Background: Concentrations of the greenhouse gas carbon dioxide have surpassed a level of 400pm in May 2013, a concentration that has not been observed in the earth’s atmosphere for a long, long time. To find similarly high concentrations, we have to go beyond times scales that we, or our ancestors, can directly study and observe. Instead we have to use the geological past and go all the way back to a time called the Pliocene epoch (2.6-5.3 million years ago). The Pliocene is the most recent time in the geological past when atmospheric carbon dioxide levels were similar to today. We can hence use it as a natural laboratory to study mechanisms and consequences of climate change.