How Canada’s wildfires are warming the stratosphere

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.

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-sales 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.

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.