Abstract
Abstract. In this study, we analyse the role of climate change in the forest fires that raged through large parts of Sweden in the summer of 2018 from a meteorological perspective. This is done by studying the Canadian Fire Weather Index (FWI) based on sub-daily data, both in reanalysis data sets (ERA-Interim, ERA5, the Japanese 55 year Reanalysis, JRA-55, and Modern-Era Retrospective analysis for Research and Applications version 2, MERRA-2) and three large-ensemble climate models (EC-Earth, weather@home, W@H, and Community Earth System Model, CESM) simulations. The FWI, based on reanalysis, correlates well with the observed burnt area in summer (r=0.6 to 0.8). We find that the maximum FWI in July 2018 had return times of ∼24 years (90 % CI, confidence interval, > 10 years) for southern and northern Sweden. Furthermore, we find a negative trend of the FWI for southern Sweden over the 1979 to 2017 time period in the reanalyses, yielding a non-significant reduced probability of such an event. However, the short observational record, large uncertainty between the reanalysis products and large natural variability of the FWI give a large confidence interval around this number that easily includes no change, so we cannot draw robust conclusions from reanalysis data. The three large-ensembles with climate models point to a roughly 1.1 (0.9 to 1.4) times increased probability (non-significant) for such events in the current climate relative to preindustrial climate. For a future climate (2 ∘C warming), we find a roughly 2 (1.5 to 3) times increased probability for such events relative to the preindustrial climate. The increased fire weather risk is mainly attributed to the increase in temperature. The other main factor, i.e. precipitation during summer months, is projected to increase for northern Sweden and decrease for southern Sweden. We, however, do not find a clear change in prolonged dry periods in summer months that could explain the increased fire weather risk in the climate models. In summary, we find a (non-significant) reduced probability of such events based on reanalyses, a small (non-significant) increased probability due to global warming up to now and a more robust (significant) increase in the risk for such events in the future based on the climate models.
Highlights
The summer of 2018 in Sweden was characterized by numerous large forest fires spread over large parts of the country
For Norrland, we find large significant differences between JRA-55, with a return time of ∼ 5 years, and ERA-I, ERA5 and MERRA-2, with return times of ∼ 30 years
In Götaland, we find differences between the products, but JRA-55 closely matches ERA5, with return times of ∼ 8–10 years, and ERA-I has a higher return times of ∼ 20 years, and MERRA-2 has even higher return times at ∼ 60 years
Summary
The summer of 2018 in Sweden was characterized by numerous large forest fires spread over large parts of the country. Though forest fires are common in Sweden (Drobyshev et al, 2012), the number of fires and total burnt area in 2018 were much higher than observed over recent years (2008–2017; Fig. 1). Spring and summer weather conditions in 2018 were anomalously dry and warm. This was caused by very persistent atmospheric blocking, especially in May and July. In July, the high surface pressure (Fig. 2a) caused high tempera-. F. Krikken et al.: Attribution of the role of climate change in the forest fires in Sweden 2018
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