A growing importance of large fires in conterminous United States during 1984-2012

Fire frequency, extent, and size exhibit a strong linkage with climate conditions and play a vital role in the climate system. Previous studies have shown that the frequency of large fires in the western United States increased significantly since the mid-1980s due to climate warming and frequent droughts. However, less work has been conducted to examine burned area and fire emissions of large fires at a national scale, and the underlying mechanisms accounting for the increases in the frequency of large fires are far from clear. In this study, we integrated remote-sensed fire perimeter and burn severity data sets into the Dynamic Land Ecosystem Model to estimate carbon emissions from large fires (i.e., fires with size larger than 1000 acres or 4.05 km²) in conterminous United States from 1984 to 2012. The results show that average area burned by large fires was 1.44 × 10⁴km²yr^-1 and carbon emissions from large fires were 17.65 Tg C yr^-1 during the study period. According to the Mann-Kendall trend test, annual burned area and pyrogenic carbon emissions presented significant upward trends at the rates of 810 km² yr^-1 and 0.87 Tg C yr^-1, respectively. Characteristic fire size (fire size with the largest contribution to the total burned area) in the period of 2004-2012 increased by 176.1% compared to the period of 1984-1993. We further found that the larger fires were associated with higher burn severity and occurred more frequently in the warmer and drier conditions. This finding implies that the continued warming and drying trends in the 21st century would enhance the total burned area and fire emissions due to the contributions of larger and more severe wildfires.