Spring is arriving earlier in northern climates as rainfall becomes less frequent, according to a new study from the National Science Foundation. The experts identified a link between a drop in the total number of rainy days each year and the early appearance of leaves.
While experts had already determined that rising temperatures have led to the first leaves coming out at earlier dates, this research shows that fewer rainy days play the second largest role in this early leaf-out, explained study co-author Desheng Liu of The Ohio State University.
“Scientists have looked mainly at how temperature affects when leaves first appear and, if they considered precipitation at all, it was the total amount,” said Liu. “But it isn’t the total amount of precipitation that matters the most – it’s how often it rains.”
The results of the study showed that a decline in rainfall frequency will cause spring to emerge an additional one to two days earlier each decade through the end of the century.
“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.”
The researchers explained that fewer rainy days lead to the earlier arrival of spring because trees and other plants are receiving more solar radiation earlier in the year, which stimulates leaf growth.
With fewer rainy days, daytime temperatures are hotter as more sunlight heating reaches the Earth’s surface and atmosphere.
The study was focused on data north of 30 degrees latitude, which includes most of the United States. The experts analyzed satellite images that recorded when vegetation started to turn green from 1982 to 2018. This data was compared with the number of rainy days during each season.
According to study co-author Jian Wang, the findings demonstrate that we should expect an even earlier spring in the future compared to what current models tell us.
The study is published in the journal Nature Climate Change.