Global warming has been expected to bring not only droughts, but also floods, because what rain you get comes hammering down harder. And the downpours of the future now look to be even more drenching than expected.

A new Nature Geoscience paper (subscription required) considers the intensity of precipitation measured hour by hour for a century in the Dutch town of De Bilt. Theoretically, it’s thought that the intensity of rainfall, including the biggest cloudbursts, should rise by 7% for each degree Celsius that the temperature goes up. That’s based on a thermodynamics equation called the Clausius-Clapeyron relation – and it’s what you see if you look at extreme rainfall on the scale of days.

But it’s the rainiest hours, not the rainiest days, that interest the paper’s authors, Geert Lenderink and Erik Van Meijgaard of the Royal Netherlands Meteorological Institute. That turns out to make a difference.

Looking at the intensity of those rainiest hours, they find that although the 7% scale holds in cold weather, it tips up to about 14% once the temperature hits 12 degrees Celsius. This pattern in the hourly De Bilt data also appears in a high-resolution regional climate model of twentieth-century Europe, they report.

Finally, the authors check what the model shows for central Europe in 2071-2100 – important because features of the climate system producing the jump from 7% to 14% might change as the world warms. Again, they find that in most places the most intensely rainy hours get at least 10-15% more intensely rainy with each degree of warming.

This doesn’t mean there will be a greater number of rainy days. The authors say there may well be more dry ones. But it suggests the heavy clouds that follow will grow more ominous than climatologists had imagined.

Anna Barnett

Photo: Rubyn von Visceral