Permafrost carbon cycle

It is estimated that the total soil organic carbon (SOC) stock in northern circumpolar permafrost region equals around 1,460–1,600 Pg.

As climate change increases mean annual air temperatures throughout the Arctic, it extends permafrost thaw and deepens the active layer, exposing old carbon that has been in storage for decades to millennia to biogenic processes which facilitate its entrance into the atmosphere.

[15]: 1283  According to the IPCC Sixth Assessment Report, there is high confidence that global warming over the last few decades has led to widespread increases in permafrost temperature.

[15]: 1237  In Yukon, the zone of continuous permafrost might have moved 100 kilometres (62 mi) poleward since 1899, but accurate records only go back 30 years.

Based on high agreement across model projections, fundamental process understanding, and paleoclimate evidence, it is virtually certain that permafrost extent and volume will continue to shrink as global climate warms.

[10] In particular, sufficient concentrations of iron oxides in some permafrost soils can inhibit microbial respiration and prevent carbon mobilization: however, this protection only lasts until carbon is separated from the iron oxides by Fe-reducing bacteria, which is only a matter of time under the typical conditions.

[18] Altogether, the likelihood of the entire carbon pool mobilizing and entering the atmosphere is low despite the large volumes stored in the soil.

Although temperatures will increase, this does not imply complete loss of permafrost and mobilization of the entire carbon pool.

[5] Moreover, other elements such as iron and aluminum can adsorb some of the mobilized soil carbon before it reaches the atmosphere, and they are particularly prominent in the mineral sand layers which often overlay permafrost.

Under the "intermediate" scenario RCP 4.5, where greenhouse gas emissions peak and plateau within the next two decades, before gradually declining for the rest of the century (a rate of mitigation deeply insufficient to meet the Paris Agreement goals) permafrost carbon emissions would increase by 17%.

[22] Notably, estimates of carbon release alone do not fully represent the impact of permafrost thaw on climate change.

[25] There's some debate over whether the observed emissions from permafrost soils primarily constitute microbial respiration of ancient carbon, or simply greater respiration of modern-day carbon (i.e. leaf litter), due to warmer soils intensifying microbial metabolism.

[26][27] On the other hand, former permafrost areas consistently see increased vegetation growth, or primary production, as plants can set down deeper roots in the thawed soil and grow larger and uptake more carbon.

[7] Moreover, thawed areas become more vulnerable to wildfires, which alter landscape and release large quantities of stored organic carbon through combustion.

[30] On the other hand, warming allows the beavers to extend their habitat further north, where their dams impair boat travel, impact access to food, affect water quality, and endanger downstream fish populations.

[31] Global warming in the Arctic accelerates methane release from both existing stores and methanogenesis in rotting biomass.

[35] Since methanogenesis requires anaerobic environments, it is frequently associated with Arctic lakes, where the emergence of bubbles of methane can be observed.

[39] Another process which frequently results in substantial methane emissions is the erosion of permafrost-stabilized hillsides and their ultimate collapse.

[41] Another example occurred in the wake of a 2020 Siberian heatwave, which was found to have increased RTS numbers 17-fold across the northern Taymyr Peninsula – from 82 to 1404, while the resultant soil carbon mobilization increased 28-fold, to an average of 11 grams of carbon per square meter per year across the peninsula (with a range between 5 and 38 grams).

[42] Nevertheless, a study from 2018, by using field observations, radiocarbon dating, and remote sensing to account for thermokarst lakes, determined that abrupt thaw will more than double permafrost carbon emissions by 2100.

[47] Another 2018 paper suggested that permafrost emissions are limited following thermokarst thaw, but are substantially greater in the aftermath of wildfires.

[50] Thus, it can be defined as "the unglaciated continental shelf areas exposed during the Last Glacial Maximum (LGM, ~26 500 BP) that are currently inundated".

They compared those figures to the extrapolated present-day emissions of Canada, the European Union and the United States or China, respectively.

[57] One of the scientists involved in that effort, Susan M. Natali of Woods Hole Research Centre, had also led the publication of a complementary estimate in a PNAS paper that year, which suggested that when the amplification of permafrost emissions by abrupt thaw and wildfires is combined with the foreseeable range of near-future anthropogenic emissions, avoiding the exceedance (or "overshoot") of 1.5 °C (2.7 °F) warming is already implausible, and the efforts to attain it may have to rely on negative emissions to force the temperature back down.

[58] An updated 2022 assessment of climate tipping points concluded that abrupt permafrost thaw would add 50% to gradual thaw rates, and would add 14 billion tons of carbon dioxide equivalent emissions by 2100 and 35 billion tons by 2300 per every degree of warming.

The annual number of scientific research papers published on the subject of permafrost carbon has grown from next to nothing around 1990 to around 400 by 2020. [ 1 ]
Permafrost peatlands under varying extent of global warming, and the resultant emissions as a fraction of anthropogenic emissions needed to cause that extent of warming. [ 11 ]
Greater summer precipitation increases the depth of permafrost layer subject to thaw, in different Arctic permafrost environments. [ 16 ]
Recent observations suggest that CO 2 absorption had been increasing at a faster rate over the areas with a lot of permafrost and limited tree cover than over the areas with extensive tree cover. [ 22 ]
Carbon cycle accelerates in the wake of abrupt thaw (orange) relative to the previous state of the area (blue, black). [ 32 ]
Methane emissions from thawed permafrost appear to decrease as bog matures over time. [ 45 ]
Carbon dioxide and methane (in CO 2 equivalent) emissions from subsea permafrost alone under the different Representative Concentration Pathway scenarios over time. [ 49 ]
Nine probable scenarios of greenhouse gas emissions from permafrost thaw during the 21st century, which show a limited, moderate and intense CO 2 and CH 4 emission response to low, medium and high-emission Representative Concentration Pathways . The vertical bar uses emissions of selected large countries as a comparison: the right-hand side of the scale shows their cumulative emissions since the start of the Industrial Revolution , while the left-hand side shows each country's cumulative emissions for the rest of the 21st century if they remained unchanged from their 2019 levels. [ 1 ]