SIMULATED EOCENE HOTHOUSE CLIMATE - A DEEPMIP STUDY

During the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma), a rapid injection of isotopically depleted carbon into the atmosphere led to a ~5 °C global temperature rise, ocean acidification, and perturbation of marine and terrestrial ecosystems. In this study, a series of DeepMIP climate sensitivity experiments has been carried out with the Community Earth System Model CESM1.2 to evaluate how changes in the radiative forcing could have contributed to Eocene hyperthermal events. The analysis of our simulations suggests a substantial warming from 3x to 12x CO₂ PAL, reaching up to 40 °C near the equator and almost 20 °C in polar regions, consistent to proxy estimates. The lower equator-to-pole temperature gradient compared to present-day is due to the lack of an ice sheet, an increase in greenhouse gases, and a lower cloud optical depth. The climate simulations suggest an intensified hydrological cycle with higher precipitation in the tropics, particularly over the Indian Eocene continent, and in mid-latitudes, whereas mega-droughts are prominent in the subtropics, particularly in Africa and South America. Topographic effects like closure of the Drake Passage and the more southern location of Australia during the Eocene as well as a lower-than-present meridional temperature gradient contribute to a much weaker surface ocean circulation near the Antarctic continent as compared to the current pronounced Antarctic Circumpolar Current. Increased stratification, reduced export production, and reduced marine dimethyl sulfide-producing plankton productivity causes a decline in DMS emissions to the atmosphere, thus contributing to reduced cloud condensation nuclei and lower cloud aerosol optical depth (anti-CLAW hypothesis), potentially leading to a positive climate feedback.