By Emily Jack-Scott, Aspen Global Change Institute and Sarah Spengeman, Energy Innovation.

Something is clearly different about the wildfires roaring in the western United States. Over the last decade, we have increasingly used terms like “megafires” and “gigafires” (fires that burn more than a hundred thousand acres or more than a million) to describe them. They’re staggering, not just in size, but also in severity. We’re facing a new reality of increasingly frequent, more intense wildfires, dense wildfire smoke, and greater loss of property and life. Fire seasons are lasting longer, and lifelong firefighters are burning out.

Fires have a long and useful place in western ecosystems, but the fires of the last couple decades are a new beast. A perfect storm is playing out — the result of accelerating climate change, long-term fire suppression initiated by Euro-American colonists, and more people than ever building their homes in fire-prone forests.

The science is now forecasting these larger, more intense fires will likely transform entire forest ecosystems, if we do not act quickly to cut greenhouse gas emissions. For those of us living in the West — breathing thick smoke for weeks on end, watching neighbors and loved ones troop off for months to battle blazes — enacting solutions now is also deeply personal.

Flames of the 2018 Lake Christine fire climb up “ladder fuels” into the crowns of dense forests atop Basalt Mountain in Colorado. This fire necessitated the evacuation of many residents of Basalt, CO, and the Aspen Global Change Institute office. Photo credit: Emily Jack-Scott.

The latest scientific research confirms that we must be prepared for a dramatically altered landscapes in the coming decades. Scientists have found even small changes in fire behavior can have widespread impacts, converting forests into non-forests over massive swaths of the West. This new pattern of wildfires could result in the fire-driven conversion of 30 to 50 percent of most western conifer-dominated forests into shrublands, grasslands, or a different forest type altogether before 2100.

Historically, forests evolved to tolerate less severe fires. Because fires burned more regularly, forests were often more open, and plants on the ground were less likely to grow tall enough to serve as “ladder fuels” (ladders for fires on the ground to climb up into the crowns of trees, escalating the fire). Even if smaller, younger trees died in those fires, larger older trees could withstand the ground fires and provide seeds for new trees to grow amidst the grasses and shrubs that often sprout up after a fire.

Top photo: View from atop Slate Peak in northeastern Washington, looking southwest, 1934, George Clisby photograph, National Archives, Seattle, Washington, USA. The 1934 panoramic view shows extensive evidence of prior wildfires, varied age classes of cold forest, and recently burned and recovering areas. In the same view nearly eight decades later (bottom photo, 2013, John Marshall Photography), note the complete absence of recent fire evidence, widespread ingrowth creating denser forests, loss of nonforest, and lack of forest successional heterogeneity. Source: Hessburg et al. 2021.

But new, higher severity fires, in denser forests, over much larger areas, could potentially wipe out many forests. Fires now climb more easily up into the crowns of mature trees and are burning too hot for even thick-barked mature trees to withstand. And because our forests have lost a great deal of variety in ages and structures of trees, megafires are scorching more uniformly over broader swaths of land. These new megafires are leaving behind tens, or even hundreds of thousands of acres scorched so badly there are not enough seed trees to contribute to the usual regrowth that would have historically regenerated after fires in the West.

Even when seed trees survive, the actual soil can be so scorched by hotter fires that their life-giving nutrients are vaporized, and instead of absorbing water the soils actually repel it. This renders the area far more prone to erosion, leaving even the most resilient of landscapes barren and dry after high severity fires. In these cases, it can take decades to return to conditions hospitable enough to support the natural regrowth of forests. And that’s assuming a more hospitable climate than the one we’re now living in.

Climate change is creating hotter and drier conditions that stop forest recovery. Seeds have a harder time germinating and growing to adult trees, and more frequent fires are sweeping through areas still in early recovery stages after the last burn.

The new reality of megafires is resulting in what ecologists refer to as conversion — a transformation of one kind of eco-system into another, with different plant and animal species, and totally different ecologies. The conversion of a forest to grassland, shrubland, or another kind of forested area, can happen because of one single large, high-severity fire, or as the result of post-fire conditions that make the burned area more likely to burn again before the forest has had a chance to regrow.

Processes that may give rise to fire-driven forest conversion. (a) Conversion is initiated by processes that result in extensive areas of adult tree mortality (the solid arrows; red represents fire, and yellow represents climate). (b) Conversion is maintained by processes that impede regeneration of pre-fire tree species (dashed red and yellow arrows) and protract vegetation change temporally. © The duration of forest conversion may be further influenced by positive and negative fire–vegetation feedbacks (dashed purple arrows), which respectively promote or inhibit additional burning. Source: Coop et al. 2020.

When dense forests burn, they leave behind partially burned dead and dying trees that can still burn in another fire. These secondary, high-severity fires can actually burn hotter than the first fire, killing off seedlings or young trees and reinforcing the forest’s conversion into a new ecosystem. This is especially true at the edges of forests, where trees are often already at the limits of tolerable growing conditions.

So what can be done? There can be no doubt that policy is urgently needed to rapidly reduce the greenhouse gas emissions that are driving climate change and creating the conditions for intensifying fires. But even as we work to cut emissions, forest managers have options to better protect our forests. They will be forced to make hard choices about which areas to protect (such as municipal watersheds or near residential areas), while allowing the conversion of other locations, given limited resources and the scale of the crisis.

Researchers who study forest management strategies are making the case that current forest management strategies will no longer be sufficient in a climate-changed world. New practices must be implemented, applying the best available science, including Indigenous knowledge. Depending on the forest type, this may translate into selective logging, prescribed burning, or managed wildfires in order to replicate the effects of a low-severity fire. Scientists point out that under current conditions, we are pouring ever more resources into the suppression of mega-fires and are stuck in reactionary mode, whereas these proactive approaches could drive “more benign fire behavior.”

Forest management is entering a new era. The heat is on to counteract the massive threats to forests we’re now watching unfold, threatening our homes and our health. Policymakers must accept that forest management must look different given new climate conditions. The alternative of nonaction will mean we lose much of our beautiful, cooling, and carbon-absorbing forests that, until now, have endured for thousands of years.

Energy Innovation: Policy and Technology LLC is a nonpartisan energy and environmental think tank, providing original climate policy analysis to policymakers.