THAT hurricanes are likely to become more powerful as the climate warms is not a matter much debated among researchers. A warmer climate will increase sea-surface temperatures relative to those of the adjacent atmosphere in some regions, leading to more evaporation, more clouds and stronger storms. You might expect this to be reflected in the geological record. But a study just published in Geology by Michael Toomey of the United States Geological Survey suggests, counter-intuitively, that the hurricanes which struck Florida during a cool period 12,000 years ago were more powerful than those during a subsequent time of warmth.

Dr Toomey’s findings stem from his analysis of sediment cores collected near the Dry Tortugas, islands off Florida’s coast. These contain strata laid down during the Younger Dryas, a time when the climate cooled, and also during the warmer years that followed it. The Younger Dryas is thought to have been caused by a freshwater lake that covered part of central Canada breaking through the glacier which had dammed it and draining into the Atlantic Ocean. This injection of fresh water altered the ocean’s circulation and triggered a cold period across much of the northern hemisphere.

Dr Toomey looked in the cores for layers of turbidite, a rock that forms when sediment gets disturbed, flows down marine slopes and is deposited as a jumbled mess on the ocean floor. Turbidites are often a result of earthquakes, but the Dry Tortugas are not prone to them. Thus, Dr Toomey argues, these particular turbidites must have been formed, as similar deposits are today, by hurricanes.

Nowadays, the bigger the storm, the bigger the grain size of the resultant turbidite. So it is curious that turbidite grains laid down during the Younger Dryas had an average diameter of 23 microns whereas those from the subsequent, warmer years averaged 19 microns. Why the Younger Dryas was so stormy, at least off the coast of Florida, in such a cold period is a mystery—and one that shows how complicated Earth’s climate actually is.