Temperatures in July were more than ten degrees above the long-term average. There was hardly any rain, 13 millimeters fell from May to July. In the summer of 2018, the largest forest fires that Sweden had experienced in more than a hundred years broke out. 30,000 hectares burned, too much for the sparsely populated country: emergency services from several European countries finally helped to bring the fires under control.
High temperatures, low humidity, dry soil, little wind: experts call conditions like those in Sweden “fire weather” because they promote the spread of forest fires. Researchers at the European Center for Medium-Range Weather Forecasts in Reading, UK, have now investigated how climate change influences such conditions. In the specialist magazine Npj Climate and Atmospheric Science reportthat the risk of extreme fire events in parts of Europe could increase around tenfold by the end of the century.
Europe has been experiencing an increase in destructive forest fires for years. It is estimated that people start 96 percent of fires, either through arson or through carelessly dropping cigarette butts. But the meteorological conditions then determine how strongly a fire can spread, according to the British researchers. One measure of this is the fire weather index FWI, a combination of precipitation, temperatures, humidity and wind.
First, the scientists used long-term weather data to analyze the fire risk in Europe from 1980 to the present. They then simulated how rising global temperatures affect fire risk. They assume a middle scenario for global warming, as expected based on current policy efforts: global warming of two degrees by the middle of the century and three degrees by the end of the century, compared to pre-industrial levels.
The forest fire season in Europe is likely to last a week longer than before
In Europe, according to the analysis, this is likely to result in the risk of forest fires doubling in around half of the area. Extreme fire weather, which currently occurs every 20 years, would then occur on average every ten years. But the risk does not grow equally everywhere. In Spain, southern France, southern Italy, Greece and Turkey, the risk of extreme forest fires could locally increase tenfold. Conditions that now occur every 20 years would then occur every other summer.
In southern Europe, rising temperatures in particular had an impact on the risk of forest fires, according to the researchers. Decreasing rainfall around the Mediterranean, however, was not as significant – which could be because the region is already very dry. In northern Europe it is the other way around: here, decreasing rainfall in summer increases the risk of forest fires, while the development of temperatures plays a subordinate role. Overall, the forest fire season in Europe is likely to extend by around a week on average.
However, the intensity of a fire does not only depend on the weather, “but also on the fuel,” points out fire ecologist Johann Goldammer from the University of Freiburg. In Greece, for example, the severe forest fires of recent years can also be attributed to the population’s rural exodus: where sheep were previously herded or olive groves were cultivated, nature is reclaiming spaces. “The land is overgrown with vegetation,” says Goldammer. As a result, there is more combustible material available for a fire. However, Goldammer criticizes that many studies on the topic do not take local vegetation into account.
The authors of the current work assume that the combustible material in Europe will remain roughly the same, but point out that shrinking vegetation would also be conceivable: If a fire destroys a particularly large amount of biomass in one year, there will be less fuel in the following years available, which reduces the risk of fire again. At the same time, hotter summers generally weaken forests, which can lead to more dead trees – easier prey in the event of a forest fire. “Prolonged drought and heat,” says fire ecologist Goldammer, “will undoubtedly make the forest fire problem worse.”