More commentary on Westerling et al's report in Science on global warming and western wildfires:
1. It is not news that there is a link between global warming and wildfires via atmospheric carbon dioxide. The source of nearly 40% of global atmospheric carbon is what Stephen Pyne (historian of wildfire) calls "open burning" of living biomass, as opposed to internal combustion of fossil fuels. The news is that not only do forest fires fuel global warming, but also that climate change is apparently causing increased wildfire activity, a feed-forward cycle that could accelerate the rate of increase in atmospheric carbon. It will be interesting to see whether these results are replicated in other areas of the globe. Similar mechanisms could be at work, for instance, in Siberia, another region of large wildfires where the timing of snowmelt could affect fire seasons.
2. The possibility of several confounding factors is not discussed in the Science report.
a. Could the increase in the incidence and size of large fires on public lands in the mid-1980s be associated with changes in federal fire control policy and with how firefighting resources were deployed?
b. The area with the greatest increase in fire activity shows a link to warm temperatures and drought because the level of fuel moisture and the length of the dry season are tied to snowmelt. These also happen to be among the most inaccessible of areas to firefighting crews and some of the hardest areas to monitor for early ignition warnings. Their inaccessibility could be linked to the growth of small fires into larger ones, accounting in part for the differences between subalpine forests and the other study areas which did not see such large increases in fire activity.
3. The authors emphasize the interactive relationship between climate as a cause of increased wildfire activity and land-use as a cause, in that increasing drought can affect land management, especially in the Southwest. I hope this complexity is not lost in news reports.
4. The study raises interesting questions about how increased fire frequency and size and decreased fire intervals may change forest structure and composition in the long term. There are also interesting questions to be addressed about effects on wildlife and the interactions between warmer and longer summers, increased fire activity, and forest pests.