Clouds obscure link between extreme rainfall and rising temperatures

Extreme rainfall is usually defined as the heaviest 5% of rain events in a specific area. As global air temperatures rise, scientists expect extreme rainfall to increase. This is because warmer air can hold more moisture. Recent examples of more extreme rainfall have become evident in more flooding scattered over the globe in the last years.

However, when looking at observations of how heavy rainfall events relate to local temperatures across regions, scientists noticed an unexpected pattern seemingly contradicting the theory. They found that in warmer tropical and mid-latitude regions, extreme rainfall rates decrease when mean daily temperatures exceed about 23°C–25°C.

A research team led by the Max Planck Institute for Biogeochemistry in Jena, Germany, has now resolved this discrepancy and found that clouds were the culprit. Rain falls from clouds, which also block the incoming sunlight, thereby cooling the surface. Mean air temperatures are thus being affected by clouds. As a result, the true correlation of extreme rainfall rates with warming air temperatures is biased—especially in warmer tropical regions, where clouds reflect significantly more sunlight.

In the study published in Nature Communications, the authors developed a method to remove the cooling effect of clouds from mean air temperatures by using satellite-derived radiation datasets. After the removal, they found that the increase of extreme rainfall rates with temperature aligns very closely with the theoretical expectations and with climate model projections.

“This confirms what is widely expected: Extreme rainfall intensifies in a globally warmer climate,” says Dr. Sarosh Alam Ghausi, lead author and PostDoc at Max Planck Institute for Biogeochemistry in Jena, Germany. He continues, “While heavy rainfall increases mostly everywhere, we found the largest increases in high temperature-driven heavy rainfall events in tropical humid areas such as in India, Northern Australia, and the Amazon.”

As extreme rainfalls are expected to become more intense, this likely increases the risk of flooding when no active measures are being taken. And the increase in extreme rainfall is expected to further continue as air temperatures rise with climate change.

Dr. Axel Kleidon, senior author and group leader at the Max Planck Institute, remarks, “These results support the physical expectations that the whole hydrological cycle becomes more intense and extreme with warmer temperatures. We will not only see more extreme rainfall rates, but also more intense and longer dry spells in the future.”