Permafrost: European scale
Frozen soils and permafrost currently hold about 1700 PgC, more than twice the carbon than the atmosphere, and thus represent a particularly large vulnerability to climate change (i.e., warming) (1,6). Permafrost temperatures have increased by 0.29°C ± 0.12°C from 2007 to 2016 averaged across polar and high-mountain regions globally (12). Permafrost at 28 mountain locations in the European Alps, Scandinavia, Canada as well as High Mountain Asia and North Asia warmed on average by 0.19 ± 0.05 °C per decade between 2007 and 2016 (13).
Soils of the northern permafrost zone cover only 15% of the Earth’s total soil area but store more than 30% of the global surface soil carbon in the form of organic matter (3,8). Now that the permafrost has started to thaw due to global warming, this vast reserve of permafrost carbon previously stabilized by low temperatures becomes vulnerable to microbial decomposition. As a result, significant amounts of carbon dioxide and methane are released into the atmosphere and further reinforce climate change (3,9). Evidence suggests that 5-15% of the vast pool of soil carbon stored in northern permafrost ecosystems could be emitted as greenhouse gases by 2100 under the current path of global warming (7).
It is virtually certain that the area of Northern Hemisphere permafrost will continue to decline over the first half of the 21st century in all climate change (RCP) scenarios (2). In the lower-end (RCP2.6) scenario, the permafrost area is projected to stabilize at a level approximately 20% below the 20th century area, and then begin a slight recovering trend. In the moderate (RCP4.5) scenario, the simulations that extend beyond 2100 show permafrost continuing to decline for at least another 250 years. In the higher-end (RCP8.5) scenario, the permafrost area is simulated to approach zero by the middle of the 22nd century in simulations that extend beyond 2100 (1).
In 2019 the IPCC concluded that widespread permafrost thaw is projected for this century and beyond. By 2100, projected near-surface (within 3-4 m) permafrost area shows a decrease of 24 ± 16% globally for the low-end scenario of climate change (RCP2.6) and 69 ± 20% for the high-end scenario (RCP8.5) (12).
Observed loss of carbon from permafrost soils
No detectable changes?
Remarkably, field measurements of thawing permafrost soil in Greenland and Alaska (10) did not reveal detectable changes in carbon content of the soil. Scientists studied this in the Alaskan tundra and found that no detectable changes were found because the field measurements were carried out the wrong way. Carbon pools were based on fixed depths. Using a fixed depth fails to detect soil carbon changes because ground subsidence and compaction due to the thawing of the permafrost brings additional carbon that was once deeper into the fixed-depth inventory and masks losses from the original carbon pool (7).
Loss of soil carbon: 5.4% per year!
The scientists, therefore, chose a different approach. They quantified the carbon content in relation to a fixed, stable mineral component of the soil as a metric for carbon pool comparisons through time. This approach revealed completely different results. Over a five-year period they showed a loss of soil carbon in the upper 55 cm of the permafrost of 5.4% per year in tundra ecosystems at a site in Alaska where permafrost is degrading due to climate change (7). This is a surprisingly large loss of carbon. Experiments show that this loss is too large to reflect emissions of carbon dioxide and methane into the atmosphere. Part of the lost carbon seems to be transported as particulate and dissolved carbon by water flowing from the melting soil into rivulets and small streams (26). However, these results indicate that the feedback of carbon dioxide and methane from melting permafrost to climate change may be occurring faster than previously thought (7).
Vulnerabilities - The permafrost carbon feedback
In high-latitude regions of the Earth, temperatures have risen 0.6 °C per decade, twice as fast as the global average (4). The resulting thaw of frozen ground exposes substantial quantities of organic carbon to decomposition by soil microbes (3). The permafrost region contains twice as much carbon as there is currently in the atmosphere (5,6). A substantial fraction of this material can be mineralized by microbes and converted to carbon dioxide (CO2) and methane (CH4) on timescales of years to decades. At the proposed rates, the observed and projected emissions of CH4 and CO2 from thawing permafrost are unlikely to cause abrupt climate change over a period of a few years to a decade. Instead, permafrost carbon emissions are likely to be felt over decades to centuries as northern regions warm, making climate change happen faster than we would expect on the basis of projected emissions from human activities alone (5).
Abrupt permafrost thaw occurs when warming melts ground ice, causing the land surface to collapse into the volume previously occupied by ice. This process, called thermokarst, alters surface hydrology. Water is attracted towards collapse areas, and pooling or flowing water in turn causes more localized thawing and even mass erosion. Owing to these localized feedbacks that can thaw through tens of metres of permafrost across a hillslope within only a few years, permafrost thaw occurs much more rapidly than would be predicted from changes in air temperature alone. Abrupt thaw is an important mechanism of rapid permafrost degradation, yet abrupt thaw is not included in large-scale models, suggesting that important landscape transformations are not currently being considered in forecasts of permafrost carbon–climate feedbacks. This is in part due to the fact that we do not know at this stage what the relative importance of abrupt to gradual thaw across the landscape is likely to be (5,6).
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Europe.
- IPCC (2014)
- Caesar et al. (2013); Koven et al. (2013), both in: IPCC (2014)
- Schuur et al. (2015)
- IPCC (2013), in: Schuur et al. (2015)
- Zimov et al. (2006); Tarnocai et al. (2009), both in: Schuur et al. (2015)
- Turetsky et al. (2019)
- Plaza et al. (2019)
- Hugelius et al. (2014), in: Plaza et al. (2019)
- Ping et al. (2015); Schaefer et al. (2014), both in: Plaza et al. (2019)
- Elberling et al. (2013); Sistla et al. (2013), both in: Plaza et al. (2019)
- Vonk and Gustaffson (2013), in: Plaza et al. (2019)
- IPCC (2019a)
- Biskaborn et al. (2019), in: IPCC (2019b)