Earlier blooming intensifies spring heat waves in Europe

The early emergence of flowers and leaves due to climate change amplifies springtime heat waves in Europe, new climate simulations suggest. While not as deadly as their summertime counterparts, spring heat waves can disrupt ecosystems and damage crops.

Simulating a 30-day head start to spring’s natural beginning, researchers discovered that the shift alters water distribution in the environment and boosts the likelihood of spring heat waves by about 0.6 events per year. Since the 1970s, spring plant growth has shifted earlier in the year by around two to five days each decade in the Northern Hemisphere.

The heat waves also become more intense, with the average one lasting a day longer and reaching 1 degree Celsius warmer, the researchers report in a paper to be published in Geophysical Research Letters.

While early plant growth affected spring heat, the new research found no influence on summer heat waves. That result suggests that although plants impact temperature, the effect is short-lived, says study coauthor Andy Pitman, a climate scientist at the University of New South Wales in Sydney.

Pitman and colleagues ran a climate simulation multiple times, each time shifting spring plant growth earlier and earlier. The simulations revealed that budding plants suck water from the soil that would otherwise evaporate and cool the air. The drier soil increased daytime temperatures and decreased low- and mid-lying clouds that block sunlight. By summer, the early spring had little effect.

Pitman suggests that by the time summer rolls around, other factors such as rainfall begin to have a much larger effect than the remnant impacts of early spring greening. Essentially the effects of an early spring get “washed out,” he says. The researchers plan to look at the effects of early spring plant growth on other parts of the world, including North America.

The results make sense, but “it’s just one model,” says Benjamin Zaitchik, a hydroclimate scientist at Johns Hopkins University. “It would be interesting to see if other modeling groups could show how the same effect plays out in the other models.”