Progress in Greenhouse Climate Modeling

Abstract

This article discusses past greenhouse climates with an emphasis on recent progress in the comparison of models and data in the Eocene. These past climates—about 10° warmer than modern—provide valuable lessons in the climate dynamics of a world with higher-than-modern greenhouse-gas concentrations, reduced temperature gradients, and a lack of polar ice. Some key points that emerge from this analysis are: 1) Past greenhouse climates are characterized by warmer-than-modern global mean temperatures and winter season warmth, as well as strong polar amplification of warming that generates weak meridional temperature gradients. 2) A trade-off exists between heat transport in a warmer world and a world with smaller temperature gradients; a range exists in which a warmer world with weak temperature gradients can transport as much heat as a cooler world with stronger gradients. 3) These features can be reproduced in models if global mean and tropical temperatures are allowed to increase significantly over modern values. 4) Local radiative-convective feedbacks play an important role, perhaps dominating over transport, or acting in conjunction with transport to maintain relatively warm, weak temperature gradient climates. 5) Past greenhouse climates appear to have many of the modern modes of variability from orbital to interannual. 6) Modeling efforts are most useful when done independently of proxy data; model output can then be compared with proxies to evaluate potential weaknesses in the model. 7) Model-data mismatch might be due to incorrect boundary or initial conditions or resolution, or to missing or incomplete representation of relevant physics. 8) Refinements and new developments in proxies for temperature and other variables offer much potential for discriminating among the next generation of modeled scenarios, thereby allowing a better understanding of the conditions under which greenhouse climates exist.