finding 11.2 : key-finding-11-2

Rising Alaskan permafrost temperatures are causing permafrost to thaw and become more discontinuous; this process releases additional carbon dioxide and methane, resulting in an amplifying feedback and additional warming (high confidence). The overall magnitude of the permafrost–carbon feedback is uncertain; however, it is clear that these emissions have the potential to compromise the ability to limit global temperature increases.

This finding is from chapter 11 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: Permafrost is thawing, becoming more discontinuous, and releasing CO2 and CH4. Observational and modeling evidence indicates that permafrost has thawed and released additional CO2 and CH4 indicating that the permafrost–carbon cycle feedback is positive accounting for additional warming of approximately 0.08º to 0.50ºC on top of climate model projections. Although the magnitude of the permafrost–carbon feedback is uncertain due to a range of poorly understood processes (deep soil and ice wedge processes, plant carbon uptake, dependence of uptake and emissions on vegetation and soil type, and the role of rapid permafrost thaw processes, such as thermokarst), emerging science and the newest estimates continue to indicate that this feedback is more likely on the larger side of the range. Impacts of permafrost thaw and the permafrost carbon feedback complicates our ability to limit global temperature increases by adding a currently unconstrained radiative forcing to the climate system.

Description of evidence base: The Key Finding is supported by observational evidence of warming permafrost temperatures and a deepening active layer, in situ gas measurements and laboratory incubation experiments of CO2 and CH4 release, and model studies.3d339c60-bdf6-44f9-900d-249676925b4f e787a738-62a2-4c16-984c-b37f225a7510 e08db6e2-291f-465b-a693-a90f6110f5af 55c65d6f-38d7-45e3-91f3-993d46bb29be 0ee6881f-0ceb-4192-bf18-9fe5f8e4d01c 19747fc7-181f-4af9-97fb-f47dd75140bf 747900dd-7e2a-42e4-8e9f-e92b34e2eed4 Alaska and arctic permafrost characteristics have responded to increased temperatures and reduced snow cover in most regions since the 1980s, with colder permafrost warming faster than warmer permafrost.3d339c60-bdf6-44f9-900d-249676925b4f 2ecb64ff-f4e0-4acd-b049-e5d04f44c57a 75d4db91-a3d6-4533-bc7d-a4c4f3d89d99 Large carbon soil pools (more than 50% of the global below-ground organic carbon pool) are locked up in the permafrost soils,05903e43-63b7-4a76-8ddf-625849add0f6 with the potential to be released. Thawing permafrost makes previously frozen organic matter available for microbial decomposition. In situ gas flux measurements have directly measured the release of CO2 and CH4 from arctic permafrost.3a1ac4af-4295-4dff-a77f-d4d58d618d62 0928307d-3733-451d-8ef4-0936eb367f02 The specific conditions of microbial decomposition, aerobic or anaerobic, determines the relative production of CO2 and CH4. This distinction is significant as CH4 is a much more powerful greenhouse gas than CO2.6c7c285c-8606-41fe-bf93-100d80f1d17a However, incubation studies indicate that 3.4 times more carbon is released under aerobic conditions than anaerobic conditions, leading to a 2.3 times the stronger radiative forcing under aerobic conditions.e08db6e2-291f-465b-a693-a90f6110f5af Combined data and modeling studies suggest a global sensitivity of the permafrost–carbon feedback warming global temperatures in 2100 by 0.52° ± 0.38°F (0.29° ± 0.21°C) alone.5b7d739a-50de-4006-811f-5a9bd469c977 Chadburn et al.29b5eac3-49d9-47aa-9f54-fa5c2501c39b infer the sensitivity of permafrost area to globally averaged warming to be 4 million km2 by constraining a group of climate models with the observed spatial distribution of permafrost; this sensitivity is 20% higher than previous studies. Permafrost thaw is occurring faster than models predict due to poorly understood deep soil, ice wedge, and thermokarst processes.0ee6881f-0ceb-4192-bf18-9fe5f8e4d01c 19747fc7-181f-4af9-97fb-f47dd75140bf 747900dd-7e2a-42e4-8e9f-e92b34e2eed4 36a37175-cb3e-463a-9259-499506b15ef3 Additional uncertainty stems from the surprising uptake of methane from mineral soils12c3ea10-a785-4e52-b2cf-ecad1c207714 and dependence of emissions on vegetation and soil properties.0992f3f4-2780-45e8-bd5c-3a1ec35a6ceb The observational and modeling evidence supports the Key Finding that the permafrost–carbon cycle is positive.

New information and remaining uncertainties: A major limiting factor is the sparse observations of permafrost in Alaska and remote areas across the Arctic. Major uncertainties are related to deep soil, ice wedging, and thermokarst processes and the dependence of CO2 and CH4 uptake and production on vegetation and soil properties. Uncertainties also exist in relevant soil processes during and after permafrost thaw, especially those that control unfrozen soil carbon storage and plant carbon uptake and net ecosystem exchange. Many processes with the potential to drive rapid permafrost thaw (such as thermokarst) are not included in current earth system models

Assessment of confidence based on evidence: There is high confidence that permafrost is thawing, becoming discontinuous, and releasing CO2 and CH4. Physically-based arguments and observed increases in CO2 and CH4 emissions as permafrost thaws indicate that the feedback is positive. This confidence level is justified based on observations of rapidly changing permafrost characteristics.

Thawing permafrost very likely has significant impacts to the global carbon cycle and serves as a source of CO2 and CH4 emission that complicates the ability to limit global temperature increases.

This finding was derived from figure -.2: Confidence / Likelihood

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