A cartoon with people hiking in the forest. Trees are illustrated as match sticks. The caption reads: I wonder how the forest is being affected by climate change?

Across the United States and around the world, wildfires are growing in intensity and frequency. Wildfire season can spark anytime throughout the year in arid regions where Mediterranean climates predominate, such as Spain, Portugal, and much of California. Dry, wooded regions where people and wild lands exist in close proximity have grown especially dangerous, as rural communities become trapped by rapidly spreading conflagrations.

Climate scientists have correlated the growing incidence and intensity of wildfires with rising global temperatures. Few places seem immune: Australia; Indonesia; Canada; Alaska; the American northwest, southwest and southeast; Chile; and Western Europe have all seen massive and destructive wildfires in recent years. In federally managed forests in the western U.S. today, wildfires larger than 1,000 acres have become nearly five times more frequent and burned areas 10 times as large as in the 1970s, according to research by LeRoy Westerling at the University of California at Merced. This time period corresponds to significant warming documented around the globe: Two-thirds of the 1.4-degree rise in average global temperatures since 1880 has occurred since 1975, according to NASA’s Earth Observatory.

How climate change affects wildfires

Climate change contributes to more and bigger wildfires in a variety of ways.

The rise in average global temperatures has led to higher spring and summer temperatures, and importantly an earlier onset of spring. This pattern has led to a rapid melting of spring snowpack, causing soils to dry out earlier and remain dry longer.

After months of drying in the longer periods of higher temperatures, stressed forests have become more susceptible to infestations by bark beetles and other insects that thrive in warmer temperatures. Throughout the western United States and Canada, bark beetles have killed off hundreds of millions of trees and devastated forestlands, turning them into kindling for catastrophic wildfires. Insect outbreaks killed more than 300 million trees in Texas in 2011, and more than 129 million trees in California from 2010 to 2017, according to the Fourth National Climate Assessment, released in 2018.

Warming temperatures have allowed populations of mountain pine beetles to explode at elevations and latitudes where winters were historically cold enough to limit their numbers. These insects have killed trees across more than 25 million acres in the western U.S. since 2010. In California, the level of tree mortality has been so high in some areas that 70 percent of trees died in a single year.

Meanwhile, the West Coast of the U.S. is particularly susceptible to a kind of weather whiplash – wet winters fueled by atmospheric river storms originating in the Eastern Pacific, followed by parched summers that dry out spring vegetation and transform it into kindling for wildfires in the fall. A 2018 study in Nature Climate Change projected a 25 percent to 100 percent increase in extreme dry-to-wet precipitation events.

How wildfires affect health

As wildfires grow in frequency, intensity, and the amount of area burned, they pose serious health risks.

Smoke from wildfires contains volatile and semi-volatile organic compounds and nitrogen oxides that form ozone and organic particulates and other toxic pollutants – all of which can be dangerous and even deadly for sensitive populations. Wildfires also impact climate change because they emit massive amounts of carbon dioxide and other pollutants that can affect regional and even global climate.

Wildfires are also expected to increase the risk for destructive mudslides, as landscapes laid bare by fire are drenched with winter rain. For example, mudslides in the Santa Barbara area in early 2018, coming after wildfires in the fall of 2017, devastated the region.

What’s expected in the future?

Over the next few decades, the wildfire forecast is grim. By mid-century, the annual area burned in the Western U.S., for example, could increase two to six times what it is today, according to the Fourth National Climate Assessment.

Uncertainties remain. Although the frequency of wildfires overall is very likely to increase as temperatures warm, where and when they’ll spark is difficult to impossible to predict. Furthermore, while scientists have studied the impact on climate warming on bark beetles, little is known about the impact on other insects or on forest pathogens. How warming affects the growth of trees is also uncertain, as are warming effects on the distribution and abundance of various species.

What can be done to reduce wildfires?

While wildfires can be natural drivers of rejuvenation in forest landscapes, the frequency and intensity of fires as the global climate warms can sterilize soils and destroy a forest’s ability to recover. To limit the damage from such destructive fires, many forest managers say it’s critical to thin dying and dead timber from forests. But in regions such as the American West, the scale of the problem is so vast and affected areas so remote that it’s inevitable that large tracts of wilderness, increasingly blanketed by fallen timber, will eventually burn. For now, the focus is on thinning forests where they border more heavily populated areas.

Tom Toro is a cartoonist and writer who has published over 200 cartoons in The New Yorker since 2010.

Bruce Lieberman, a long-time journalist, has covered climate change science, policy, and politics for nearly two decades. A newspaper reporter for 20 years, Bruce worked for The San Diego Union-Tribune...