How Canada’s wildfires are warming the stratosphere

BBCfuture \ By India Bourke17th August 2023

Extreme wildfires are increasing due to rising emissions, but they also disrupt the climate in return. Weighing up the overall impact, however, is tricker than it seems.

Apocalyptic images of wildfire devastation – from charred homes to cities shrouded in deadly smoke – are fast coming to embody the world’s unfolding climate disaster.

In Hawaii this August, the death toll is still rising after the deadliest US wildfire in over a century ripped through Maui. In Canada, extreme fires blazing across the country are more widespread than at any other time on record.

Research has shown that wildfires’ likelihood and intensity have already increased due to human-caused global temperature rise. But there is still so much we don’t yet understand about these powerful phenomena. Not least, wildfires’ own ability to alter and disrupt climate systems long after their flames die out.

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.

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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.

Understanding how wildfires’ various impacts interact is therefore key to understanding their overall impact on the climate – and thus to guiding humanity’s attempts to limit dangerous climate change.

Super outbreaks

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.

In Canada, extreme fires blazing across the country are more widespread than at any other time on record (Credit: BC Wildfire Service/Getty Images)

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.

The big thing we’ve already learnt is that wildfires can be important for several effects in the stratosphere – Matthias Stocker

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. (Read more about what happened to the world’s ozone hole).

Charred surfaces after wildfires can reduce the albedo of landscapes, leading to an increase in warming on the surface (Credit: Getty Images)

Exactly how stratospheric warming could affect weather, almost all of which occurs in the troposphere, is not yet known, however. Changes in the heat of the stratosphere in turn changes the circulation of winds, says Karen Rosenlof of the National Oceanic and Atmospheric Administration (NOAA)’s Chemical Sciences Laboratory. “Changing the winds can change how waves propagate in the stratosphere, and that provides the feedbacks on the troposphere.” But the impact on surface weather is “going to depend on the distribution of the heating in the stratosphere, so it’s not really possible to generalise”, she says.

“The big thing we’ve already learnt is that wildfires can be important for several effects in the stratosphere,” says Stocker. “It’s a big experiment. And in my opinion, I don’t want to try out the changes. Researchers see there can already be harmful effects.”

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.”

‘It‘s a big experiment‘

With wildfires impacting the climate system in so many ways, understanding the different interactions and timescales is essential for understanding their overall impact in the long term. “We need to understand the net outcomes because we need to understand how fast to reduce our human CO2 emissions,” says Stocker.

There are suggestions that solar geoengineering, which would simulate the cooling effects of volcano and wildfire smoke, could help address some of climate change’s worst effects by artificially injecting aerosols into the stratosphere.

But doing so when we cannot yet fully model wildfires’ myriad impacts would be unwise, Stocker says. “At the moment, we don’t know what geoengineering would do. [Studying wildfires] helps us to understand what changes there could be – but hopefully we won’t try a big geoengineering scenario.”

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