Potential forest fire danger over Northern Eurasia: Changes during the 20th century

Fire is one of nature's primary carbon-cycling mechanisms but human activity interferes with the natural component of this mechanism causing by some estimates more than half of the occurrences of boreal forest fires. When the weather conditions are conducive to the expansion of forest fires, this anthropogenic effect becomes especially dangerous. In this study, we target only the meteorological component of the changes in potential forest fire danger. Using meteorological information for the past century, we investigate changes in indices that characterize the weather conditions conducive to forest fires over Northern Eurasia. Significant climatic changes over Northern Eurasia during the 20th century have been reflected in numerous variables of economic, social, and ecological interests, including the natural frequency of forest fires. The following rationale for changes in this frequency for high- and mid-latitudes (thus, for Northern Eurasia) can be formulated as follows. Warming, spring snow cover retreat, extension of the growing season and a reduction of the “warm temperatures deficit” acting together promote vegetation growth and, therefore transpiration. At the same time, warmer surface air temperatures themselves increase the “atmospheric demand” for water vapor and increase of potential evaporation. If precipitation increases insufficiently to match this growing demand, we should witness drier surface conditions and so called “summer dryness” and thus, an increase in potential forest fire danger. This scenario is, however, not inevitable. It well can be that in some regions changes in the atmospheric circulation pattern and/or changes in local factors may increase precipitation “sufficiently” to oppose summer dryness.

We are using the Global Daily Climatology Network and a new Global Synoptic Data Network archive, GSDN, created jointly by U.S. National Climatic Data Center, Meteorological Service of Canada, and Russian Research Institute for Hydrometeorological Information. Data from these archives (approximately 1500 of them having sufficiently long meteorological time series suitable for participation in our analyses for Northern Eurasia) are employed to estimate systematic changes in indices used in the United States and Russia to assess potential forest fire danger. We use three indices: (1) Keetch-Byram Drought Index, (this index was developed and widely used in the United States); (2) Modified Nesterov, and (3) Zhdanko Indices (these indices were developed and widely used in Russia). Analyses show that after calibration, time series of the days with increased potential forest fire danger constructed using each of these three indices (a) are well correlated and (b) deliver similar conclusions about systematic changes in the weather conditions conducive to forest fires. Specifically, over the Eastern half of Northern Eurasia (Siberia and the Russian Far East) statistically significant increases in indices that characterize the weather conditions conducive to forest fires were found. These areas coincide with the areas of most significant warming during the past several decades south of the Arctic Circle. West of the Ural Mountains, the same indices show a steady decrease in the frequency of “dry weather summer days” during the past sixty years. This study is corroborated with available statistics of forest fires and with observed changes in drought statistics in agricultural regions of Northern Eurasia. Potential for projections of these trends will be discussed at the Symposium.