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One of the most far-reaching ways fires impact the climate is their ability to release vast quantities of carbon stored in trees and soils into the atmosphere. In a vicious feed-back loop, the additional CO2 then contributes to the same long-term warming of the planet that makes the fires themselves more likely. In 2020 alone, California's wildfires were estimated to have negated 16 years of the state's cuts to greenhouse gas emissions. Forest regrowth may occur, the researchers suggest, but not fast enough to help keep global warming under the 1.5C limit.
Not all of wildfires' impacts on climate are so long-lasting, however. Nor do all produce warming. By blocking sunlight and attracting additional water droplets that brighten clouds, smoke aerosols can reflect sunlight back into space, leading to localised cooling in the lower atmosphere.
This cooling effect typically only lasts until rain washes the aerosols back to earth. Yet as wildfires increase in scale, even these more temporary impacts are expanding their reach and duration. Australia's 2019-2020 fire season, for instance, produced a widespread smoke-induced cooling that may have influenced the recent "triple dip" in the La Niña weather pattern, research suggests.
Calculating the net warming or cooling effect of wildfires means considering their impact across various time-scales and levels of the atmosphere, from surface up. One avenue of research has thus focused on the stratospheric reactions that take place 4-31 miles (6-50km) up in the air.
Beneath this level, the lower troposphere is warming due to rising levels of CO2. Yet the same trend is also cooling the stratosphere, where thinner air allows the carbon dioxide to release its energy into space.
Until recently, it was thought only volcanoes or nuclear explosions were powerful enough to interrupt this cooling process by propelling smoke up into the stratosphere. But when large wildfires meet with the right meteorological conditions, they can produce vast dirty thunderstorms which darken the sky, create erratic winds and tornadoes, and inject large plumes of wildfire smoke five to nine miles (8-14km) above the surface. Known as pyrocumulonimbus clouds, or pyroCbs, these thunder-clouds release aerosols that can travel thousands of miles across the globe.
Once airborne, the black carbon in these wildfire aerosols absorb heat, causing them to rise and warm the surrounding stratosphere, says Matthias Stocker from the Wegener Center for Climate and Global Change at the University of Graz, Austria.
His research on large wildfires' stratospheric impact has shown that smoke from the pyroCb super outbreak in Australia in 2019-20 caused the stratosphere to warm very strongly (by up to 10C/18F) during the plumes' early development. Over the next few months, it remained an average of 3.5C (6.3F) warmer, before the aerosols sank back to earth.
Canada has this year seen by far its most active pyroCb year over the last decade, says David A Peterson, a meteorologist with the US Naval Research Laboratory in Washington DC, which is attempting to create a prediction system for the movement of pyroCb smoke, and has been building a global dataset since 2013.
"At least 133 pyroCbs have been observed in Canada since early May, with 153 observed worldwide," he adds – more than doubling the country's previous seasonal maximum. (Read about the latest science on whether climate change caused Canada's wildfires).
However, none of the many pyroCb events observed in 2023 rival the stratospheric impact of the 2019-20 Australia super outbreak, or the 2017 Pacific Northwest event in Canada, says Peterson. Both produced stratospheric smoke plumes that "rival or exceed the impact from the majority of volcanic eruptions over the past decade", he says – persisting at high altitudes for many months.
STRATOSPHERE VS TROPOSPHERE
Models clearly show that the conditions for pyroCb wildfires are set to increase, meaning there is the potential for the effects of such aerosols to become significant enough "to change dynamics in the stratosphere and have consequences," Stocker says.
One particular concern is that the recovery of the ozone layer, which blocks harmful ultraviolet radiation, could be delayed – and research has already demonstrated some negative impacts.
ALBEDO AND EVAPORATION
Wildfires can also influence climate back on the ground.
One mechanism involves changes to a landscapes' albedo, or ability to reflect light. In the aftermath of a fire, charred surfaces can reduce albedo, leading to an increase in surface warming. Conversely, a reduced forest canopy can raise albedo by exposing more reflective entities such as grass or snow, leading to a cooling effect.
Another process involves the evaporation of water. Thriving plants release water from their leaves in a process known transpiration, and water also evaporates directly from the soil and canopies. The surrounding air is cooled as a result. But when wildfires suppress this, warming increases.
A 2019 study which investigated the interplay of these factors found that the average surface temperature can warm for at least five years after flames are extinguished. Reduced transpiration was found to be the main cause of this, says Zhihua Liu, an ecology researcher at the University of Montana and lead author of the study.
"If there are more frequent and severe fires in the future, this land surface warming may contribute to climate warming," he adds. "However, the interactions among climate warming, vegetation dynamics, and fire are very complex, and yet to be fully understood."
