The unknown impacts of fires and how they can change climate over the long term

Devastating images of the destruction caused forest fire — from charred homes to cities shrouded in deadly smog — are fast becoming the epitome of the climate catastrophe unleashing on the world.

In Hawaii, the death toll continues to rise more than a century after the worst US wildfire in Maui this August. In Canada, extreme fires burning across the country are more widespread than at any time on record.

Research shows that the probability of fires and their severity have already increased due to global warming. Caused by humans.

But there are still many things we don’t know about these powerful phenomena. For example, the ability of wildfires to change weather patterns long after their flames are out.

One of the most long-lasting effects of fire on climate is its ability to release large amounts of carbon stored in trees and soil into the atmosphere.

In a vicious cycle, the extra CO2 contributes to the same long-term warming of the planet, fueling fires.

By 2020 alone, California wildfires are estimated to cancel out the effect of 16 years of government reductions in greenhouse gas emissions.

Forests may regrow, but not fast enough to help keep global warming below the 1.5°C limit, researchers suggest.

However, not all wildfire effects on climate are long-lasting. Not all of them produce heat.

Smoke blocks sunlight and reflects it back into space, causing cooling in the lower atmosphere.

This cooling effect usually only lasts until the rain smoke particles return to Earth. However, as wildfires increase in size, even these temporary impacts expand in scope and duration.

For example, the 2019-2020 fire season in Australia produced widespread smoke-induced cooling, which the research suggests may have played a role in the unusually prolonged La Niña.

Therefore, understanding how the various effects of wildfires interact is key to understanding their overall impact on climate and guiding humanity’s efforts to limit dangerous climate change.

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Calculating the net effect of wildfires on temperature requires consideration of their impact on different timescales and states of the atmosphere., upwards from the surface. One line of research focuses on stratospheric reactions occurring between 6 and 50 km altitude.

Below this level, the lower troposphere warms due to rising CO2 levels. But at the same time, the stratosphere cools, where the thin air allows carbon dioxide to release its energy into space.

Pyrocumulonimbus clouds and stratospheric warming

Until recently, it was believed that only volcanoes or nuclear explosions were powerful enough to disrupt this cooling process.

But when large wildfires meet the right weather conditions, they can create vast, dirty storms that darken the sky, create erratic winds and tornadoes, and send plumes of smoke 5 to 9 miles above the surface. Known as Pyrocumulonimbus, these clouds release aerosols that can travel thousands of kilometers across the planet..

Matthias Stocker of the Wegener Center for Climate and Global Change at the University of Graz, Austria, explains that once airborne, the black carbon in these forest fire aerosols absorbs heat, causing them to rise and warm the surrounding stratosphere.

His research on the stratospheric impact of large wildfires shows that smoke from pyrocumulonimbus superwaves in Australia in 2019 and 2020 caused strong warming (up to 10°C/18°F) during early stratospheric growth. In the following months, the aerosols warmed by an average of 3.5°C (6.3°F) before sinking again.

For Canada, it was the busiest year for pyrocumulonimbus in the past decade, said meteorologist David A. Peterson says he’s trying to develop a system to predict the movement of smoke. From the said clouds, it has been building a global database since 2013.

“Since the beginning of May, at least 133 pyrocumulonimbus have been observed in Canada and 153 pyrocumulonimbus globally,” it added, more than double the country’s previous seasonal high.

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However, none of the many pyrocumulonimbus events seen in 2023 rival the stratospheric impact in Australia in 2019 and 2020 or Canada in 2017, Peterson says. These produced stratospheric plumes that “rivaled or exceeded the impact of most volcanic eruptions of the past decade,” persisting at high altitudes for months.

The models clearly show that conditions for wildfires (which produce pyrocumulonimbus) will increase.That means the effects of these aerosols “can be significant enough to change the dynamics in the stratosphere and have consequences,” Stocker says.

It is worth noting that recovery of the ozone layer, which blocks harmful UV radiation, is slow and research has already shown some negative effects.

However, it is still unclear exactly how stratospheric warming might affect climate, which occurs almost entirely in the troposphere.

Changes in stratospheric heating alter air circulation, explains Karen Rosenloff of the National Oceanic and Atmospheric Administration’s (NOAA) Chemical Sciences Laboratory.

“Changing winds can change how waves propagate in the stratosphere, and that drives responses in the troposphere.” But the impact on surface weather “depends on the distribution of warming in the stratosphere, so it’s not really possible to generalize,” he says.

“The biggest thing we’ve learned already is that wildfires can be important for many effects on the stratosphere,” Stocker says.

“It’s a big test. And in my opinion, I don’t want to prove changes. Researchers have already seen that harmful effects can occur.”

Effects on the surface

Wildfires can also affect surface weather.

One mechanism involves changes in landscapes’ albedo, or ability to reflect light.

Following fire, burned surfaces can reduce albedo and increase surface temperature. Conversely, reduced canopy can increase albedo by exposing more reflective features such as grass or snow, which can have a cooling effect.

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Another process is evaporation of water. Plants release water from their leaves in a process called transpiration, and water evaporates directly from the soil and from the tops of trees. As a result, the surrounding air cools down. But when wildfires suppress this, heat increases.

A 2019 study examining the interaction of these factors found Average surface temperatures are likely to be warmer for at least five years after the flames are extinguished.

The main reason for this is reduced sweating, says Zhihua Liu, an ecologist at the University of Montana and lead author of the study.

“If more frequent and severe fires occur in the future, this warming of the landscape could contribute to climate warming,” he adds.

“However, the interactions between climate warming, vegetation dynamics and fire are complex and still not fully understood.”

Because wildfires affect the climate system in multiple ways, understanding the different interactions and time scales is essential to understanding their long-term global impact. “We need to know the net results to know how quickly we need to reduce human CO2 emissions,” Stocker explains.

It has been suggested that solar geoengineering, which simulates the cooling effects of smoke from volcanoes and forest fires, can help combat some of the worst effects of climate change by artificially injecting aerosols into the stratosphere.

Stocker says it’s unwise to do so when we can’t yet fully model the many effects of wildfires.

“At this point, we don’t know what geoengineering will do. [Estudiar los incendios forestales] It helps us understand what changes there might be, but we don’t attempt a large geoengineering scenario.”

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BBC-NEWS-SRC:, Import Date: 2023-08-19 00:00:11

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