Plankton cooled a greenhouse

experiments are lucky — a megalomaniac climatologist can only dream of putting an Earth-like planet in a giant test tube, pumping billions of tonnes of CO2 into its atmosphere, and registering the effects on life and climate. Fortunately, there are other approaches. At the Palaeocene/Eocene (P/E) boundary 55 million years ago, nature appears to have done the greenhouse experiment for us. Bains et al. (page 171 of this issue) now report that they have identified a rather unexpected response of the oceanic biosphere to dramatically high concentrations of atmospheric CO2, and temperatures, at this boundary — one that can account for a subsequent reduction in atmospheric CO2 and cooling. In the early Palaeogene, 65 to 41 million years ago — a period that includes the Palaeocene and the first half of the Eocene — the Earth was generally much warmer than today. The polar regions were free of continental ice sheets, alligators and turtles thrived on Ellesmere Island at 75° N, and palm trees grew as far north as Kamchatka. For a period of about 60,000 years, ‘superwarm’ conditions developed at what is known as the P/E thermal maximum. Oxygen isotopic (O/O) analyses of shells from marine microorganisms called foraminifera show that surface-water temperatures off the coast of Antarctica rose from about 13 °C to 20 °C (ref. 2). Subtropical regions also became warmer; but the higher the latitude, the greater was the effect. The warming coincides with some of the most dramatic biotic changes since the mass extinctions at the Cretaceous/Tertiary boundary, 65 million years ago. Deep-sea, bottom-dwelling foraminifera suffered major extinctions, while terrestrial mammals and different oceanic plankton groups underwent considerable diversification. Events at the P/E boundary coincide with a large decline in the C/C ratio of the carbon dissolved in the ocean. The decline happened rapidly, within a few tens of thousands of years. It is recorded in the shells of both planktonic and deep-sea, bottomliving foraminifera, as well as in the teeth of land mammals, indicating that the entire ocean–atmosphere carbon reservoir altered in composition. How could such a big change have happened so quickly? Several years ago, Dickens et al. provided a plausible explanation. Under normal temperature conditions, enormous amounts of carbon are stored in ocean sediments as gas hydrates — solid news and views