Three to five million years ago, global temperatures were significantly higher than expected from atmospheric carbon dioxide levels at the time, according to a study published online this week in Nature Geoscience. The authors conclude that the higher long-term sensitivity of the Earth system should be taken into account when defining dangerous anthropogenic climate change.
Daniel Lunt and colleagues combine a global climate model with a reconstruction of the Earth’s environment three to five million years ago, when temperatures were 3 to 5℃ warmer than today. They use the model to separate various contributions to global temperatures, and find that atmospheric carbon dioxide concentrations caused more warming than would be expected from the estimates of climate sensitivity that are used for future climate projections. They conclude that the discrepancy arises because in the earlier period the Earth system had a long time to adjust to the environmental conditions, whereas climate models for future projections do not fully include feedbacks that involve slowly changing parts of the system, such as ice sheets or vegetation patterns.