"key-finding-11-2" . "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."^^ . . . "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."^^ . "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. 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. Large carbon soil pools (more than 50% of the global below-ground organic carbon pool) are locked up in the permafrost soils, 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. 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. 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. 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. Chadburn et al. infer the sensitivity of permafrost area to globally averaged warming to be 4 million km 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. Additional uncertainty stems from the surprising uptake of methane from mineral soils and dependence of emissions on vegetation and soil properties. The observational and modeling evidence supports the Key Finding that the permafrost–carbon cycle is positive."^^ . "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.\r\n

\r\nThawing 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.\r\n

"^^ . "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"^^ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .