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Abstract Arctic and Antarctic sea ice has undergone significant and rapid change with the changing climate. Here, we present preindustrial and historical results from the newly released Community Earth System Model Version 2 (CESM2) to assess the Arctic and Antarctic sea ice. Two configurations of the CESM2 are available that differ only in their atmospheric model top and the inclusion of comprehensive atmospheric chemistry, including prognostic aerosols. The CESM2 configuration with comprehensive atmospheric chemistry has significantly thicker Arctic sea ice year‐round and better captures decreasing trends in sea ice extent and volume over the satellite period. In the Antarctic, both CESM configurations have similar mean state ice extent and volume, but the ice extent trends are opposite to satellite observations. We find that differences in the Arctic sea ice between CESM2 configurations are the result of differences in liquid clouds. Over the Arctic, the CESM2 configuration without prognostic aerosol formation has fewer aerosols to form cloud condensation nuclei, leading to thinner liquid clouds. As a result, the sea ice receives much more shortwave radiation early in the melt season, driving a stronger ice albedo feedback and leading to additional sea ice loss and significantly thinner ice year‐round. The aerosols necessary for the Arctic liquid cloud formation are produced from different precursor emissions and transported to the Arctic. Thus, the main reason sea ice differs in the Arctic is the transport of cloud‐impacting aerosols into the region, while the Antarctic remains relatively pristine from extrapolar aerosol transport.
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Arctic Sea Ice in Two Configurations of the CESM2 During the 20th and 21st Centuries
Abstract We provide an assessment of the current and future states of Arctic sea ice simulated by the Community Earth System Model version 2 (CESM2). The CESM2 is the version of the CESM contributed to the sixth phase of the Coupled Model Intercomparison Project (CMIP6). We analyze changes in Arctic sea ice cover in two CESM2 configurations with differing atmospheric components: the CESM2(CAM6) and the CESM2(WACCM6). Over the historical period, the CESM2(CAM6) winter ice thickness distribution is biased thin, which leads to lower summer ice area compared to CESM2(WACCM6) and observations. In both CESM2 configurations, the timing of first ice‐free conditions is insensitive to the choice of CMIP6 future emissions scenario. In fact, the probability of an ice‐free Arctic summer remains low only if global warming stays below 1.5°C, which none of the CMIP6 scenarios achieve. By the end of the 21st century, the CESM2 simulates less ocean heat loss during the fall months compared to its previous version, delaying sea ice formation and leading to ice‐free conditions for up to 8 months under the high emissions scenario. As a result, both CESM2 configurations exhibit an accelerated decline in winter and spring ice area, a behavior that had not been previously seen in CESM simulations. Differences in climate sensitivity and higher levels of atmospheric CO2by 2100 in the CMIP6 high emissions scenario compared to its CMIP5 analog could explain why this winter ice loss was not previously simulated by the CESM.
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- PAR ID:
- 10445212
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 125
- Issue:
- 9
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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