The extreme wildfires sweeping across parts of North America, Europe and Siberia this year are not only wreaking local damage and sending choking smoke downwind. They are also affecting the climate itself in important ways that will long outlast their flames.
Wildfires emit carbon dioxide and other greenhouse gases that will continue to warm the planet well into the future. They damage forests that would otherwise remove CO2 from the air. And they inject soot and other aerosols into the atmosphere, with complex effects on warming and cooling.
To be sure, the leading cause of global warming remains overwhelmingly the burning of fossil fuels. That warming lengthens the fire season, drying and heating the forests. In turn, blazes like those scorching areas across the Northern Hemisphere this summer have a feedback effect—a vicious cycle when the results of warming produce yet more warming.
How Bad Is the Climate Feedback from Fires?
Although the exact quantities are difficult to calculate, scientists estimate that wildfires emitted about 8 billion tons of CO2 per year for the past 20 years. In 2017, total global CO2 emissions reached 32.5 billion tons, according to the International Energy Agency.
When they calculate total global CO2 output, scientists don't include all wildfire emissions as net emissions, though, because some of the CO2 is offset by renewed forest growth in the burned areas. As a result, they estimate that wildfires make up 5 to 10 percent of annual global CO2 emissions each year.
There have always been big wildfires, since long before humans began profoundly altering the climate by burning fossil fuels. Those historical emissions are part of the planet's natural carbon cycle. But human activities, including firefighting practices, are resulting in bigger, more intense fires, and their emissions could become a bigger contributor to global warming.
Extreme fires can release huge amounts of CO2 in a very short time. California fire experts estimate that the blazes that devastated Northern California's wine country in October 2017 emitted as much CO2 in one week as all of California's cars and trucks do over the course of a year. This year's fires have also been extreme; two of the state's largest fires on record are burning right now, including the Mendocino fire complex, which exceeded 400,000 acres this week.
According to NOAA scientist Pieter Tans, head of the carbon cycle greenhouse gases group with the Greenhouse Gas Reference Network, a very large, very hot fire destroying 500,000 acres could emit the same total amount to CO2 as six large coal-fired power plants in one year.
That suggests that California's wildfires in recent years may be releasing enough CO2 to endanger the state's progress toward meeting its greenhouse gas reduction targets.
While fires have been worsening in some regions, globally the total burned area and emissions from wildfires have actually decreased over the past 20 years, said Guido van der Werf, a Dutch researcher who analyzes trends for the Global Fire Emissions Database. The global decline is because burned savannas and rainforests in the tropics are being converted to agricultural lands, which are less fire-prone.
In regions of the world drying out with global warming, like the U.S. West and the Mediterranean, however, extreme fire seasons have increased in recent years.
"If we start to see a higher level of fire activity than in the past because of global warming, they become part of a climate feedback loop," van der Werf said. That means warming causes more fires, which causes more warming.
In addition to their CO2 emissions, wildfires can affect the climate in other important ways.
Dead Wood and Changes to the Land
Fires don't just burn up trees and shrubs and emit smoke. They leave behind long-lasting changes on the ground, and those changes also have effects on the climate.
Over the course of several decades after a big fire, emissions from decomposing dead wood often surpass by far the direct emissions from the fire itself. But at the same time, new growth in burned areas starts to once again take CO2 from the atmosphere and store it.
Fires also change the reflectivity of the land, called albedo. As burned forest areas start to regrow, lighter-colored patches of grasses and shrubs come in first, which, because they reflect more solar radiation, can have a cooling effect until the vegetation thickens and darkens again.
Scott Denning, an atmospheric scientist at Colorado State University, says site-specific studies show that the cooling effect in northern forests can last for decades. In a tropical rainforest, on the other hand, the dark canopy can regrow within a few years.
When new trees grow fast, they can start stashing away significant amounts of carbon quickly. But some recent research suggests that global warming is preventing forest regrowth after forest fires, including along the Front Range of Colorado and in the forests of the Sierra Nevada. If that emerges as a widespread trend in the coming decades, it means less forests available to take CO2 out of the atmosphere. Forests are estimated to absorb up to 30 percent of human greenhouse gas emissions.
Aerosols' Cooling and Warming Effects
Scientists can't say for certain whether the global level of fire activity in recent years is warming or cooling the atmosphere overall. Part of the reason that they don't have a definitive answer is because, along with CO2, wildfires also produce many other volatile organic particles called aerosols, including substances like black carbon and gases that form ozone.
One recent study suggests that wildfires emit three times more fine particle pollution than estimated by the Environmental Protection Agency. This pollution creates health problems, and scientists are also working to better understand its impact on the climate.
Some of those aerosols can make the atmosphere more reflective. They block sunlight, which cools the atmosphere, similar to the effect attributed to emissions from volcanic eruptions. In general, the climate effect of aerosols is short-lived, lasting from a few months to a couple of years.
But black carbon, an aerosol and short-lived climate pollutant, can actually absorb heat while floating around in the air, and that heats the atmosphere. Recent research shows that the heat-trapping potency—though it is short-lived—is much higher than previously thought, roughly two-thirds that of carbon dioxide, according to Alfred Wiedensohler, with the Leibniz Institute for Tropospheric Research.
Megafires may intensify these emissions and sends them higher into the atmosphere. A study published this week found that wildfires in Canada in 2017 resulted in extreme levels of aerosols over Europe, higher than those measured after the 1991 Mt. Pinatubo eruption.
An increase in megafires, driven at least partly by global warming, could change the wildfire carbon cycle, said Mark Parrington, a senior scientist with the European Centre for Medium-Range Weather Forecasts Copernicus Atmosphere Monitoring Service.
"In general, if we're seeing an increase in megafires, with direct injections (of pollutants) into the upper atmosphere, the effects can linger for weeks or months, and that could have more of a climate-cooling effect," he said.
More pieces to the wildfire-climate puzzle will fall into place later this summer after scientists evaluate data gathered by a C-130 airplane that's making daily cruises near Western U.S. wildfires to take detailed measurements of wildfire emissions. The mission is sponsored by the National Center for Atmospheric Research and the National Science Foundation.
With the explosion of wildfires in the region the past few decades, the data will help evaluate impacts to human health and the environment, including nutrient cycling, cloud formation and global warming, said University of Wyoming atmospheric scientist Shane Michael Murphy, one of the project researchers.
Wind-Blown Soot Can Affect the Ice Sheets
Eventually, the skies will clear once again, but all that smoke doesn't just magically disappear. The CO2 will heat the atmosphere for centuries; the methane for a few decades. Some of the aerosols and other particles are heavy enough to drift earthward, and others will wash to the ground with the first good rains of autumn or winter, but not before spreading out over the Northern Hemisphere's oceans and continents.
Those tiny remnants of burned plants can also affect the climate when they land on mountain glaciers and especially on the snow and ice in the Arctic. In some years, scientists have traced soot from wildfires in Canada to Greenland, where they darken the ice and snow and speed up melting. Wildfire pollution was a significant factor in the record surface melting of the Greenland ice sheet in 2012, said climate scientist Jason Box.
The overall effect of wildfire fallout on Arctic melting is difficult to quantify, partly because of sparse sampling across the remote area, and partly because of the great annual variations in wildfire emissions. But a growing body of research suggests that wildfire soot will contribute to accelerating the Arctic meltdown in the decades ahead, he said.
With wildfires burning farther north, emissions from wildfires in Greenland or Sweden could add significantly to the load of snow-darkening pollution in the Arctic because the sources are so close to the ice sheets. A 2016 study in Alaska estimated that risk of tundra fires will increase fourfold in the decades ahead.
Accounting for Wildfires' Climate Impact
Once the spark is lit, humans can't do much to change wildfires' greenhouse gas emissions. But it's still important to include them in the calculations for reaching the greenhouse gas reduction targets in the Paris climate agreement. With time running out to try and cap global warming at well-below 1.5 degrees Celsius, every ton of CO2 counts, and knowing how extreme wildfire seasons affect greenhouse gas emissions lets the world know how much it will have to cut emissions elsewhere.
Understanding how emissions form during wildfires could also help in the design of mitigation strategies to reduce their impact, said Christine Wiedinmyer, associate science director of the Cooperative Institute for Research In Environmental Sciences (CIRES) in Boulder, Colorado.
For example, a recent CIRES research project suggests that hotter, uncontrolled fires produce more harmful substances. Introducing controlled fires that generally are not as hot could help reduce emissions.
The current increase in extreme fires in some regions is part of a global ecosystem shift driven by human-caused global warming, Denning said. He warned that societies need to adopt strong policies to prevent huge regions of carbon-storing forests from being replaced by lower-carbon grasslands and shrubs.
Based on the best estimates of CO2 emissions from wildfires, Denning said they are dwarfed by emissions from the burning of coal, oil, and gas, and that's where the focus should be on reducing emissions.
"Without very strong climate policy, industrial emissions are likely to triple in this century. Against that backdrop, the climate effects of increased wildfires are smaller than the error bars in the climate effects of all that coal, oil, and gas," he said.