Tourism in numbers - Europe
Europe is the most important tourist region in the world. According to UNWTO, in 2006 nearly 55% of all international tourist arrivals (461 million) were on the “old continent”.Southern Europe and the Mediterranean region are the favourite holiday destinations in Europe. According to UNWTO, in 2006 about 165 million tourists visited these regions (25).
At present, the predominant summer tourist flows in Europe are from north to south, to the coastal zone. However, coastal and mountain tourism are the segments that are most vulnerable to climate change, and the Mediterranean region is the world's most popular holiday region: it attracts some 120 million visitors from northern Europe each year, the largest international flow of tourists on the globe, and their spending is in excess of EUR 100 billion (23).
Tourism in numbers - Switzerland
Tourism accounted for 2.8-2.9% of the Gross Domestic Product (GDP) in the period 2005-2009 in Switzerland (27). Foreign tourists accounted for approximately 5.3% of the income generated by exports in Switzerland (27). This places tourism in the 3rd position of Swiss exports. Tourism in Switzerland is also an important employer: in 2009, 4.1 % of jobs were directly generated by this industry (27). In some tourist destinations in the Alps, the value added of tourism amounts to more than 80% of the regional GDP (26).
Tourism in numbers - The Alps
The Alps are also among the most visited regions. About 60-80 million people visit the Alps each year as tourists. Tourism activities in the Alps generate close to EUR 50 billion in annual turnover and provide 10-12% of the jobs (5,9). There are over 600 ski resorts and 10,000 ski installations in the Alps. France, Switzerland, Austria and Italy provide over 85% of Europe’s skiing area. France has the highest winter season turnover of all these four countries (5,10).
19 % of the area faces increasing economic problems. For 18 % of the area, the economy, settlements and cultural heritage are breaking down as people leave (12). This is particularly the case in southern France, some parts of Italy (e.g. Piemont) and Slovenia. Only tourism can reverse this trend but the number of tourists visiting the Alps has either been constant or decreasing since the 1980s. In these areas forests move into grassland and the area becomes less attractive for tourism (13).
Vulnerabilities – In general
There are four broad categories of climate change impacts that will affect tourism destinations, their competitiveness and sustainability (24):
- Direct climatic impacts
- Indirect environmental change impacts. Changes in water availability, biodiversity loss, reduced landscape aesthetic, altered agricultural production (e.g., wine tourism), increased natural hazards, coastal erosion and inundation, damage to infrastructure and the increasing incidence of vector-borne diseases will all impact tourism to varying degrees.
- Impacts of mitigation policies on tourist mobility. Policies that seek to reduce GHG emissions will lead to an increase in transport costs and may foster environmental attitudes that lead tourists to change their travel patterns.
- Indirect societal change impacts. Climate change is thought to pose a risk to future economic growth and to the political stability of some nations. Climate change is considered a national and international security risk that will steadily intensify, particularly under greater warming scenarios. Tourists, particularly international tourists, are averse to political instability and social unrest.
Vulnerabilities - Trends in the past
It was the winters with little snow at the end of the 1980s (1987/88–1989/90) that caused a stir (1,8). The big difference to earlier periods with little snow was that the capital intensity of ski tourism had considerably increased. The snow shortage at the end of the 1980s left a clear mark on the tourist trade. The earnings of cable-way companies decreased by 20% compared to the ‘normal’ winter of 1986/1987. Particularly affected were smaller companies at lower and medium altitudes (1).
Due to changing climate conditions, the overall number of lifts in the Alps is slightly decreasing and in areas at low altitudes ski-resorts are already closed or will be closed in the near future (11).
A number of companies in regions above 1700 m achieved good and even first-rate results due to the lack of snow at lower and medium altitudes. The slump in the hotel and holiday-apartment trade was less pronounced, as they also accommodate non-skiers. Hotel rooms and holiday apartments also tend to be booked quite a long time in advance. The question does arise, however, as to how long tourists will remain loyal to a location and keep returning to it if they are repeatedly confronted with inadequate snow conditions (1).
The experience acquired by Swiss ski resorts shows that a ski resort can be considered snow-reliable if, in 7 out of 10 winters, skiing is possible on at least 100 days between December 1 and April 15 (2). It is estimated that the snowline, as well as the line of natural snow-reliability, will rise by 150 m with 1°C warming (6). On this basis, climate change could result in a 150 m, 300 m and 600 m increase in the altitude of the natural snow-reliability for 1, 2 and 4°C of warming (5).
The duration of snow cover is expected to decrease by several weeks for each °C of temperature increase in the Alps region at middle elevations (14). At the most sensitive elevation in the Austrian Alps (600 m in winter and 1400 m in spring) and with no snowmaking adaptation considered, a 1°C rise leads to four fewer weeks of skiing days in winter and six fewer weeks in spring (15). A 2°C warming with no precipitation change would reduce the seasonal snow cover at a Swiss Alpine site by 50 days/yr, and with a 50% increase in precipitation by 30 days (16).
Vulnerabilities – Projections for the future - the Alps
A recent Europe-wide assessment has identified increasing losses in winter tourism due to reduced snow cover and the increased exposure of settlements and infrastructure to natural hazards as the primary vulnerabilities to climate change in the Alps (5). According to research among stakeholders (30), the most important impacts of climate change on tourism in Switzerland are snowpack reduction, melting glaciers, and water scarcity.
Under present climate conditions, 609 out of the 666 (or 91%) Alpine ski areas in Austria, France, Germany, Italy, and Switzerland can be considered as naturally snow-reliable. The remaining 9% are already operating under marginal conditions. The number of naturally snow-reliable areas would drop to 500 under 1°C, to 404 under 2°C, and to 202 under a 4°C warming of climate. This is the first systematic cross-country analysis of snow-reliability under climate change for the Alps and covers more than 80% of the skiing domain (5).
Sensitivity to climate change varies markedly among the Alpine countries. Germany is most sensitive, with only a 1°C warming leading to a 60% decrease (relative to present) in the number of naturally snow-reliable ski areas. Practically none of the ski areas in Germany will be left naturally snow-reliable under a 4°C warming. Switzerland, meanwhile, is the least sensitive of the five countries, with a 1°C warming leading to only a 10% decrease, while a 4°C warming would lead to a 50% decrease (relative to present) in the number of naturally snow-reliable areas. There will also be “winners” and “losers”, both in terms of regions and in terms of the ski areas themselves, with low-lying ski areas being considerably more vulnerable than areas with high altitudinal range (5).
Many of the Austrian ski areas have low base points. With climate change the natural snow-reliability of Austrian ski areas will reduce substantially. The lack of higher altitudes in many of the Austrian mountain ranges makes it impossible to operate on high-elevation sites. The Swiss ski areas will be least affected in the Alps. For Germany, the low-lying ski areas of Bavaria will be highly affected. In France and Italy many ski areas operate at fairly high altitudes (5).
Changes in mean winter snow water equivalent (SWE), the seasonal evolution of snow cover, and the duration of the continuous snow cover season in the European Alps have been assessed from an ensemble of regional climate model (RCM) experiments under the IPCC SRES A1B emission scenario (35). The assessment was carried out for the periods 2020–2049 and 2070–2099, compared with the control period 1971–2000. The strongest relative reduction in winter mean SWE was found below 1,500 m, amounting to 40–80 % by mid century relative to 1971–2000 and depending upon the model considered. At higher elevations the decrease of mean winter SWE is less pronounced but still a robust feature. Major impacts for winter tourism in the Alps are expected. Many ski-regions have mean elevations below 2,000 m and are therefore especially vulnerable to climate change (35). This could result into millions of overnight stays lost during the winter seasons (36).
The danger of rockfall and landslides will also be increased by the melting permafrost. Alpinism (walking, hiking and rock climbing routes) is especially affected by the danger of rockfall. Settlements are hardly at risk from these processes (26).
Vulnerabilities – Projections for the future - Switzerland
Swiss winter sports tourism could even benefit from climate change up to 2030. Winter sports tourism in Switzerland should be less negatively affected by climate change than Austria. The ski resorts in the Swiss part of the Alps are generally higher. The regions with reliable snow could be successful in attracting tourists both from other parts of Switzerland and from the more seriously disadvantaged areas, such as Germany and Austria (25, 34).
High Alpine regions (mainly in Valais, Ticino, and Uri) are particularly vulnerable, mainly due to the combination of future glacier surface loss, increased water scarcity and drought, changes in scenic beauty, and potential permafrost melting. Also regions at mid-elevation are vulnerable, primarily due to snowpack reduction. The lakes regions, the cities, the Plateau and Southern Ticino may benefit from climate change (30). It can be assumed that the rather small ski resorts at mid elevations, which are already endangered today, will disappear in the course of this century. At mid-elevation stations, where most resorts are, the economically important Christmas-New Year period might often not be snow covered anymore (37).
From a study on six Swiss ski resorts at different altitudes it was concluded that all of these resorts – from low to high altitudes – were projected to face reductions in winter recreation season lengths, exceeding 26% by the near-future (2020–2050), 53% by mid-future (2050–2080), and 78% in the far-future (2070–2100). As a result, these ski resorts would face a reduction of up to 13%, 33%, and 51% of their reference period (1981–2010) revenue from winter skiing activities in the near-, mid-, and far-future horizons (39).
Future snowpack changes, four snow indices
Future snowpack changes in the Swiss Alps were estimated with a snowpack model and input data from climate models. The input data are climate change projections made with respect to the reference period 1984-2010 for three future time steps: 1935 (covering the 30-year time slot 2020-2049), 2060 (covering 2045-2074), and 2085 (covering 2070-2099). These projections are based on a large number of climate models (10 combinations of high-resolution regional climate models driven by low-resolution global climate models), and a mean and high-end scenario of climate change (the so-called A1B and A2 scenarios, respectively). A distinction was made between low-elevation (below 500 metre), mid-elevation (1000-1700 metre) and high-elevation stations (above 2000 metre) (37).
Four snow indices were considered: (1) Mean winter snow depth during December – February; (2) Fraction of winters with continuous snow cover of at least 5 cm during a continuous period of at least 30 days; (3) Number of snow days with at least 5 cm (necessary for sledding or cross-country skiing) and 50 cm (for skiing on rough terrain), respectively; (4) New snow days, defined as the number of days per year with a height of new snow greater than 1 cm (37).
- Changes in mean winter snow depth. Low-elevation stations show the strongest (relative) reduction in mean winter snow depth: 59% in 2035, 78% in 2060, and 85% in 2085. For mid-elevation stations mean reductions are 34 % in 2035, 60 % in 2060, and 74 % in 2085. Moreover, all studied mid-elevation stations have a mean winter snow depth of less than 20 cm toward the end of the century. The highest elevation station was least affected by predicted climate change.
- Decrease in continuous snow cover. Low-elevation stations already do not have a continuous snow cover for 30 days today. At low elevations, it is likely that winters without any snowfall at all occur toward the end of the century. Most mid-elevation stations show a decrease of continuous snow cover between 4 and 22 % in 2035 and between 7 and 30 % in 2060. In 2085 half of the studied mid-elevation stations no longer have a continuous snow cover for at least 30 days. The highest elevation station preserves a continuous snow cover even at the end of the century.
- Decrease in snow days. Lowest elevation stations show the highest reduction in number of snow days: the number of days with at least 5 cm of snow may on average reduce from 10 days per year today to 1 day per year at the end of this century. On average, low elevations currently do not experience snow days over 50 cm. For mid-elevation stations the number of days with at least 5 cm of snow reduces by 20, 40, and 60 % in 2035, 2060, and 2085, respectively. Mid-elevation stations show strong reductions in the number of snow days over 50 cm: on average these stations are projected to only experience up to 10 snow days >50 cm per year toward the end of the century. In fact, days with high snow depths are already strongly reduced at mid-elevation stations in 2060. At the highest elevation station, however, the number of snow days will still be high for even the high threshold of 50 cm at the end of this century (161 days). What would be considered a snow-sparse winter of today’s climate is projected to become quite average or even snow abundant under future climate conditions.
- Decrease in new snow days. As for the reduction of new snow days, the relative reductions are highest at the low-elevation stations, up to 75 % in 2085. On average, these low elevations are projected to experience only 2 days with snowfall per year toward the end of the century. At mid-elevation stations relative reductions range between 16-24%, 30-46%, and 41-59% in 2035, 2060, and 2085, respectively. Relative reductions are lowest at the highest elevation station (26% in 2085); at this station, reduction in days with snowfall is highest in absolute numbers (30 in 2085), however.
In the Swiss Alps, the altitude at which snow is sufficient for skiing is 1200 m above sea level (a.s.l.) (17). Thus, 85% of Switzerland’s 230 ski resorts can be considered to be snow-reliable today, while the figure for the 122 individual ski-lifts located at much lower altitudes is 40% (1).
If the line of snow reliability were to rise up to 1500 a.s.l. as a result of climate change, the number of snow-reliable ski resorts would drop to 63% and to 9% for individual ski-lifts at much lower altitudes, respectively. If this line were to rise to 1800 a.s..l., then there would be a further serious deterioration in conditions: only 44% of skiing regions and 2% of individual ski-lifts could be designated as snow-reliable. … The only areas with good prospects will be those with transport facilities that provide access to altitudes higher than 2000 a.s.l. (1).
The regions at higher altitudes may experience greater demand, prompting a further expansion in quantitative terms. The pressure on ecologically sensitive high-mountain regions will increase. The call for snow-reliable ski resorts constitutes the main reason for the current boom in concept studies and plans for opening up high-mountain regions, or, expressed in different terms: climate change is an argument for opening up high-mountain regions to tourism (1).
Tourism is the economic sector that would be most affected by climate change in Switzerland. The influence is of an order of magnitude that cannot be neglected. The potential annual costs of climate change in Switzerland was estimated to be 0.6 to 0.8% of the Swiss gross national product for 1995 (2), more than two thirds of it being related to tourism. Winter tourism industry accounts for 4% of Austria's GNP (19).
Climate change is not regarded as a catastrophe for winter tourism in Switzerland, however (1). The tourism representatives think that climatic change is presented in a highly exaggerated form by the media — and also in science and politics. Although climate change could intensify the problems that already exist in ski areas at lower altitudes and speed up structural changes in the sector, the majority of ski resorts at medium and high altitudes, however, would scarcely be affected (1).
Alpine tourist resorts could benefit from hotter temperatures at lower elevations under future climates. Tourists already react on a short-term basis to hot days and spend more nights in hotels in mountain resorts. If heat waves become more regular, it seems likely that tourists will choose to stay at alpine resorts more frequently and for longer periods (28). Numerous places at lakes and rivers might become an alternative to seaside holiday resorts at the Mediterranean Sea. However, more tourists in summer will not compensate for the loss of income of mountain resorts in winter (4).
Glacier tourism is a vital summer tourism niche dramatically affected by the glacier retreat due to climate change. Glacier tourism sites in the Alps bring in more than one million visitors per year (40). Adverse effects of climate change for tourism include greater difficulty accessing glaciers, more dangerous activities, or loss of attractiveness of the site (41). As permafrost thaws, rock falls on and off glaciers are increasingly observed, threatening the safety of hikers and mountaineers, e.g. in Switzerland (38).
Adaptation strategies – In general
Climate change is slowly entering into decision-making of a range of tourism stakeholders (e.g., investors, insurance companies, tourism enterprises, governments, and tourists); studies that have examined the climate change risk appraisal of local tourism officials and operators have consistently found relatively low levels of concern and little evidence of long-term strategic planning in anticipation of future changes in climate (24).
There is also some evidence that local tourism operators may be overestimating their adaptive capacity (e.g., capacity to make snow under the warmest scenarios). The incorporation of adaptation to climate change into the collective minds of private and public sector tourism decision-makers (‘mainstreaming’) remains several steps away (24).
The capacity of the tourism sector to adapt to climate change is thought to be relatively high due to its dynamic nature and therefore there will be important opportunities for tourism to reduce the vulnerability of communities to climate change (24).
Adaptation strategies - Switzerland
While some regions may be able to maintain their winter tourism with suitable adaptation strategies, others would lose all winter tourism due to a diminishing snow pack. Despite global warming, it is impossible to exclude the possibility of winters with heavy snowfall in the future. Climate change must be viewed as a catalyst that is reinforcing and accelerating the pace of structural changes in tourism (1).
A first ‘wave of adaptation’ started in the 1990s’, after some successive years of very bad winter conditions. This mainly concerned the further development and securing of snow sports activities. Now, a new wave seems to take place, mostly focusing on the promotion of year-round tourism, innovation, and diversification of the tourism offer. This trend, however, is not only determined by climate change, but also by changes in tastes, globalization, population structure, and mobility patterns (30).
Focused on 2030, a large number of possible adaptation measures has been proposed for Switzerland (29):
- Promoting research, innovation and diversification of tourism offers: Diversification of the offer, with the development of new tourism activities; Development of wellness activities; Revalorization and popularization of the summer season.
- Further developing and securing snow sport activities: Elaboration of a general concept for artificial snowmaking in order to optimize planning; Upward spatial development of ski resorts; Creation of accumulation lakes and targeted artificial snowing of slopes; Investment in water security; Extension of retention lakes for water storage.
- Improving natural hazard management:Systematic and subsequent monitoring of areas at risk.
- Risk reduction through organizational measures.
- Clear positioning and targeted marketing: Common development of a strategy among regions; Specialization on specific segments; Communication on climate-friendly tourism; Communication on snow security when this exists; Marketing of regional strength, making use of the ‘summer freshness potential’.
- Awareness rising among the population.
Generally, the promotion of year-round tourism, innovation, and the diversification of the tourism offer are seen as the most implemented and most effective measures. The further development and securing of snow sport activities has also been largely implemented and is generally seen as effective. Nonetheless, in some areas this is only a short- and middle-term solution (30). Research shows that economic diversification (up to 40 days) is indispensable for ski resorts to survive in low elevation mountains after the 2050s (39).
Climate change is already affecting the strategies and plans of the winter sport resorts today. Climate change and global warming, together with international competition, have been used as the key arguments for constructing artificial snow-making facilities, as well as for extending existing ski runs and opening new ones in high alpine regions (at above 3000 a.s.l.) (1).
The winter tourism industry has responded to the implications of observed changes, and a range of technological and behavioural adaptation measures have been put into practice. Artificial snow-making remains the dominant adaptation strategy. Other measures include grooming of ski slopes, moving ski areas to higher altitudes and glaciers, protecting against glacier melt with white plastic sheets, diversification of tourism revenues, and the use of insurance and weather derivatives (5).Also, withdrawal from ski tourism at lower elevations may be necessary. Swiss Banks, for example, now only provide very restricted loans to ski areas at altitudes below 1500 m (7).
Adaptation measures also have costs, as well as limits. Snow-making has proven cost-effective, but such estimates are based only on the direct financial costs to ski operations and do not include the potential externalities of such practices on water consumption, energy demand, landscape, or ecology. Furthermore, snow-making costs will increase non-linearly as temperatures increase – and if ambient temperatures increase beyond a certain threshold snow-making will simply not be viable (5).
Likewise, grooming of ski slopes can reduce the minimum snow-depth required for ski operations by 10 or 20 cm. However, no amount of grooming can overcome significant declines or the total absence of snow cover. Similarly, plastic sheets have been shown to be cost-effective in protecting glacier mass, but there are limits to the area that can be covered by such sheets and they cannot prevent the eventual disappearance of glaciers if warming trends continue (5).
Insurance, meanwhile, can reduce the financial losses from occasional instances of snow-deficient winters, but cannot protect against systemic long-term trends towards warmer winters (5).
Climate change will cause shifts in offer and demand as well as shifts in the regions of origin of guests. Some touristic destinations will lose in attractiveness, others will profit from new opportunities. By adjusting their offer, tourist destinations may develop new core competences, and new guest groups may be attracted (4).
Promising development models for the Swiss tourism industry are the concentration of winter sports in top destinations, the promotion of wellness-centres in the mountain region, and the diversification of attractions offered for summer recreation. These examples illustrate that adaptation takes place in a dynamic context, where minimising the cost of damage caused by climate change goes hand in hand with the search for innovative business opportunities (4).
Examples of shut down of ski resorts
An Example of successful withdrawal from ski tourism is Gschwender Horn in Immenstadt, Bavaria. At the beginning of the 1990s, after a series of snow-deficient winters, the municipality, together with the Allianz Umweltstiftung, decided to withdraw from the non-profitable ski operation. The facilities (two ski lifts and a transportable children’s lift) were dismantled, the ski runs (approximately 40 hectares) re-naturalised. Today, the area is used for summer and winter tourism, namely hiking, mountain biking, snowshoeing and ski touring (17).
The city council of Abondance in the French Alps - its name a cruel reminder of the generous snowfall it once enjoyed – has decided to shut down the ski station that has been its economic raison d'etre for more than 40 years. The reason: not enough snow. Abondance (at 930 meters) falls in the altitude range climate scientists say has seen the most dramatic drop in snowfall in recent generations (20). Whether this shut down is due to climate change or not is disputed (21,22). Other factors, such as incomplete investments (€ 5.3 million in a mixed chairlift/telecabine without snowmaking on the pistes below) may be far more important than global warming (21).
There are 20 other lowlying Haute Savoie resorts that some say are on the brink of closure. A report last winter by the Organisation for Economic Cooperation and Development suggests that the number of ski resorts in the department will fall from 37 to 18 if poor conditions continue (22).
Adaptation costs - Switzerland
Examples of adaptation costs of technical measures have been summarized by (30):
- protection of glaciers with white sheets: 3 Euros/m2 (31);
- extension of ski areas to higher elevations in Switzerland: 25-30 million Euros (32);
- investment costs for snowmaking material in Switzerland up to 2003: 500 Mio. CHF (32); in 2006-2007 only: 409 Mio CHF (33); for each km of slopes: 750,000-1,000,000 CHF (33);
- production of artificial snow: 1-5 Euros/m3 (31);
- operational costs for producing artificial snow: in Switzerland, 7-10 Mio CHF are spent yearly to cover the energy costs of artificial snow (33);
- construction costs for reservoir lakes: 1.5-2.5 Mio CHF for lakes with a capacity of 30- 50,000 m3 and 3.0-3.5 Mio CHF for lakes with a capacity of 80,000 m3 (33).
Barriers to adaptation - Switzerland
From research among stakeholders (30) it was shown that the public sector has an important role to play in the process by coordinating adaptation measures, and generally, by designing a common strategy, which must promote sustainable and long-term solutions, in particular in relation to energy consumption and mobility. Barriers to adaptation can be social, economic, technological, and institutional, and can be overcome by (30):
- providing more and/or better information and communication on the consequences of climate change;
- fostering better cooperation among stakeholders, among political parties at the different levels, and higher public participation and in general by putting stakeholders willing to adapt in contact;
- providing financial support and more and/or better information and communication on possible adaptation measures.
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Switzerland.
- Elsasser and Bürki (2002)
- Bürki (2000), in: Elsasser and Bürki (2002)
- Meier (1998), in: Elsasser and Bürki (2002)
- Federal Office for the Environment FOEN (Ed.) (2009)
- Agrawala (2007)
- Föhn (1990); Haeberli and Beniston (1998), in: Agrawala (2007)
- Elsasser and Burki (2002),in: Agrawala (2007)
- Marty (2008)
- BMU (2004), in: European Environment Agency (EEA) (2005)
- Bätzing (2003) , in: European Environment Agency (EEA) (2005)
- Güthler (2003), in: Alcamo et al. (2007)
- Bätzing ( 2003); Kanatsching and Weber (1998), in: Alcamo et al. (2007)
- Bader and Kunz (1998), in: Alcamo et al. (2007)
- Hantel et al. (2000); Wielke et al. (2004); Martin and Etchevers (2005), all in: Alcamo et al. (2007)
- Hantel et al. (2000), in: Alcamo et al. (2007)
- Beniston et al. (2003), in: Alcamo et al. (2007)
- Föhn (1990), in: Elsasser and Bürki (2002)
- Allianz Umweltstiftung (2005),in: Agrawala (2007)
- Federal Ministry of Agriculture, Forestry, Environment and Water Management (2010)
- EEA, JRC and WHO (2008)
- UNWTO, UNEP and WHO (2008)
- Deutsche Bank Research (2008)
- OcCC/ProClim- (2007)
- STV-FST (2011), in: Matasci (2012)
- Serquet and Rebetez (2011)
- Müller and Weber (2008), in: Matasci (2012)
- Matasci (2012)
- Agrawala and Fankhauser (2008), in: Matasci (2012)
- Mathis et al. (2003), in: Matasci (2012)
- Lang (2009), in: Matasci (2012)
- Gonseth (2013)
- Steger et al. (2013)
- Damm et al. (2016), in: Klein et al. (2016)
- Schmucki et al. (2017)
- Temme (2015), in: IPCC (2019)
- Ashraf Vaghefi et al. (2021)
- Salim et al. (2021b), in: Salim et al. (2021)
- Salim et al. (2021)