Warm springs increase risk of severe summer droughts10 June 2020
Warm and sunny springs - like the one just seen in the UK - can create conditions that pave the way for severe summer droughts, a new study has shown.
Researchers from the universities of Reading and Exeter, along with institutions around the world, studied the drought during the 2018 summer heatwave in Europe, which caused forest fires and damaged harvests. They found the drought was partly caused by the unusually warm spring that preceded it, as this triggered early and rapid plant growth that depleted soil moisture.
The findings, published today [Wednesday 10 June] in Science Advances, serve as a warning to countries when they experience warm springs, particularly as hot conditions and droughts are expected to become more common in future under climate change.
“Heatwaves and extremely dry conditions will be more common in a warmer world, so the possibility of spring providing warning signs that drought is on the way is hugely significant" - Dr Patrick McGuire, University o fReading
Dr Patrick McGuire, an environmental scientist at the University of Reading and NCAS, one of two Reading scientists who ran one of the computer models in the study, said: “Droughts can impact supplies of drinking water, harm ecosystems and mean less carbon dioxide can be absorbed by plants than normal due to restricted growth or wildfires.
“Heatwaves and extremely dry conditions will be more common in a warmer world, so the possibility of spring providing warning signs that drought is on the way is hugely significant. This could allow us to take early and effective action to reduce impacts on society and the environment.”
The 2018 summer heatwave caused extremely dry conditions in Europe – particularly northern and central Europe – causing a significant economic impact.
The team, led by Ludwig Maximilian University (LMU) of Munich, compared the 2018 drought with those of 2003 and 2010. The 2018 summer drought differed in that it was preceded by an unusual spring heatwave, which was found to have amplified its effects.
Mitigating effects of droughts
The study showed that around half of the reduction in water availability in summer 2018 was found to be caused by the rapid, early growth of plants in the spring. The impact on different countries depended on the plants and trees that dominate their landscapes.
Professor Pierre Friedlingstein, a climate scientist at the University of Exeter who co-authored the report, said: “The coupled dynamics between weather and land ecosystems is quite complex. One could assume that a warmer spring, would lead to an earlier bloom of the vegetation which surely should be good news. However, this is only partly true. Stronger vegetation activity in spring also means more soil water being needed, leading to more severe drought in the summer, as it happened during the 2018 summer heatwave.”
The scientists say studying growth rates of plants in spring could act as an early warning of impending summer droughts. Adopting alternative land management strategies, such as planting trees next to crops to provide shade, could mitigate and the effects of droughts.
Dr Ana Bastos, lead author of the study, who now heads a research group at the Max Planck Institute for Biogeochemistry in Jena, said: “Little is known about whether and how preceding weather parameters influence the response of ecosystems to extreme conditions during the summer.
“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.
“When plants resume growth earlier in the year, they use more water. 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.”
A. Bastos, P. Ciais, P. Friedlingstein, S. Sitch, J. Pongratz, L. Fan, J.P. Wigneron, U. Weber, M. Reichstein, Z. Fu, P. Anthoni, A. Arneth, V. Haverd, A. K. Jain, E. Joetzjer, J. Knauer, S. Lienert, T. Loughran, P.C. McGuire, H. Tian, N. Viovy, S. Zaehle (2020); ‘Direct and Seasonal Legacy Effects of the 2018 Heatwave and Drought on European Ecosystem Productivity’; Science Advances 2020