Spring heatwave intensified the 2018 European drought

In a new study led by Ludwig-Maximilians-Universitaet Munich (LMU), experts have determined that a spring heatwave contributed to the severity of the summer 2018 drought in Europe. Unusually warm spring temperatures accelerated the timing and rate of plant growth, which depleted soil moisture. 

The 2018 drought hit northern and central Europe particularly hard, triggering wildfires and causing devastating crop losses. The researchers found that the summer drought was intensified by the spring heatwave that preceded it.     

According to the study authors, the drought impacts varied on a regional scale depending on the dominant type of vegetation. 

“Little is known about whether and how preceding weather parameters influence the response of ecosystems to extreme conditions during the summer,” said study co-lead author Ana Bastos. “To answer this question, we used the year 2018 in Europe as a case-study and carried out climate simulations incorporating 11 different vegetation models.”

The study showed that warmer springtime conditions led to more vigorous vegetation growth, which increased rates of carbon dioxide uptake. However, the impact on annual productivity and the overall carbon balance was found to be highly variable across different regions. 

“When plants resume growth earlier in the year, they use more water,” said Bastos. “In Central Europe, rapid plant growth in the spring significantly reduced the water content of the soil. By the summer, the level of soil moisture was already insufficient to maintain the biomass that had accumulated, making ecosystems more vulnerable to the effects of the drought.” 

In Central Europe, hot spring temperatures had a negative impact on the productivity of ecosystems and net uptake of carbon dioxide later in the year.

In Scandinavia, however, the early plant growth compensated for the drought-induced losses later in the summer. Ecosystem productivity and the annual carbon balance were either unaffected by the spring heatwave or slightly improved. 

The study authors linked the variations in regional outcomes to specific differences in vegetation. Arable land and pastures dominate the landscape in central Europe, while forests cover much of Scandinavia. 

“Trees use water somewhat more economically,” said Bastos. “If they grow faster in the spring, they also consume more water than they otherwise would. But they can control water loss from transpiration by adjusting the opening of stomatal pores in their leaves.” 

In addition, trees have deeper roots that allow them to tap into water at greater depths. This explains how the boreal forests of Northern Europe maintained near-normal levels of carbon fixation during the intense drought.

Overall, the research shows that the spring heatwave of 2018 worsened the vulnerability of ecosystems to summer drought in central Europe, but mitigated the drought impacts in Scandinavia. The findings suggest that monitoring growth rates of vegetation in spring could serve as an early indicator of upcoming summer droughts. 

The team has identified alternative land management strategies that may help mitigate the effects of future droughts.

“In the long term, owing to climate change, spring vegetation will regularly grow at faster rates, consuming more water and increasing the risk of summer droughts,” said study co-lead author Julia Pongratz. 

“It might be possible to make ecosystems more resilient by altering the plant cover – for example, by planting stands of trees in the immediate vicinity of cropland.” 

“But more extreme water shortages in summer will themselves alter the nature of ecosystems, if threshold levels of mortality and fire incidence are more frequently exceeded. So it is not at all clear whether Europe’s ecosystems will continue to serve as carbon dioxide sinks in the future.”

The study is published in the journal Science Advances.

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By Chrissy Sexton, Earth.com Staff Writer