Significant climatic changes over the high latitudes in the 20^th century have been reflected in many atmospheric, oceanic, and terrestrial variables. Changes in surface air temperature, precipitation, growing season duration, and snow cover cause changes in numerous derived variables of economic, social and ecological interest, including the natural frequency of forest fires. Fire is one of nature’s primary carbon-cycling mechanisms but human activity interferes with 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. Using meteorological information for the past century (data from more than 1500 stations with a daily time resolution), we found a significant (sometimes a twofold) increase in indices that characterize the weather conditions conducive to forest fires. The areas, where this increase was statistically significant, coincide with the areas of most significant warming during the past several decades in Central Alaska and in Siberia south of the Arctic Circle.

The figure below shows an example of one of these indices (KBDI) regionally averaged over southern Siberia (south of 55°N, east of 85°E and west of 128°E). To characterize the level of potential fire danger, the Keetch-Byram Drought Index (KBDI; Keetch and Byram 1968) uses only daily temperature and precipitation information and estimates soil moisture deficiency. The logic behind the index is that wet soil suppresses wild fires while dry soil organic matter enhances these fires and makes them difficult to control. Our analysis indicates that the frequency of unusually dry summer and spring conditions (days with anomalously high KBDI values) has increased in Siberia and Alaska during the past century (50 years in Alaska). These changes, when overlapping with anthropogenic factors (such as reckless behavior in the forest, logging, and agricultural burning), have already aggravated the negative consequences of forest fires and (if continued) may be devastating in the future.

Ref. Keetch, J.J. and G.M. Byram, 1968: A drought index for forest fire control. U.S.D.A. Forest Service Research Paper SE-38. 35 pp. [Available from: http://www.srs.fs.fed.us/pubs/]