experienced substantial climate warming showed 49% and 27% reductions in insect abundance and species richness, respectively, compared with those in natural habitats with no climate warming. Under the same level of climate warming, low-intensity agriculture experienced 30% and 23% reductions in insect abundance and species richness, respectively (225).

More natural habitats = more resilience to climate warming: Insects are highly sensitive to climate warming and need close access to shady conditions to survive the warmest parts of the year. Agriculture reduces the availability of these nearby conditions and makes insects more vulnerable to the effects of climate warming (226). When the intensity of agriculture is low, the effects of climate warming can be largely offset by making sure that enough shady conditions – natural habitats – are available nearby. In low-intensity agricultural areas where the surrounding natural habitat covers 75% of the land, insect abundance and species richness are reduced by 7% and 5%, respectively, compared with natural habitats with no climate warming. When only 25% of the natural habitat cover was present, these reductions were 63% and 61%, respectively. This suggests that nearby natural habitats can mitigate the effect of climate warming in low-intensity agriculture if these habitats cover a sufficiently large area (225). In low-intensity agriculture, incorporating natural habitat into agricultural landscapes can promote the maintenance of local insect biodiversity (Carvalheiro et al., 2010; Garibaldi et al., 2011; Dainese et al., 2015; Fourcade et al., 2021). This does not hold for high-intensity agriculture areas, however. According to observations in these areas, a buffering effect of nearby natural habitats was smaller or non-existent (225).

Preserve insect biodiversity: less intensive agriculture + more natural habitats: As the global demand for food increases, it is likely that the expansion of agricultural systems will continue. If this agricultural expansion is associated with a reduction in the availability of natural habitats within production landscapes or a move toward higher-intensity agriculture, our results indicate that large declines in insect biodiversity will occur, particularly as climate warming accelerates. On the other hand, the ability of natural habitats to mitigate the negative impacts of agriculture and climate change in low-intensity agriculture presents a management option that could help to maintain insect populations (225).

Insect biodiversity is essential for many ecosystem functions and services, including key services for agriculture. Ensuring that these services remain available in agricultural landscapes will benefit people and nature. Under the highest-emissions scenarios, most of the terrestrial surface will be exposed to novel high temperatures by 2070. Reducing the magnitude of changes in insect biodiversity will require strong climate mitigation and implementation of land-management strategies that increase the availability of natural habitats (225).

The range boundary of the pine processionary moth has shifted by 87 km northwards between 1972 and 2004, and (in the Alps of northern Italy) 110-230 m upwards between 1975 and 2004. This observed latitudinal and altitudinal shift was attributed to increased winter survival due to a warming trend over the past three decades (25). The trend of improved survival in previously prohibitive environments is expected to continue, causing further latitudinal and altitudinal expansion.

About 21,000 years ago the ice sheets reached their maximum extension cover during the last ice age. Global warming between this Last Glacial Maximum and the early Holocene, about 10,000 years ago, was on the order of 4 to 7°C. This is roughly comparable to the magnitude of global warming that is projected for the next 100 to 150 years if the emission of green house gasses is not reduced substantially (155). The magnitudes of changes in vegetation composition and structure since this last glacial period provide an index of the magnitude of ecosystem change that may be expected under warming of similar magnitude in the coming century (156). Although the rate of projected future global warming is at least an order of magnitude greater than that of the last glacial-to-interglacial transition (155), a glacial-to-modern comparison provides a conservative estimate of the extent of ecological transformation to which the planet will be committed under future climate scenarios.

Therefore, 594 published paleoecological records were reviewed to examine compositional and structural changes in terrestrial vegetation since the last glacial period and to project the magnitudes of ecosystem transformations under alternative future emission scenarios (154). The results indicate that terrestrial ecosystems are highly sensitive to temperature change and suggest that, without major reductions in greenhouse gas emissions, terrestrial ecosystems worldwide are at risk of major transformation, with accompanying disruption of ecosystem services and impacts on biodiversity.

The results indicate that the magnitude of past glacial-to-interglacial warming was large enough to drive changes in vegetation composition that were moderate (27% of investigated sites) to large (71%) at most locations across the globe. Changes in vegetation structure were also moderate (28%) to large (67%). Combining these results with future projections of climate change shows that over most of the globe the probability of large compositional change and of large structural change of the vegetation is less than 45% and less than 30%, respectively, under a low-end scenario of climate change (RCP 2.6). By contrast, under a high-end scenario (RCP 8.5), the probabilities of large compositional change and large structural change of the vegetation are both greater than 60%. This high-end scenario projects global warming for a so-called business-as-usually scenario: a scenario in which we do not succeed in reducing our emissions of green house gasses. Under this scenario, the rate of warming will be on the order of 65 times as high as the average warming during the last deglaciation (155).

The analogs of global warming since the last glacial period for on-going and future climate changes are crude. Nevertheless, these results provide concrete evidence that vegetation composition and structure are sensitive to changes in mean annual temperature of the magnitudes forecast for the coming century and that vegetation transformations will become increasingly extensive as temperatures increase. The authors of this study therefore conclude that terrestrial vegetation over the entire planet is at substantial risk of major compositional and structural changes in the absence of markedly reduced emissions of green house gasses. Much of these changes could occur during the 21st century. In fact, observations from around the globe indicate that current climate change may already be driving substantial changes in vegetation composition and structure (65). Impacts on planetary-scale biodiversity, ecological functioning, and ecosystem services will increase substantially if warming exceeds 1.5°C.

Climate change will generally enhance phytoplankton blooms (14) and increase dominance of cyanobacteria in phytoplankton communities, resulting in increased threat of harmful cyanobacteria and enhanced health risks, particularly in water bodies used for public water supply and bathing (15). More frequent extreme precipitation and runoff events are also expected to increase the load of nutrients to waters and in turn result in more eutrophication. Phytoplankton and zooplankton blooms in several European lakes are occurring one month earlier than 30–40 years ago (16).

Environmental impacts of agriculture under a changing climate are considered more and more important. In particular, the role of nitrate leaching on the quality of aquifers, rivers and estuaries is a global problem, now generally recognized (53). Projections made at European level for winter wheat showed for the 2071–2100 period that decreases in N-leaching predominate over large parts of eastern Europe and some smaller areas in Spain, whereas increases occur in the United Kingdom and in smaller regions over many other parts of Europe (54).

Changes in river streamflow and water temperature

Climate change will affect future flow and thermal regimes of rivers. This will directly affect freshwater habitats and ecosystem health. In particular fish species that are strongly adapted to a certain level of flow variability will be sensitive to future changes in flow regime. In addition, all freshwater fish species are exotherms, and increasing water temperatures will therefore directly affect fishes’ biochemical reaction rates and physiology (59). Rising water temperatures also decreases oxygen solubility and increases organic matter decomposition, causing lower dissolved oxygen concentrations (13,60). In addition, remobilisation and bioaccumulation of toxic substances (e.g. mercury) in fish also increase under higher water temperatures (59). Water temperature rises could therefore also affect fish populations and distributions indirectly due to its impacts on other water quality parameters (58).

Climate change significantly affects river flow regimes for 90 % of the global land surface area, compared to 25 % of the land area where flow regime is affected by dams and human water withdrawals (61). Researchers showed that water quality in rivers worldwide deteriorated during droughts and heat waves during the period 1980-2021. The water was warmer, salinity had increased while dissolved oxygen had decreased (232).

Projections have been made of global streamflow and water temperature changes for 2031–2060 and 2071–2100, compared with the control period 1971–2000, for the SRES A2 and B1 emissions scenarios and 3 Global Climate Models (58). According to these projections, overall, climate change will result in an earlier onset of the high flow (snowmelt) peak, especially in the high northern latitude zone. For 2031–2060 and 2071–2100 this earlier onset is projected to be more than 30 days for 8–10% and 10–12 % of the global surface area respectively, compared with 1971–2000. Substantial declines in mean streamflow are projected for the U.S., Europe, Southeast Asia and southern parts of South America, Africa and Australia. Climate change will further increase river temperatures during the 21st century and will result in strong declines in low (summer) flow for the end of the 21st century in large parts of the world. This combination could in particular threaten fish populations. The loss of areas with suitable freshwater habitat conditions may partly be compensated, however, by areas that will become suitable for freshwater species in the future. A northwards movement of the 21.5 °C-isoline was quantified of approximately 190–400 km for the northern hemisphere; besides, the global surface area where high water temperatures are below 21.5 °C for 95% of the time was shown to reduce from 39 % in 1971–2000 to 32–35 % in 2071–2100 (58). Hence, climate change is likely to reduce the spatial area of suitable habitats of cold water fishes, which could be invaded by cool or warm water species if other habitat requirements (e.g. food availability) are also fulfilled (62).

Changes in lake water temperature and mixing regime

Climate change is affecting the shifting of seasons in lakes, globally. Between 1980 and 2021, in Northern Hemisphere lakes, spring and summer temperatures have arrived earlier by 2.0 and 4.3 days per decade, respectively. The arrival of autumn has been delayed by 1.5 days per decade. As a result, the summer season has lengthened by 5.6 days per decade. These changes will benefit some species, by prolonging the growing season, but negatively impact others, by leading to phenological mismatches in critical activities. These changes will also affect water quality, for instance through the growth of toxic, bloom-forming cyanobacteria. This seasonal shift will proceed this century. For this century, projections for a high-end scenario of climate change indicate an additional earlier arrival of current spring and summer temperatures by 3.3 and 8.3 days per decade, respectively, and a later arrival of current autumn temperatures by 3.1days per decade. As a result, the summer season is projected to lengthen further by up to 12.1 days per decade (229).

World wide, lake's summer surface temperatures showed an average warming trend of 0.34 °C per decade for the period 1985 - 2009 (83), and an average warming trend of 0.35 °C per decade for annual minimum (winter) lake surface temperatures for the period 1973 - 2014 (194). A long-term study of 24 European lakes revealed a decline in the abundance of cold-stenothermal fish species, particularly in shallow lakes, and an increase in the abundance of eurythermal fish species, even in deep, stratified lakes (84).

Winter temperature is of great importance to lake ecosystems, and the annual minimum temperature sets a strong control on many processes occurring in lakes. Higher winter temperatures have been shown to have a striking and long-lasting influence on lake ecology (195), including the distribution and population dynamics of species (196). Winter lake surface temperatures influence mixing of deep and shallow water layers, and thus the transport of oxygen from surface to deep waters and the transport of dissolved nutrients in the opposite direction. This may affect phytoplankton and zooplankton production, and even fish (197) and could also result in a mismatch of species interactions (198). Lake ecosystem processes can also be influenced through changes in the formation of ice cover (199).

The trend of annual minimum lake surface temperatures in Europe was assessed by studying temperature observations over the period 1973 - 2014 for eight lakes in Europe, situated in Ireland, the UK, Austria and Switzerland. All of these lakes experienced a relatively steady increase in annual minimum lake surface temperature, with an average warming rate of + 0.35 °C per decade (range + 0.12 to + 0.51 °C per decade) (194). According to an analysis of observational data for ten European lakes from 1966 to 2015, annual maximum lake surface temperature has increased at an average rate of +0.58 °C per decade (218). 

Model simulations of climate change impacts on the mixing of lakes have been carried out for 635 lakes worldwide for a low-end and a relative high scenario of climate change (the so-called RCP 2.6 and RCP 6.0 scenarios, respectively). Changes are projected for 2080-2100 relative to the historic period 1985-2005 (161). Under the high scenario, many lakes are projected to have reduced ice cover; about one-quarter of seasonally ice-covered lakes are projected to be permanently ice-free by 2080-2100. Surface waters are projected to warm, with a median warming across lakes of about 2.5 °C, and the most extreme warming about 5.5 °C, under the high scenario. As a result of these changes, many lakes will mix less frequently in the future in response to climate change. The model simulations suggest that around 100 of the studied lakes are projected to undergo changes in their mixing regimes. About one-quarter of these 100 lakes currently mix once a year and will become permanently stratified systems; about one-sixth currently mix twice per year and will mix once a year in the future.

A lack of vertical mixing by the end of the twenty-first century will result in reduced upwelling of nutrients from deep to shallow waters and a decrease in deep-water oxygen concentrations, which can lead to reduced lake productivity (162) and the formation of deep-water dead zones (163), respectively. There is scattered evidence of mixing regime alterations already taking place, and the ecological consequences of these changes are starting to appear (164).

Since at least 1979, lake cold spells have become shorter‐lasting and weaker. Worldwide, the occurrence – annual number of days – of lake cold spells has declined by 3.96 days per decade from 1979 to 2022. In this period, the intensity of lake cold spells has decreased by 0.26°C per decade, globally. In Europe, the decline in lake cold spells is due to increases in the mean of surface water temperatures. Between now and 2100, lake cold spells are projected tocontinue to decrease in terms of their frequency and occurrence (242). 

Groundwater and dependent ecosystems

Future climate change will affect recharge rates and, in turn, the depth of groundwater levels and the amount of available groundwater for ecosystems (10). Ecosystems will show a more contrasted variation in temperature and nutrient concentrations when fed by local groundwater systems compared with regional ones (64). As a consequence, it is likely that larger systems will be more resilient to climate change (63).

The warming of the upper layers of the ocean drives greater stratification of the water column, reducing mixing in some parts of the ocean and consequently affecting nutrient availability and primary production. These changes have increased the size of the nutrient-poor “ocean deserts” of the Pacific and Atlantic by 6.6 million km2, or 15%, over the period 1998 to 2006 (42). General circulation models also predict that oxygen concentrations in the upper layers of the ocean are likely to decrease as a consequence of increasing stratification (44); this is supported by recent observations (43). The uneven distribution of heating of the world’s oceans also strongly influences the behavior of ocean currents (45), which play critical roles in the dynamics, local climates, and biology of the ocean.

The oceans have absorbed approximately one-third of the carbon dioxide produced by human activities (46). A quantification of the oceanic sink for anthropogenic carbon dioxide (CO2) over the period 1994 to 2007 shows an average uptake rate that represents 31 ± 4% of the global anthropogenic CO2 (169). The absorption of anthropogenic CO2 has acidified the surface layers of the ocean, with a steady decrease of 0.02 pH units per decade over the past 30 years and an overall decrease since the pre-industrial period of 0.1 pH units (46). The IPCC concludes that the ocean has taken up between 20-30% of total anthropogenic CO2 emissions since the 1980s causing further ocean acidification (200).

Global satellite observations of daily marine coastal algal blooms show that the spatial extent and frequency of blooms increased significantly, globally, between 2003 and 2020. This increase is partly due to recent increases in sea surface temperature (227).

Compound extremes

Triple threat extremes of heat, acidification and oxygen loss in oceans have expanded 39‐fold, now last 3‐times longer, and have become 6‐times more intense since the early 1960s (234).

Climate change is increasing the temperature of the ocean’s surface water and its acidity and decreasing its oxygen content. These changes are already affecting marine life and fisheries. Fish are migrating from the warmed parts of the oceans, leaving local populations with a sharp reduction in fish catches. The extra CO2 absorbed by the oceans is making seawater more acidic, dissolving the shells of marine creatures. Too little oxygen can induce large-scale mortality of marine life. And heated oceans also increase the risk of hurricanes. In the last 100 years, the frequency of marine heat waves has increased by tens of percents, and they last much longer now. Ocean acidification is in a nearly permanent state of extremes. Low oxygen content locally even leads to dead zones.

These changes are worrying. What’s even more worrying is the fact that these extremes – high temperature, high acidity and low oxygen content – can occur at the same spot at the same time. These compound marine extremes have occurred in recent years, are likely to happen again in the future, and will disrupt marine ecosystems and fisheries around the world. Such compounded extreme events can have a much larger impact than that of individual stressors (235).

The ’triple threat’ of compound extremes of heat, acidification and little oxygen in ocean waters used to be relatively rare, but that’s no longer the case. Since the 1960s, these triple threat extremes ‘have expanded 39‐fold, now last 3‐times longer, and have become 6‐times more intense since the early 1960s’, researchers conclude in a recent publication in the journal Advancing Earth and Space Sciences (234). 2.3% of the volume of the global top 300 meters of the oceans is now under conditions of triple compound extremes. The top 300 meters reflects the habitat range of vertically migrating organisms (236). These compound extremes are the most severe in the tropics and north Pacific Ocean.

Projected changes in the oceans by 2100

The IPCC concluded that mean ocean surface temperature has increased since the 1970s at a rate of 0.11 (range 0.09 to 0.13) °C per decade, and forms part of a long-term warming of the surface ocean since the mid-19th century (201). By 2100, the top 2000 m of the ocean are projected to take up 2-4 times (low-end scenario of climate change, RCP2.6) or 5-7 times (high-end scenario, RCP8.5) more heat than the observed accumulated ocean heat uptake since 1970 (200)

On a global scale, ocean animal biomass consistently declines with climate change from the year 1970 to 2100. This was concluded from model simulations of six global marine ecosystem models. These models were forced with two global climate models based on four scenarios of climate change: a low-end scenario, two intermediate scenarios and a high-end scenario (RCP2.6, RCP4.5 and RCP6.0, and RCP8.5, respectively). According to the authors of this study, their results represent the most comprehensive outlook on the future of marine animal biomass to date (170).

By 2100, the model simulations indicate a decrease of mean global animal biomass by 5% (±4% SD) under the low-end scenario of climate change and 17% (±11% SD) under the high-end scenario. This agrees with an average 5% decline for every 1 °C of global warming. According to these results, limiting future warming to 1.5 °C to 2.0 °C above preindustrial levels would limit biomass declines to 4 to 6% by 2100, underscoring the potential impact of climate change mitigation according with the Paris Agreement. A comparison with biomass trends of scientifically assessed fish stocks (171) shows that the model simulations reflect observed trends for assessed fish stocks, providing confidence in the future projections.

Increasing temperature and decreasing primary production primarily drive the projected biomass declines. Among others, ocean warming causes increased ocean stratification, which reduces nutrient availability in the upper ocean, leading to decreased primary production and lower energy supply for higher trophic levels (172). Climate change impacts are amplified at higher trophic levels. Fishing at the current levels of intensity does not seem to alter the effects of climate change substantially (170).

Generally, warming waters and enhanced primary production are expected to facilitate species expansions and biomass increases in polar regions, while tropical areas may experience pronounced species losses as thermal thresholds are exceeded. In temperate regions, warming is expected to change species composition, and reduced primary production due to enhanced stratification will result in biomass declines (173). Thus, projected decreases in animal biomass are largest at middle to low latitudes, where many nations depend on seafood and fisheries, and where marine biodiversity is already threatened by multiple human activities (174). In turn, the largest increases are projected at high latitudes, highlighting new opportunities for, and potential conflict over, resource use, but also an urgent need for protecting sensitive species and rapidly changing ecosystems (170).

Declines are similar for all scenarios of climate change through to mid-century, after which they begin to diverge. Thus, the consequences of different scenarios of green house gasses emissions may not be distinguishable over the next two to three decades but differ markedly in the long term (170).

Projected changes in the Mediterranean Sea

Winter warming is occurring at a faster rate than summer warming for ecosystems around the world. For projections for the Mediterranean Sea by 2040, a far greater effect on coastal communities was found because of winter warming compared with summer warming (221).

Marine heat waves - globally

The global ocean has warmed substantially over the past century. But that’s not all. In addition to this long-term persistent warming, discrete periods of extreme regional ocean warming, called ‘marine heat waves’, have increased in frequency (166,220).

Marine heatwaves are prolonged (>5 days) anomalously high sea surface temperature events. They can have strong ecological and socioeconomic impacts, such as mass coral bleaching in tropical regions, biological regime shifts in temperate regions and enhanced coastal urban heat islands (239).

Satellite observations reveal that marine heatwaves have very likely doubled in frequency between 1982 and 2016, and that they have also become longer-lasting, more intense and extensive. In the first decade of the satellite record, 27 large marine heatwaves occurred with an average duration of 32 days and an average peak temperature anomaly of 4.8°C. By contrast, 172 large marine heatwaves occurred in the past decade, with an average duration of 48 days and an average peak temperature anomaly of 5.5°C (220). Globally, the duration, maximum intensity, and total magnitude and area of marine heatwaves have increased in the period 1982–2020 (228).

From September 1981 to December 2017, >30,000 marine heatwaves have occurred globally. The majority (90%) of these heatwaves lasted <14 days and covered <0.2 million km2. The 300 largest marine heatwaves covered, on average, 1.5 million km2, lasted 40 days, and had a peak temperature anomaly of 5.0°C above climatology (220).

Between 2006 and 2015, 84% to 90% of all globally occurring marine heat waves are attributable to the temperature increase since 1850 - 1900 (201). Marine heat waves have negatively impacted marine organisms and ecosystems in all ocean basins in the first two decades of this century, including critical foundation species such as corals, seagrasses and kelps (201,220). 

The IPCC concluded that marine heatwaves are projected to further increase in frequency, duration, spatial extent and intensity (maximum temperature). Climate models project increases in the frequency of marine heatwaves by 2081 - 2100, relative to 1850 - 1900, by approximately 20 to 50 times under a low-end (RCP2.6) and high-end scenario of climate change (RCP8.5), respectively (200), largely due to increases in mean ocean temperature (201). The intensity of marine heatwaves is projected to increase about 10-fold under the high-end scenario (RCP8.5) by 2081 - 2100, relative to 1850 - 1900 (200).

Similar to atmospheric heat waves, marine heat waves are defined as periods when daily sea-surface temperatures exceed a threshold for a number of days. In a recent study in Nature Climate Change, the global increase in marine heat wave events and the implications for marine ecosystems have been inventoried (165). The authors defined a marine heat wave as a period of at least 5 consecutive days when sea-surface temperatures exceed the 10% highest temperatures at that part of the ocean at that time of year. Previous research has already shown that, following this definition, the frequency and duration of marine heat waves have increased significantly over the past century across most of the global ocean: on average, there were over 50% more marine heat wave days per year in the period 1987-2016 compared to 1925-1954 (166). The IPCC also concluded that marine heatwaves have very likely doubled in frequency since 1982 and are increasing in intensity (200).

The study shows that marine ecosystems in the southwest Pacific and the mid-west Atlantic are particularly at risk. In addition, regions are at risk where rapid increases in the annual number of marine heat wave days overlap with existing high-intensity non-climate human stressors, such as overfishing and pollution. These regions include the central west Atlantic, the northeast Atlantic and the northwest Pacific. In Europe, recent marine heat waves in the Mediterranean Sea have been linked to local extinctions, decreased rates of natural carbon sequestration, loss of critical habitat and diminished socioeconomic value. Globally, observed increased coral bleaching, decreased sea grass density and decreased kelp biomass were correlated with the increased annual number of heat wave days.

Future projections indicate that marine heat waves will become more frequent, more intense and longer lasting throughout the twenty-first century (167,220). They may affect entire ecosystems such that they disrupt the provision of ecological goods and services in coming decades. Hundreds of millions of people benefit from these services (168).

Marine heatwaves were extremely rare events during preindustrial times, with expected return periods of hundreds to thousands of years. Under the condition of 1.5°C of global warming, their return periods are reduced to, mostly, ten to hundreds of years. Under the condition of 3°C of warming, their return periods are only, mostly, 1 to 10 years (220).

Observations 1982–2022

A study on global changes in the frequency, intensity, and duration of marine heatwaves over the period 1982–2022, using satellite observations of sea surface temperatures, showed that, on average, the frequency, intensity, and duration of 20‐, 50‐ and 100‐year marine heatwaves have increased from 1982 to 2022. The duration of heatwaves has increased on about half of the world’s oceans, especially in the Arctic (243).

During 1982–2022, marine heatwaves with a 100‐year return period have increased in frequency by 67%, have become more intense by 23% and last 137% longer, on average. These changes are for more than 80% due to man-made climate change. Heatwaves will become more frequent, intense, and persistent in the future (243).

The scientists used the following definition for a marine heatwave: an event where sea surface temperature exceeds a critical temperature for at least 5 consecutive days. The critical temperature was defined as the 90th percentile of the distribution of sea surface temperature registrations during the period 1982–2022 (243).

Marine heat waves - Mediterranean Sea

In the Mediterranean Sea, unprecedented mass mortality events and changes in marine ecosystems were reported in the summers of 1999 (181), 2003 (182), 2006 (183) and 2008 (184). These impacts were due to extreme warming of sea surface water, called a ‘marine heat wave’. The duration of the 2022/23 marine heatwave was unprecedented; its peak intensity was comparable with the exceptional event in summer 2003 (230).

Marine heat waves are extreme events of abrupt but prolonged periods of high sea surface temperatures at a particular location (185). They can occur anywhere, at any time, with the potential to propagate deeper to the water column (186). A recent study has already identified significant increases in marine heat waves globally over the last century, including the Mediterranean Sea (187). These events may substantially disrupt marine ecosystems, including biodiversity die-offs and tropicalisation of marine communities (188), extensive species migrations (189), strandings of marine mammals and seabirds, toxic algal blooms (190) and extensive coral bleaching (191). In addition, there are major implications for fisheries as well (189).

The current situation

One of the first-detected marine heat waves worldwide occurred in the Mediterranean in the hot summer of 2003. Surface seawater was 2-3 °C warmer than normal at that time of the year, and this event lasted for over a month (192). In 2007, in the eastern Mediterranean, a marine heat wave developed with sea surface temperature more than 5 °C above the mean for the time of year (193).

From observations during 1982 – 2012, characteristics of current marine heat waves were derived. Currently, these events occur 0.8 time a year and last for up to 1.5 months between July and September. Their mean intensity varies from 0.3 to 0.9 °C, which means they are up to 0.9 °C warmer than the mean situation for that time of the year. They cover up to 20-90% of the Mediterranean Sea surface (180).

These events seem have become a regular phenomenon; extreme marine heatwaves also occurred in the Mediterranean in the summers of 2018 and 2022 (231). 

At middle and high latitudes, phytoplankton massively grows (i.e., blooms) once a year at the sea surface, and introduces a major carbon flux into the ecosystem that sustains larger animals. Marine heatwaves in the winter intensify water column stratification thus hindering the deep‐ocean nutrient transport to the surface, which is essential for phytoplankton to bloom (240). Reduced carbon stocks within the upper layer and smaller phytoplankton sizes potentially diminish the energy source to support deep‐water ecosystems, which may trigger cascading effects on the entire ecosystem of the area (241).

Warming sea water

The temperature change projected for the Mediterranean Sea by the end of the century ranges between 0.81 and 3.71 °C in the upper layer (0–150 m), between 0.82 and 2.97 °C in the intermediate layer (150–600 m) and between 0.15 and 0.18 °C in the deep layer (600 m—bottom). These results are based on the emissions scenarios RCP8.5 (high-end), RCP4.5 (moderate) and RCP2.6 (low-end) (205).

Future evolution of summer marine heat waves

The future evolution of summer marine heat waves (frequency, duration, intensity and severity) in the Mediterranean Sea was assessed using an ensemble of high-resolution regional climate system models including ocean-atmosphere interactions, driven by GCMs and a multi-scenario approach (low-, moderate and high-end scenarios RCP2.6, RCP4.5 and RCP8.5). The assessment was made for the near future (2021 - 2050) and the far future (2071 - 2100), compared with 1976 - 2005 for reference. In this assessment, an anomalously warm water event is called a marine heat wave when it fulfils certain conditions. It has to last for at least 5 days, for instance, and it must cover at least 20% of the Mediterranean Sea surface (180).

In general, marine heat waves become stronger and more intense in response to increasing greenhouse gas forcing and especially towards the end of the century. By 2100 and under RCP8.5, model simulations project at least one long-lasting marine heat wave event every year, up to three months longer, about 4 times more intense and 42 times more severe than present-day events. They are expected to occur from June-October and to affect at peak the entire Mediterranean basin (180).

These events will impact the vulnerable Mediterranean Sea ecosystems. The widespread consequences from the event of 2003 could become the “new normal”. Under a moderate or high-end scenario of global warming (RCP4.5 and RCP8.5), events with characteristics similar to the observed exceptional 2003 event seem to become the new standard already in the period 2021 - 2050. Especially under the high-end scenario (RCP8.5) and at the end of this century, marine heat waves can become three times longer than the one of 2003, with mean intensities three times higher (180).

Marine heat waves - Baltic Sea

Marine heatwaves can cause devastating impacts in coastal marine ecosystems, particularly in shallow marginal seas such as the Baltic Sea. Direct impacts of marine heatwaves in the Baltic Sea were observed during the summer of 2018. Anomalous high temperatures up to 21 °C were registered in coastal bottom waters that likely led to increased emissions of carbon dioxide and methane (237). A prominent example of a negative effect of these heatwaves for the Baltic Sea are harmful cyanobacteria blooms. These blooms negatively affect human recreation and aquaculture near the coasts and stimulate oxygen deficiency in the open sea (238).

Marine heat waves - northwest European shelf (North Sea, Irish Sea)

In June 2023, a marine heatwave developed in the North Sea and the coastal seas of Ireland and Great Britain. The event was unprecedented in terms of intensity and duration. Sea surface temperatures were on average +2.9 °C higher than the long-term average at this time of year, and these extremely high temperatures lasted for 16 days, which is unprecedented in – at least – the last 40 years. During the peak week of this heatwave, sea surface temperatures locally showed +5 °C anomalies in the central North Sea and the Irish shelf. Persistent anticyclonic weather patterns with weak winds and long hours of sunshine were responsible for first increasing the ocean stratification and then sea surface temperatures in the first half of June. In the second week of June, warm, moist air advected over the shelf by a remote low-pressure system additionally reduced ocean cooling (239).

Impacts on marine ecosystems

Some of the major impacts of climate change on marine ecosystems are (50, see also 80):

• Effects on Ecosystem Function. The distribution and abundance of phytoplankton communities throughout the world, as well as their phenology and productivity, are changing in response to warming, acidifying, and stratifying oceans (46, 42). The annual primary production of the world’s oceans has decreased by at least 6% since the early 1980s, with nearly 70% of this decline occurring at higher latitudes (47).
• Reduced Habitat Complexity. Among the most clear and profound influences of climate change on the world’s oceans are its impacts on habitat-forming species such as corals, sea grass, mangroves, salt marsh grasses, and oysters. Collectively, these organisms form the habitat for thousands of other species. For example, mass coral bleaching and mortality, the result of increasing temperatures, is already reducing the richness and density of coral reef fishes and other organisms (48).
• Exotic Species. Recent accelerated warming of high-latitude environments has increased the chances that “exotic” marine species being transported from lower latitudes are able to establish themselves and spread (49). A rising number of species are expanding their ranges, often with large-scale impacts on ecosystems at the destination.
• Diseases.There are large uncertainties about the interplay between ecological dynamics and potential vectors or disease-causing agents, making it difficult to predict whether the incidence of particular diseases will increase or decrease in a warmer and more acidic world (50).
• Thresholds. Recent evidence suggests that there is now a growing risk that several thresholds will soon be exceeded (51). For example, increasing ocean temperatures and acidities associated with atmospheric CO2 concentrations of 450 parts per million (ppm) represent a serious threat to ecosystems such as coral reefs. In the latter case, temperatures that exceed 2°C above pre-industrial temperatures are very likely to drive an unsustainable frequency of mass coral bleaching and mortality, whereas ocean acidification associated with atmospheric CO2 above 450 ppm will push coral reefs into a negative carbonate balance (48).

Changes in the distribution of fish species associated with European tidal estuaries along the northeast Atlantic seaboard were studied by comparing the mean latitude of distributions according to fish survey data from the 1970s with data from 2004–2007. 55 tidal estuaries from Portugal to Scotland were studied. Among the 15 most common species, 11 displayed a positive difference between current and past mean latitudes suggesting a northward shift of the populations. These results indicate that a number of fish species associated to estuaries seem to have migrated northwards over the last 30 years, possibly due to water warming (55).

Impacts on the Mediterranean Sea

More than 700 non-indigenous marine plant and animal species have been recorded so far in the Mediterranean (157), many of them are favoured by the warmer conditions (158,159). More than 50% of these have entered through the Suez Canal. The eastern Mediterranean is the area displaying the most severe environmental effects from invasive species (160). During the coming decades, more tropical invasive species are expected to find suitable environmental conditions to colonize the entire Mediterranean, spreading the ecological consequences already observed in some areas (160).

In addition to higher temperature of Mediterranean seawater, the salinity is also projected to increase due to the increase of the freshwater deficit (i.e. the excess of evaporation over precipitation and river runoff) and the related increase in the net salt transport at the Gibraltar Strait (205).

Seafood quantity and quality changes, causing malnutrition

More than 4.5 billion people obtain at least 15% of their animal protein intake from fish (209). In some countries, the share of protein from fish can be greater than 50% (210). Ocean acidification not only affects the available seafood, but the nutritional qualities of this food as well: when the food source of fish species changes, the species that are harvested for consumption also change.

Direct negative effects are damage to the survival of eggs and early larval stages for some fish species, and the survival, growth, development, and reproduction across different groups of organisms (211). The culture of shellfish in coastal waters is already experiencing decline in some regions because of the reduced carbonate saturation and mortality of juveniles unable to grow shells (212). Many aquaculture species and systems are thought to be less susceptible to ocean acidification, however.

Indirect effects of ocean acidification can be driven by changes in food source, especially phytoplankton, which form the foundation of the marine food web. Also, degradation or loss of coral reefs or macro algal forests will induce ecosystem effects that alter the characteristics of dependent fisheries (213). Many fish and invertebrates rely on structures such as coral reefs for ecosystem services, including the provisioning of habitat and feeding grounds. The persistence of these dependent species will likely be threatened under ocean acidification as coral reefs bleach and transition to more simple habitats. Tropical fisheries will, therefore, likely be degraded by ocean acidification, with profound consequences for the people that depend upon them.

Pollutants accumulate in human tissue

Ocean acidification affects the propagation of contaminants through the marine environment. These contaminants, such as heavy metals, pharmaceuticals, and active ingredients in personal care products, mainly enter the environment via municipal effluent discharges, aquaculture, animal husbandry, horticulture runoff and waste disposal. They can be taken up by organisms and then transferred through the food chain, accumulating in tissues of organisms at higher trophic levels, including humans. Ocean acidification may intensify this bioaccumulation (214).

Natural toxins also accumulate

Ocean acidification can modify the abundance and chemical composition of harmful algal blooms. These algae are food to shellfish, their natural toxins accumulate in shellfish, and this may in turn negatively affect human health. Ocean acidification has been found to lead to an increased algal growth rate (215), a change that could accelerate the production of toxic algae. Moreover, the toxicity of the blooms is likely to increase. These toxins not only enter the human body through food, but also trough the air. They can be releases and mixed into seawater aerosols when the algae are broken up in the surf (216).

Loss of habitats impacts mental health

The livelihoods of approximately 300 million people depend on marine fisheries. In the past, the collapse of regional fishing industries in Europe has caused unemployment, financial pressures, societal stress, and a decline in the mental health of individual fishers and the fishing community. The oceans also provide nature-based recreation and exercise opportunities that promote “nature-connectedness”, all of which have been shown to be a fundamental of mental health (217). The anticipated loss of (access to) habitats due to ocean acidification reduces recreation opportunities and the formation of associated social connections.

Loss of potential medical resources

The vast biodiversity of the oceans offers opportunities for medicines and other natural products to counter the impacts of environmental stressors on human health. So far, the exploration of these opportunities has been limited. As ocean acidification is expected to negatively impact biodiversity, it will also impair our ability to prospect for new medications in such areas (206).

How to deal with health effects of ocean acidification

For the sake of our health it is important to conserve and restore biodiversity and ecosystems. This starts close to our homes, where outputs from industry and wastewater facilities, agricultural discharge, and urban runoff deteriorate the water quality of receiving marine ecosystems and thereby reinforce the effects of global changes, such as ocean acidification. Monitoring and managing water quality at a local scale reduces the effects of these global-scale changes. In addition, marine-protected areas, for instance, could encourage the resistance and resilience to the loss of species due to acidification (206).

It looks like we have to adapt to some disturbance of ecosystems and biodiversity due to ocean acidification. One way to adapt is to boost the human consumption of species that benefit from ocean acidification, such as fast-growing macro algae. This may compensate for the loss of parts of the seafood diet and help satisfy future demands. Still, changes in the availability of fish species may lead to malnutrition and increase inequalities in societies. Those who are able to afford the price of an increasingly rare resource will still have access, many others will not (206).

Plankton in the Greater North Sea have shown a northerly movement of about 250 km per decade over the past 40 years which appears to have accelerated since 2000 (145). Very fast rates of northwards movement were observed in the coastal waters of southern Norway from 1997 and 2010. About 1600 benthic marine species were found, and of these 565 species had expanded their distribution northwards along the coast, at rates of 500-800 km per decade (146). Phytoplankton and highly mobile pelagic species are the fastest migrating organisms; their migration rate can be an order of magnitude faster than those of terrestrial species (147). 

Large-scale changes in the biogeography of calanoid copepod crustaceans have been detected in the northeastern North Atlantic Ocean and adjacent seas: a northward extension of warm-water species associated with a decrease in the number of colder-water species. These changes were attributed to a regional increase in sea surface temperature. In addition it was shown that long-term changes in phytoplankton, zooplankton and salmon are highly significantly related to sea surface temperature in the northeastern Atlantic, Northern Hemisphere temperature, and the North Atlantic Oscillation (8).

All biological variables show a pronounced change which started after circa 1982 for euphausiids (decline), 1984 for the total abundance of small copepods (increase), 1986 for phytoplankton biomass (increase) and Calanus finmarchicus (decrease) and 1988 for salmon (decrease). This cascade of biological events led to an exceptional period, which is identified after 1986 to present and followed another shift in large-scale hydro-climatic variables and sea surface temperature. This regional temperature increase therefore appears to be an important parameter that is at present governing the dynamic equilibrium of northeast Atlantic pelagic ecosystems with possible consequences for biogeochemical processes and fisheries (8).

The Mediterranean basin is considered to be one of the regions that will face the largest changes in climate worldwide. The expected temperature increase of 3-5 °C in the Mediterranean this century will strongly increase potential evapotranspiration while annual rainfall will decrease. The expected shifts in vegetation belts resulting from increased aridity and a 3 °C increase in temperature will be an upward shift of ca. 545 m and a 50-80 km northwards shift in latitude (144). These impacts will be exacerbated on islands where no (or insufficient) areas are available for such shifts. The flora and vegetation of the alpine areas and the spatially restricted summit areas of mountain ranges will probably be the most threatened.

So far, there is little evidence for direct depletion or extinction of populations due to climate change on these islands, however. Species appear to be able to cope with drastic climate change thanks to differences in small-scale habitats. The high habitat heterogeneity of Mediterranean-type ecosystems may thus represent an ‘ecological insurance’ for the future persistence of plant species at local scale, allowing species to migrate locally in more favourable ecological niches. Nevertheless, at the scale of small islands, this may not be sufficient to ensure the survival of highly specialized plants (142). 

At this stage it is extremely difficult to put forward indicators for the economic effects on ecosystems associated with climate change. The problem is potentially severe and economically significant, but that we know relatively little both ecologically and economically about the impacts of future biodiversity loss (13).

Because both the changes in the physical oceanographic environment and marine biological response to the physical changes are poorly understood, it is not possible to make projections for changes in marine biodiversity and ecosystems (13).

According to the IPCC (67), climate change translates into a key risk of large-scale loss of ecosystem services, including water purification by wetlands, removal and sequestration of carbon dioxide by forests, crop pollination by insects, coastal protection by mangroves and coral reefs, regulation of pests and disease, and recycling of waste nutrients.

The increase in winter temperatures associated with a shift of NAO towards its positive phase in recent years has resulted in a relief from winter stress for many species and populations (29). This has reduced mortality rates during winter, thereby influencing local population dynamics and allowing, for example, the northward expansion of many species. In contrast to winter warming, the recent increase in summer temperature has had fewer ecological consequences, as it has not been large enough to cause an increase in heat stress to critical levels during summer (32).

Consequently, the number of studies reporting range expansions far exceeds the number reporting retractions and local extinctions at southern or low elevation species boundaries (30).

However, things may change. The European summer of 2003 was an example of summer temperatures that did indeed result in widespread drought and heat stress (31). Species and populations might suffer increasingly from heat stress in future, thus increasing the relative importance of range reductions and other adverse effects of global warming in temperate regions (32).

• Strategy “benign neglect” refers to the most adaptable and/or insensitive species and those with low exposure that will need minimal interventions with low-level monitoring.
• As exposure and sensitivity increase and autonomous-response capability decreases, substantial benefits may result from simply designating new protected areas and undertaking low-level habitat management to reinforce species’ intrinsic dispersal and migration mechanisms.
• Dynamic placing of buffer zones, removal of barriers, and establishment of corridors or “stepping stones” within a wider landscape may be necessary.
• In any habitat or community, some species may require specific actions for their conservation or to retain critical biological interactions. Intermediate strategies—including intervention to arrest or divert natural succession or ecosystem regime shifts, maintenance of specific habitats or habitat diversity, and targeted interventions to restore disrupted species interactions (e.g., pollinator or plant-herbivore networks)—are now widely used.
• Intensive intervention strategies include assisted migration and translocation of species outside their native range. Reestablishment and rewilding involve intensive habitat management to restore critical habitat types, with whole communities recreated from populations surviving elsewhere.
• Finally, controlled ex situ conservation, involving captive breeding and genetic manipulation in zoological and botanical gardens and recently developed cryogenic seed banks, may contribute to conserving species or populations with a view to future release or reintroduction.

Observed responses to paleoclimatic change emphasize the importance of refugia—both macro- and microrefugia—as key landscapes to protect (57).

Within the context of conservation practice, vegetation intactness is more significant than climate stability for ecosystem vulnerability: in terms of ecosystem degradation or species extinctions, reduction in vegetation intactness is a greater threat than climate change at present, and is likely to be in future. Conservation practitioners will have a much greater chance to influence the intactness of an ecosystem rather than its robustness to future climatic conditions (which can only be changed through international mitigation efforts), and therefore a focus on maintaining ecosystem integrity should always be a primary conservation objective (58).

Strengthen reserve networks

• Increase connectivity
• Increase number of reserves
• Protect large areas, increase reserve size
• Create/manage buffer zones around reserves
• Create ecological reserve networks, large reserves, connected by small reserves and stepping stones
• Institute flexible zoning around reserves
• Secure boundaries of existing reserves
• Protect many small reserves rather than single large
• Create linear reserves oriented longitudinally

Promote the resilience and adaptive capacity of ecosystems

• Practice adaptive management
• Improve techniques for and do more restoration wetlands, rivers, matrix
• Soften land use practices in the matrix
• Re-asses conservation goals
• Adopt long-term and regional perspective in planning, modeling, and management
• Broaden genetic and species diversity in restoration and forestry
• Develop adaptation strategies now; early adaptation is encouraged
• Manage for flexibility, use of portfolio of approaches, maintain options
• Preserve genetic diversity in populations
• Represent each species in more than one reserve
• Institute government reform (i.e. adaptive governance)
• Maintain natural disturbance dynamics of ecosystems
• Proactive habitat management to mitigate warming
• Manage the matrix
• Anticipate surprises and threshold effects i.e. major extinctions or invasions
• Design biological preserves for complex changes in time, not just directional change
• Study and protect metapopulations
• Protect functional groups and keystone species
• Adjust park boundaries to capture anticipated movement of critical habitats
• Create institutional flexibility
• Manage for landscape asynchrony
• Manage populations to reduce temporal fluctuations in population sizes

Establishing additional conservation areas to better account for climate change impacts

• Protect full range of bioclimatic variation
• Protect current and predicted future refugia
• Locate reserves in areas of high heterogeneity, endemism
• Locate reserves at northern boundary of species’ ranges
• Protect mountains
• Protect primary forests
• Protect urban green space
• Increase wetland protection
• Focus protection on sensitive biomes
• Locate reserves so that major vegetation transitions are in core
• Locate reserves at core of ranges

Reduce pressures on ecosystems

• Mitigate other threats, i.e. invasive species, fragmentation, pollution
• Do not implement CO2 emission mitigation projects that negatively impact biodiversity
• Limit CO2 emissions
• Strategic zoning of land use to minimize climate related impacts
• Schedule dam release to protect stream temperatures

Heavy human intervention (‘kiss-of-life’ options)

• Practice intensive management to secure populations
• Translocate species
• Protect endangered species ex situ
• Establish neo-native forests plant species where they were in the past, but are not found currently

Other

• Integrate climate change into planning excercises
• Improve inter-agency, regional coordination
• Increase and maintain monitoring programmes
• Increase interdisciplinary collaboration
• Promote conservation policies that engage local users and promote healthy human communities
• Do regional impact assessments
• Leadership by those with power senior management, government agencies
• Create education programmes for public and land use practices and effects on and with climate
• Develop best management practices for climate change scenarios
• Increase investment in climate related research
• Create culturally appropriate adaptation/management options
• Initiate dialogue among stakeholders
• Use predictive models to make decisions on where to situate new reserves
• Provide education opportunities and summaries of primary literature for management staff to learn and network about climate change
• Action plans must be time-bound and measurable
• Establish cross-national collaboration
• Focus on annuals rather than perennials near climate boundaries
• Institutional capacity enhancement to address climate change
• Institute reform to improve support for interdisciplinary, multi-institutional research
• Manage human-wildlife contact as change occurs
• Increase social acceptance of shared resilience goals
• Promote personal action plans among employees to reduce emissions
• Use caution in predictive modelling because the responses of some species are not well predicted
• Use simple decision rules for reserve planning
• Use social networks for education about climate change
• Use triage in short-term to priotize action