A Sensitive Earth: Implications of Eocene Climate for Future Greenhouse Projections

Matthew Huber
Purdue University

In the Eocene (55 to 34 Mya) climate was much warmer than modern and climate models have long found it difficult to reproduce this pattern accurately. Recent progress in improving the model-proxy data comparison provides some intriguing conclusions that have implications for predicting future climate change. It appears that climate models that warm by 2°C per doubling of atmospheric carbon dioxide concentrations can reproduce the pattern of global warmth evidenced in the Eocene, but only if greenhouse gas forcing is stronger than suggested by proxy records. This implies that 2°C warming per doubling is too low, and that a value of this parameter (called climate sensitivity) closer to 4°C is more likely to be valid. Both values are compatible with observations over the past 1,000 years and climate models produce a range of sensitivities between typically between 2 and 4.5°C, so a value of 4°C per doubling of carbon dioxide is not difficult to believe. While this difference (between 2 and 4°C per doubling) may seem small, its implications for future climatic conditions and safe emissions pathways are enormous. Eocene climate model results also indicate that sensitivity to carbon dioxide variations increase with increasing temperature, which suggests that positive feedbacks in the climate system may render Earth's climate less stable under conditions much warmer than modern. Assuming that the Eocene estimates of climate sensitivity are correct, estimates of future climate are made. These indicate a small, but finite probability of lethal heat stress becoming widespread across many of Earth's currently most-inhabited regions.