Climate model sensitivity to atmospheric CO_2 levels in the Early-Middle Paleogene

摘要

This study examines the sensitivity of the slab ocean version of the national Center for Atmospheric Research Climate System Model with revised Eocene geography, orography, and vegetation to changing carbon dioxide (CO_2) levels. We compare model results with temperature proxies from the geologic record for the Early-Middle Paleogene, orography, and vegetation to changing carbon dioxide (CO_2) levels. We compare model results with temperature with temperature proxies from the geologic record for the Early-Middle Paleogene. We ran three modeling experiments with CO_2 levels at 500, 1000, and 2000 ppm, and all with atmospheric methane levels of 3.5 ppm. Surface temperatures in the two higher CO_2 scenarios are warmer than those of the 500 ppm scenario. The largest warming with increasing CO_2 occurred in the high latitudes, particularly in the Northern Hemisphere, during the wintertime. Compared to the 500 ppm case, Arctic wintertime temperature increased by ～10 ℃ for the 1000 ppm scenario, and ～20 ℃ for the 2000 ppm scenario. The 1000 and 2000 ppm scenarios produced mean annual and cold month mean temperatures in mid- and high latitudes that are much more compatible with the climate interpretations from Eocene flora, especially for data from the Southern hemisphere. Tropical sea surface temperatures (SSTs) in the 2000 ppm scenario, however, are still ～4 ℃ higher than the warmest temperatures inferred from proxy data. The better match between temperatures in the high CO_2 modeling scenario and high latitude climate interpretations is consistent with the idea that the CO_2 levels during the Eocene were high, at least 3-4 times the pre-industrial value of 280 ppm, but the discrepancies in the tropics suggest that SST estimates from proxies are too low or that the models lack some tropical cooling mechanism that was important at this time.