Heterogeneous response of circumboreal wildfire risk to climate change since the early 1900s.
Martin-Philippe Girardin, Adam Ali, Christopher Carcaillet, Manfred Mudelsee, Igor Drobyshev, Christelle Hely-Alleaume, Yves Bergeron.
We investigated changes in wildfire risk over the 19012002 (AD) period with an analysis
of broad-scale patterns of July monthly drought code (MDC) variability on 28 forested
ecoregions of the North American and Eurasian continents. The MDC is an estimate of
the net effect of changes in evapotranspiration and precipitation on cumulative moisture
depletion in soils, and is well correlated with annual fire statistics across the circumboreal
(explaining 25–61% of the variance in regional area burned).We used linear trend and
regime shift analyses to investigate (multi-) decadal changes in MDC and percentage
area affected by drought, and kernel function for analysis of temporal changes in the
occurrence rates of extreme drought years. Our analyses did not reveal widespread
patterns of linear increases in dryness through time as a response to rising Northern
Hemisphere land temperatures. Instead, we found heterogeneous patterns of drought
severity changes that were inherent to the nonuniformly distributed impacts of climate
change on dryness. Notably, significant trends toward increasing summer moisture in
southeastern and southwestern boreal Canada were detected. The diminishing wildfire
risk in these regions is coherent with widely reported decreases in area burned since
about 1850, as reconstructed by dendrochronological dating of forest stands. Conversely,
we found evidence for increasing percentage area affected by extreme droughts in
Eurasia (10.57% per decade; Po0.05) and occurrence rates of extreme drought years in
Eurasian taiga (centered principally on the Okhotsk–Manchurian taiga, P50.07).
Although not statistically significant, temporal changes in occurrence rates are sufficiently
important spatially to be paid further attention. The absence of a linear trend in
MDC severity, in conjunction with the presence of an increase in the occurrence rate of
extreme drought years, suggest that fire disturbance regimes in the Eurasian taiga could
be shifting toward being increasingly pulse dependent.