Constraining the climate sensitivity in CESM2 through simulation of the Last Glacial Maximum
Abstract
Equilibrium climate sensitivity (ECS) is the long-term response of global mean surface temperature to a doubling of atmospheric CO 2 . ECS remains poorly constrained with values ranging from 1.8 to 5.6 ° in the Coupled Model Intercomparison Project phase 6 (CMIP6) models, a range that is much larger than that in previous CMIP intercomparisons. The ECS in the Community Earth System Model version 2 (CESM2) is 5.3°C, which is considerably higher than 3-4°C in the preceding CESM1 and the Community Climate System Model version 4 (CCSM4). Determining whether this high ECS is realistic is paramount to constraining future climate change and crafting effective policies and adaptation plans. The Last Glacial Maximum (LGM; 21 ka before present) with lower levels of atmospheric greenhouse gases (GHGs) and the presence of high-latitude land ice sheets (LISs) is one of the most-studied paleo-climatic intervals, and thus offers a prime opportunity to inform ECS. Here we compare the CESM2 LGM simulation with its predecessors and with temperature reconstructions using novel proxy synthesis and paleoclimate data assimilation. We find that while the LGM global cooling in CESM1 and CCSM4 compares reasonably well with proxy evidence, the CESM2 cooling is much larger and falls outside of the data uncertainty range. The LGM climate forcing and feedbacks are quantified in CESM2 and compared with those in CESM1. The forcing-feedback analysis and additional sensitivity experiments suggest that the greater LGM cooling in CESM2 is attributed to cloud-related physical parameterizations. Our study illustrates the importance of paleoclimate constraints in informing ECS in climate models.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMA130...04Z
- Keywords:
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- 3310 Clouds and cloud feedbacks;
- ATMOSPHERIC PROCESSES;
- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES;
- 3337 Global climate models;
- ATMOSPHERIC PROCESSES;
- 3360 Remote sensing;
- ATMOSPHERIC PROCESSES