Crocodiles once roamed the Arctic, during the Eocene about 50 million years ago. Polar regions were lush and warm. Greenhouse gas concentrations were higher than today, but at most about 4 times higher - not enough, according to current climate models, to have warmed the Arctic sufficiently. Something appears to be missing in current models to account for the warmth of the past.
The likely culprits are clouds, especially the low clouds that cover vast areas of tropical oceans. These clouds cool Earth by reflecting sunlight back to space. It is possible that the cloud cooling may have been absent or strongly diminished in past greenhouse climates, raising questions about our climate future. To predict our climate future more accurately, breakthroughs in the modelling of clouds and in the accuracy of climate predictions are needed. They are now within reach, thanks to advances in computing and Earth observations from space and our ability to fuse models with massive amounts of data.