Direct effect of orbital-insolation variation on long-term wildfire activity in central Japan demonstrated using a fuel moisture model

Orbital-insolation variation has directly and indirectly influenced long-term wildfire activity in some regions. However, the direct effects of insolation variation on long-term wildfire activity as a function of Milankovitch cycles have not been investigated yet. In central Japan, charcoal records in lake sediments indicate that long-term wildfire activity may be determined by spring insolation. Thus, in this study, we used a fuel moisture model to simulate high fire risk scenarios (i.e., hours with low fuel moisture) in spring based on maximum and minimum spring insolation periods since the last interglacial period. Our simulation results demonstrated that the duration of high fire risk under maximum spring insolation period is 10–60% longer than that in the minimum spring insolation period, although the duration depends on vegetation conditions. We conclude that orbital-insolation variation can directly influence the long-term wildfire activity via changes in evaporation rate arising from fuel or forest litter.