finding 2.7 : key-message-2-7

In the Arctic, annual average temperatures have increased more than twice as fast as the global average, accompanied by thawing permafrost and loss of sea ice and glacier mass (very high confidence). Arctic-wide glacial and sea ice loss is expected to continue; by mid-century, it is very likely that the Arctic will be nearly free of sea ice in late summer (very high confidence). Permafrost is expected to continue to thaw over the coming century as well, and the carbon dioxide and methane released from thawing permafrost has the potential to amplify human-induced warming, possibly significantly (high confidence).

This finding is from chapter 2 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

This chapter is based on the collective effort of 32 authors, 3 review editors, and 18 contributing authors comprising the writing team for the Climate Science Special Report (CSSR),75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 a featured U.S. Global Change Research Project (USGCRP) deliverable and Volume I of the Fourth National Climate Assessment (NCA4). An open call for technical contributors took place in March 2016, and a federal science steering committee appointed the CSSR team. CSSR underwent three rounds of technical federal review, external peer review by the National Academies of Sciences, Engineering, and Medicine, and a review that was open to public comment. Three in-person Lead Authors Meetings were conducted at various stages of the development cycle to evaluate comments received, assign drafting responsibilities, and ensure cross-chapter coordination and consistency in capturing the state of climate science in the United States. In October 2016, an 11-member core writing team was tasked with capturing the most important CSSR key findings and generating an Executive Summary. The final draft of this summary and the underlying chapters was compiled in June 2017.

The NCA4 Chapter 2 author team was pulled exclusively from CSSR experts tasked with leading chapters and/or serving on the Executive Summary core writing team, thus representing a comprehensive cross-section of climate science disciplines and supplying the breadth necessary to synthesize CSSR content. NCA4 Chapter 2 authors are leading experts in climate science trends and projections, detection and attribution, temperature and precipitation change, severe weather and extreme events, sea level rise and ocean processes, mitigation, and risk analysis. The chapter was developed through technical discussions first promulgated by the literature assessments, prior efforts of USGCRP,75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 e-mail exchanges, and phone consultations conducted to craft this chapter and subsequent deliberations via phone and e-mail exchanges to hone content for the current application. The team placed particular emphasis on the state of science, what was covered in USGCRP,75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 and what is new since the release of the Third NCA in 2014.dd5b893d-4462-4bb3-9205-67b532919566

Description of evidence base:

Annual average near-surface air temperatures across Alaska and the Arctic have increased over the last 50 years at a rate more than twice the global average. Observational studies using ground-based observing stations and satellites analyzed by multiple independent groups support this finding. The enhanced sensitivity of the arctic climate system to anthropogenic forcing is also supported by climate modeling evidence, indicating a solid grasp of the underlying physics. These multiple lines of evidence provide very high confidence of enhanced arctic warming with potentially significant impacts on coastal communities and marine ecosystems.

This aspect of the Key Message is supported by observational evidence from ground-based observing stations, satellites, and data model temperature analyses from multiple sources and independent analysis techniques.e2086a52-de43-4628-97f8-05fb1c8e1e45,e1ea418d-9ff7-4869-a09e-30672e492a64,275c5633-c650-4405-b6c7-3a8151e11b51,47a5196b-4fba-4fdb-8647-8945627725bb,e8458879-e69d-4464-a6cb-c1ee60d0cbf1,a9664a29-554f-4680-a6cd-9264b475d17b,241f3efd-0405-4fcf-995c-b6c5058cf5c7 For more than 40 years, climate models have predicted enhanced arctic warming, indicating a solid grasp of the underlying physics and positive feedbacks driving the accelerated arctic warming.b3bbc7b5-067e-4c23-8d9b-59faee21e58e,3fe708bb-96e6-4612-b91a-4a981974b900,74b2ec95-20d0-4af6-8f4c-08f7a0ae8981 Lastly, similar statements have been made in NCA3,dd5b893d-4462-4bb3-9205-67b532919566 IPCC AR5,47a5196b-4fba-4fdb-8647-8945627725bb and in other arctic-specific assessments such as the Arctic Climate Impacts Assessment56f7d484-3935-4b75-b3c0-2265edae42c2 and the Snow, Water, Ice and Permafrost in the Arctic assessment report.e4385436-dcc5-45ca-89a2-e281d025545d

Permafrost is thawing, becoming more discontinuous, and releasing carbon dioxide (CO2) and methane (CH4). Observational and modeling evidence indicates that permafrost has thawed and released additional CO2 and CH4, indicating that the permafrost–carbon feedback is positive, accounting for additional warming of approximately 0.08ºC to 0.50ºC on top of climate model projections. Although the magnitude and timing of the permafrost–carbon feedback are 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 complicate our ability to limit future temperature changes by adding a currently unconstrained radiative forcing to the climate system.

This part of the Key Message is supported by observational evidence of warming permafrost temperatures and a deepening active layer, in situ gas measurements, laboratory incubation experiments of CO2 and CH4 release, and model studies.e787a738-62a2-4c16-984c-b37f225a7510,3d339c60-bdf6-44f9-900d-249676925b4f,19747fc7-181f-4af9-97fb-f47dd75140bf,55c65d6f-38d7-45e3-91f3-993d46bb29be,e08db6e2-291f-465b-a693-a90f6110f5af,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,e4385436-dcc5-45ca-89a2-e281d025545d,75d4db91-a3d6-4533-bc7d-a4c4f3d89d99 Large carbon soil pools (approximately half of the global below-ground organic carbon pool) are stored in permafrost soil,05903e43-63b7-4a76-8ddf-625849add0f6,cd322ff1-e622-40eb-b39f-686c8a9afbd1 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, determine 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 stronger radiative forcing under aerobic conditions.e08db6e2-291f-465b-a693-a90f6110f5af Combined data and modeling studies suggest that the impact of the permafrost–carbon feedback on global temperatures could amount to +0.52° ± 0.38°F (+0.29° ± 0.21°C) by 2100.5b7d739a-50de-4006-811f-5a9bd469c977 Chadburn et al. (2017)29b5eac3-49d9-47aa-9f54-fa5c2501c39b infer the sensitivity of permafrost area to globally averaged warming to be 1.5 million square miles (4 million square km), 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 Message that the permafrost–carbon feedback is positive (i.e., amplifies warming).

Arctic land and sea ice loss observed in the last three decades continues, in some cases accelerating. A diverse range of observational evidence from multiple data sources and independent analysis techniques provides consistent evidence of substantial declines in arctic sea ice extent, thickness, and volume since at least 1979, mountain glacier melt over the last 50 years, and accelerating mass loss from Greenland. An array of different models and independent analyses indicate that future declines in ice across the Arctic are expected, resulting in late summers in the Arctic very likely becoming ice free by mid-century.

This final aspect of the Key Message is supported by observational evidence from multiple ground-based and satellite-based observational techniques (including passive microwave, laser and radar altimetry, and gravimetry) analyzed by independent groups using different techniques reaching similar conclusions.3d339c60-bdf6-44f9-900d-249676925b4f,aaf023b5-1ca5-455c-b203-8d5db9d33d7e,566e80d8-e05c-4be1-a90c-788f328629bc,a9664a29-554f-4680-a6cd-9264b475d17b,94117a50-acc5-4dbf-8029-368aa3fc9680,25342659-e41d-47c9-b230-c48cbac4361c,08047702-47b0-4401-ab44-a0f46a16efe5,6b65bead-0bf2-443b-8cc7-540607984b3cAdditionally, the U.S. Geological Survey repeat photography database shows the glacier retreat for many Alaska glaciers (Taylor et al. 2017,61d6757d-3f7a-4e90-add7-b03de796c6c4 Figure 11.4). Several independent model analysis studies using a wide array of climate models and different analysis techniques indicate that sea ice loss will continue across the Arctic, very likely resulting in late summers becoming nearly ice-free by mid-century.b3bbc7b5-067e-4c23-8d9b-59faee21e58e,6e730a84-66a2-4e74-96cb-c9e6824cf185,df11db8a-acca-4f76-ab80-b1bf7b1ee9f7

New information and remaining uncertainties:

The lack of high-quality data and the restricted spatial resolution of surface and ground temperature data over many arctic land regions, coupled with the fact that there are essentially no measurements over the Central Arctic Ocean, hampers the ability to better refine the rate of arctic warming and completely restricts our ability to quantify and detect regional trends, especially over the sea ice. Climate models generally produce an arctic warming between two to three times the global mean warming. A key uncertainty is our quantitative knowledge of the contributions from individual feedback processes in driving the accelerated arctic warming. Reducing this uncertainty will help constrain projections of future arctic warming.

A lack of observations affects not only the ability to detect trends but also to quantify a potentially significant positive feedback to climate warming: the permafrost–carbon feedback. Major uncertainties are related to deep soil and thermokarst processes, as well as the persistence or degradation of massive ice (e.g., ice wedges) 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.

Key uncertainties remain in the quantification and modeling of key physical processes that contribute to the acceleration of land and sea ice melting. Climate models are unable to capture the rapid pace of observed sea and land ice melt over the last 15 years; a major factor is our inability to quantify and accurately model the physical processes driving the accelerated melting. The interactions between atmospheric circulation, ice dynamics and thermodynamics, clouds, and specifically the influence on the surface energy budget are key uncertainties. Mechanisms controlling marine-terminating glacier dynamics, specifically the roles of atmospheric warming, seawater intrusions under floating ice shelves, and the penetration of surface meltwater to the glacier bed, are key uncertainties in projecting Greenland ice sheet melt.

Assessment of confidence based on evidence:

There is very high confidence that the arctic surface and air temperatures have warmed across Alaska and the Arctic at a much faster rate than the global average is provided by the multiple datasets analyzed by multiple independent groups indicating the same conclusion. Additionally, climate models capture the enhanced warming in the Arctic, indicating a solid understanding of the underlying physical mechanisms.

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.

There is very high confidence that arctic sea and land ice melt is accelerating and mountain glacier ice mass is declining, given the multiple observational sources and analysis techniques documented in the peer-reviewed climate science literature.

References :

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