finding 11.4 : key-finding-11-4

It is very likely that human activities have contributed to observed arctic surface temperature warming, sea ice loss, glacier mass loss, and Northern Hemisphere snow extent decline (high confidence).

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

Process for developing key messages: Evidenced by the multiple independent studies, analysis techniques, and the array of different climate models used over the last 20 years, it is very likely that human activities have contributed to arctic surface temperature warming, sea ice loss since 1979, glacier mass loss, and Northern Hemisphere snow extent decline observed across the Arctic. Key uncertainties remain in the understanding and modeling of arctic climate variability; however, many independent studies indicate that internal variability alone cannot explain the trends or extreme events observed in arctic temperature and sea ice over the satellite era.

Description of evidence base:

The Key Finding is supported by many attribution studies using a wide array of climate models documenting the anthropogenic influence on arctic temperature, sea ice, mountain glaciers, and snow extent.772b18ec-300e-4531-867b-beac17bc0ebd 57fea764-f852-4539-ab1e-8010701383c7 559ac612-3dd1-41d3-921c-0a77d4ff366d de0cccc8-faff-4214-9057-a52da62e2abc aa75c0a9-fe6d-43cc-938c-62328d5ff643 541fc57b-d7ad-4617-97de-2df91f99afc0 02cefa48-7195-4d2e-aff7-f460824987ec fbae726d-5c20-4abf-9567-2d2539990f21 feb5303e-2e2a-4476-9495-d74ffc174cf4 7e602d53-52c5-47b3-847e-26293d76b622 194b98dd-59c1-403c-8b8f-a2475bbeb05f 6e730a84-66a2-4e74-96cb-c9e6824cf185 6d2a6ea3-2248-43a2-ad47-064c9fe3315a e2234cdf-e361-477d-82af-ef4a344c5890 f8ba9ecd-1983-4a78-8fd7-f59e1ab07908 5d53feca-729f-4b68-ba22-8022f94af9b4 dc9d8bb2-9388-4e4a-8976-88c6b825fafc Observation-based analyses also support an anthropogenic influence.2f6d49a9-d59a-43db-8e0e-8a3d6e881c54 9c6fbb6f-30a4-4b15-a2a1-6b6bb7960020 Najafi et al.aa75c0a9-fe6d-43cc-938c-62328d5ff643 show that the greenhouse warming signal in the Arctic could be even stronger, as a significant portion of greenhouse gas induced warming (approximately 60%) has been offset by anthropogenic aerosol emissions. The emerging science of extreme event attribution indicates that natural variability alone could not have caused the recently observed record low arctic sea ice extents, such as in September 2012.6d2a6ea3-2248-43a2-ad47-064c9fe3315a e2234cdf-e361-477d-82af-ef4a344c5890 Natural variability in the Arctic is significant,e22c21b2-3019-4394-9579-bb1d4c3ec73a c7f78878-ec71-47fe-b5d0-e7b64241740f however the majority of studies indicate that the contribution from individual sources of internal variability to observed trends in arctic temperature and sea ice are less than 50%ea982647-594e-4911-9cd2-4330ba1f97c4 02cefa48-7195-4d2e-aff7-f460824987ec fbae726d-5c20-4abf-9567-2d2539990f21 and alone cannot explain the observed trends over the satellite era. This Key Finding marks an increased confidence relative to the IPCC AR557fea764-f852-4539-ab1e-8010701383c7 moving from likely to very likely. In our assessment, the new understanding of the anthropogenic forcing,aa75c0a9-fe6d-43cc-938c-62328d5ff643 its relationship to arctic climate change,9c6fbb6f-30a4-4b15-a2a1-6b6bb7960020 arctic climate variability,ea982647-594e-4911-9cd2-4330ba1f97c4 e22c21b2-3019-4394-9579-bb1d4c3ec73a c7f78878-ec71-47fe-b5d0-e7b64241740f and especially extreme event attribution studies6d2a6ea3-2248-43a2-ad47-064c9fe3315a e2234cdf-e361-477d-82af-ef4a344c5890 reaffirms previous studies and warrants the increased likelihood of an anthropogenic influence on arctic climate change. Multiple lines evidence, independent analysis techniques, models, and studies support the Key Finding.

New information and remaining uncertainties: A major limiting factor in our ability to attribute arctic sea ice and glacier melt to human activities is the significant natural climate variability in the Arctic. Longer data records and a better understanding of the physical mechanisms that drive natural climate variability in the Arctic are required to reduce this uncertainty. Another major uncertainty is the ability of climate models to capture the relevant physical processes and climate changes at a fine spatial scale, especially those at the land and ocean surface in the Arctic.

Assessment of confidence based on evidence: There is high confidence that human activities have contributed to arctic surface temperature warming, sea ice loss since 1979, glacier mass loss, and Northern Hemisphere snow extent given multiple independent analysis techniques from independent groups using many different climate models indicate the same conclusion.

Arctic sea ice and glacier mass loss impacts the United States by affecting coastal erosion in Alaska and key Alaskan fisheries through an increased vulnerability to ocean acidification. Glacier mass loss is a significant driver of sea level rise threatening coastal communities in the United States and worldwide, influencing marine ecology, and potentially altering the Atlantic Meridional Overturning Circulation.061347c5-780e-4419-a1bd-89b645241daf

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

References :

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