Monica Ionita, a climatologist and expert on weather forecasting at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), can still remember it clearly: in late April 2018, it was so hot in Bremen that she put a paddling pool in the garden for her daughter, although it should have been much too early to splash around outdoors. She now knows that the spring heat wave at the time provided the impetus for the following summer being one of the driest in the history of Central Europe.
“Since the turn of the century, Central Europe has experienced repeated summer heat waves and droughts, which have caused damage amounting to millions of Euros. To date, attempts to correctly predict such extreme events have been unsuccessful because the influence of the spring has been underestimated. That is why we decided to closely investigate the links between the weather in spring and that in the subsequent summer – for the entire period for which sufficient records are available. In other words, the last 140 years,” the expert reports.
April’s pivotal role: Lack of rain coupled with heat causes soils to dry out
For the analysis, Monica Ionita and her colleagues employed climate (and hydrologic) model outputs as well as statistical methods that the AWI researchers have developed; and had already successfully applied in long-term forecasts of river water levels. The findings show: in the last 14 years, the temperature and precipitation trends in April have changed fundamentally. “While there was little change in the months of March and May in the period 2007 to 2020, April was on average 3 degrees Celsius warmer compared to the reference period 1961 to 2000. In extreme years, like 2018, it was so warm in April that the snow that had fallen in winter virtually evaporated before it had the chance to drain into the soil in the form of meltwater. Furthermore, since 2007, in most regions of Central Europe there has only been half as much rain as in the reference period,” Ionita explains.
In the past 14 years, the absence of precipitation has only been one of the problems: “Rising April temperatures have led to the moisture stored in the soil evaporating. As a result, in spring there was already a marked lack of moisture in the soils of Central Europe, especially in Germany. As a rule, this deficit couldn’t be compensated for in the summer. In other words: the summer drought in the soils was pre-programmed back in April,” adds Rohini Kumar, a hydrologist at the Helmholtz Centre for Environmental Research (UFZ) in Leipzig and co-author of the new study.