By Tim Radford
Patterns of Europe’s floods and droughts are starting to change: each could be more extreme, and far likelier with rising heat.
Climate change has begun to affect the pattern of Europe’s floods. The past three decades have seen “exceptional” flooding, say Austrian scientists who have worked their way through documentary records for the last 500 years.
At the same time, heat and drought affecting the continent are on the increase. The summer of 2018 broke all records for Germany, Austria and Switzerland, and by 2019 many trees in Europe’s forests were partly or entirely dead. And by 2085 rainfall could decline by a fifth, Swiss ecologists report, to alter the make-up of the forests dramatically.
Both findings are consistent with the big picture of climate change worldwide: wet seasons will become ever wetter; dry seasons too will become more extreme, according to US researchers in a third separate study.
All attempts to establish climate records involve careful interrogation of the past. Günter Blöschl of Vienna’s University of Technology and colleagues report in Nature that they sifted evidence from mountain lake beds, floodplains and 500 years of contemporary documents to identify decades more than usually rich in floods.
The floods of 1990 to 2016 in Western and Central Europe have been among the worst in history. To make sure of such a claim, the researchers identified periods of calamitous inundation across the whole region in the late 16th century and again in the 17th; and in the 18th and early 19th centuries.
“We should be preparing for the future by improving the technology to efficiently use water for crops”
If these episodes had anything in common, bygone floods happened when air temperatures were lower: fewer of them, too, happened in the summer.
“This finding seems to contradict the observation that, in some areas such as in the northwest of Europe, the recent warmer climate is aligned with larger floods,” Professor Blöschl said. “Our study shows for the first time that the underlying mechanisms have changed.
“While in the past floods have occurred more frequently under colder conditions, the opposite is the case now. The hydrological conditions of the present are very different from those in the past.”
Now, 55% of Central European floods happen in the summer, compared with 41% in previous centuries. It’s a message for planners, city chiefs and governments across the region: flood management is going to have to adapt.
So, too, is forest and woodland management, say scientists in Switzerland and Germany, who have been measuring changes in the canopies of their forests.
For most of Europe, the single most extreme heatwave has been that of 2003: that is, until 2018. The sustained heat and aridity made temperatures in the growing season of 2018 on average 1.2°C higher than 2003, and 3.3°C higher than the average from 1961-1990.
Woodland foliage showed signs of drought stress. Leaves wilted, aged and dropped much earlier, and by 2019 many trees were dead, or partly dead. Those that survived were more vulnerable to beetle or fungal pests. Losses included beech, long considered the most drought-resistant.
Ten out of the 12 hottest growing seasons in the last 120 years have all happened this century. Climate forecasts already predict more of the same, with precipitation falling by a fifth by 2085. Foresters will have to think again about woodland design.
“Spruce was most heavily affected. But it was a surprise for us that beech, silver fir and pine were also damaged to this extent,” said Ansgar Kahmen of the University of Basel in Switzerland.
“We still need to study which tree species are good in which combinations, including from a forestry perspective. That will take time.”
Keeping Paris promise
And worldwide, farmers, foresters and water managers can also expect more of the same. As temperatures rise worldwide, dry seasons will tend to become drier, and wet seasons wetter.
US researchers report in the journal Nature Communications that they divided the world into nine land regions, and looked at annual rain or snowfall and how this fluctuated through the seasons in each of the nine from 1971 to 2000. They then looked at future temperature predictions for the rest of the century to see what happened to water availability.
The best outcome for relatively stable water supplies would be if nations could act to limit the planet’s average global temperature rise to no more than 2°C by 2100, in line with a promise made by 195 governments in Paris in 2015.
At higher temperatures the predicted scatter of flood and drought became more extreme. Once again, the message is: start planning. “We need to take precautions to optimally use how much water we have,” said Ashok Mishra of Clemson University in South Carolina.
“As the climate changes and population increases, we should be preparing for the future by improving the technology to efficiently use water for crops.” − Climate News Network