Fewer rainy days and earlier springs linked in northern climates

A drop in the total number of rainy days each year is contributing to an earlier arrival of spring for plants in northern climates, a new study finds.

Scientists have known that warmer temperatures due to climate change have led to the first leaves coming out at earlier dates. But this study shows that fewer rainy days play the second largest role in this early leaf-out, said Desheng Liu of The Ohio State University, a co-author of the research.

"Scientists have looked mainly at how temperature affects when leaves first appear and, if they considered precipitation at all, it was the total amount," Liu said. "But it isn't the total amount of precipitation that matters the most – it’s how often it rains."

In the study, supported by the U.S. National Science Foundation and published in the journal Nature Climate Change, the researchers calculated that a decline in rainfall frequency will lead to spring arriving an additional one to two days earlier each decade through 2100.

"This study exemplifies the complex consequences of climate change," said Rainer Amon, a program director in NSF's Office of Polar Programs. "We are not simply facing a warmer atmosphere, but also a shift in the diurnal and seasonal timing of biological processes on a global scale."

Why do fewer rainy days lead to the earlier arrival of spring? Rainy days are also cloudy days. Fewer rainy days in late winter and early spring means that trees and other plants are receiving more solar radiation earlier in the year, which stimulates leaf growth.

Fewer days with clouds also mean daytime temperatures will be higher with more sunlight heating the ground and atmosphere. Nighttime temperatures will cool more rapidly without clouds to trap the heat.

The researchers analyzed data from the United States, Europe and China (north of 30 degrees latitude, which includes most of the United States). The data included the dates each year when observers noted the first evidence of leaves. The scientists also used satellite images from 1982 to 2018, which recorded when vegetation started to turn green.

They compared the satellite images with data on how many rainy days there were each month at the sites studied. The findings demonstrated that "we should expect an even earlier spring in the future compared to what current models tell us," said co-author Jian Wang, also at Ohio State.

The results showed that as rainy days declined over the years, spring arrived earlier for most of the areas in the Northern Hemisphere. The one exception: grasslands in semi-arid regions, where fewer rainy days delayed spring slightly.