Substantial increases in burned area in circumboreal forests from 1983 to 2020 captured by the AVHRR record and a new autoregressive burned area detection algorithm

Wildfire maintains boreal forest health by catalyzing nutrient cycling and forest succession. However, increased annual burned area due to climate warming may facilitate forest loss and soil carbon release, which makes it important to monitor circumboreal burned area. Our goal was to characterize regional changes in circumboreal burned area from 1983 to 2020 using Advanced Very High Resolution Radiometer (AVHRR) data, and to identify the ecoregions where increases in burned area represent significant trends. We accomplished this by developing and applying a new burned area mapping algorithm that is based on an autoregressive analysis of the AVHRR and MODIS MOD09CMGv061 time series. Our algorithm worked well, and resulting burned area totals were similar to those of the MODIS MCD64A1v61 burned area product for years where both were available (2001−2020); however, the advantage of our AVHRR burned area dataset is that it extends back to 1983. Based on the resulting burned area maps, we evaluated circumboreal burned area changes and tested for significant trends. Net changes were substantial: while only 5.37 % of the circumboreal biome burned in the 1980s, 8.22 % did during the 2010s, an increase of 2.85 % (= 8.22–5.37 %) that corresponds to a proportional increase in area burned of 0.53 (= 2.85/5.37). In one ecoregion, Muskwa Slave Lake Forests, burned area more than quadrupled from the 1980s to the 2010s, and in three it more than tripled (Northern Canadian Shield Taiga, Yukon Interior Dry Forests, and Northeast Siberian Taiga). Furthermore, despite interannual variability in burned area typically being high, we found statistically significant increasing trends in burned area in seven of the twenty-three boreal ecoregions, corresponding to 19.6 % of boreal forests (35 % of North American and 11 % of Eurasian boreal forests), while only one ecoregion (Eastern Canadian Shield Taiga) had a decreasing trend. By analyzing the long-term AVHRR record, we were able to capture much larger increases in burned area than from the shorter MODIS record, allowing us to quantify how widespread and substantial these increases have been. By analyzing ecoregions, we found that north-eastern Siberian, north-western Canadian, and Alaskan boreal forests have experienced the most increases in burned area. These increases in burned area may have implications for future forest persistence and carbon storage within Eurasian and North American boreal forests.