Potential future changes of climatic drivers for the European Spruce bark beetle outbreaks in Northern Baltic Sea area under CMIP5 projections

Recently, spruce bark beetle (SBB) damage has been on an extreme record-breaking level in East Central Sweden. It is considered a result of the increase in temperatures and severe droughts propagated by climate change. At the same time, SBB outbreaks have been less severe in the eastern side of the Baltic Sea, in Estonia and Finland, than in Sweden. This study investigated to what extent precipitation, temperature sum, and droughts (hydrothermal index) have varied in land areas surrounding the Baltic Sea. We studied recent meteorological observations (from 1950 to 1999) and modelled the effect of climate change on precipitation and temperature, using three representative pathway scenarios for greenhouse gas emissions (RCP2.6, RCP4.5, RCP8.5) and multiple (17–23) climate models. Future climate projections (up to 2100) were made for Southeastern Estonia, Southern Finland, and East Central Sweden. Weather data showed that temperature sum has been high and drought severe, particularly in East Central Sweden, in the 2010s, where SBB outbreaks have been a larger problem than on the eastern shores of the Baltic Sea. Future climate projections suggest that an increase in thermal sum will further enhance the reproduction of SBB, especially in RCP4.5 and RCP8.5 scenarios. In all climate change scenarios, drought continues to be a problem in Central Eastern Sweden, potentially facilitating SBB outbreaks. In addition, moderate and severe droughts can become more frequent in Southeastern Estonia and Southern Finland if climate change proceeds as predicted in RCP4.5 and RCP8.5 scenarios.