A warm future in the past

To understand the impact of a possibly warmer future climate, geologists are searching the past for warmer-than-present interglacial intervals — examples of what we may expect in a world with more greenhouse gases than ours1. A common tactic is to look at the last interglacial (around 120,000 years ago), but a period 423,000 to 362,000 years ago may fit the bill better, because the Earth's orbital geometry has been similar during the Holocene (the present interglacial period) to what it was then. This period is known to palaeoclimatologists as stage 11 or ‘MIS 11’, according to a numbering system for glacial advances and retreats marked in the marine oxygen isotope record (Fig. 1). At a recent symposium, evidence emerged of extreme climatic variation and peculiar interplays between ocean temperature, thermohaline circulation, plankton ecology, sea level and reef growth during MIS 11, all of which may provide insight into the response of the natural carbon cycle to future climate change.