Study finds individual extreme forest fires can lead to global impacts

The radiation effects of smoke from individual extreme forest fires can apparently lead to global consequences that affect the energy balance of the atmosphere and thus the global climate in a complex way.

This is the conclusion drawn by a team from the Leibniz Institute for Tropospheric Research (TROPOS) from an analysis of the 2019/20 extreme bushfires in Australia using simulations with a global aerosol climate model. The simulated effects of the smoke led to a temperature increase of several degrees Celsius in the upper atmosphere and to a weakening of the circulation in the lower stratosphere.

As a result of further adjustment mechanisms, there has been a decrease in relative humidity of approximately 0.2 percent and precipitation of a similar magnitude. A global impact of the Australian fires on our weather and climate cannot therefore be ruled out, the researchers write Atmospheric chemistry and physics.

The Black Summer bushfires in Australia at the turn of the year 2019/2020 were extraordinary: extreme pyroconvective smoke clouds (pyroCb) transported unprecedented amounts of smoke into the lower stratosphere. Between December 29, 2019 and January 4, 2020, several such events transported between 0.3 and 2 million tons of smoke particles to heights of 12 to 14 kilometers.

The smoke layer spread over a large area of ​​the Southern Hemisphere and could still be detected in the stratosphere by lidar measurements two years later. The intensity of the Australian fires was comparable to that of the last major volcanic eruptions, and the emitted smoke even affected the Earth’s climate: smoke particles heat the atmosphere significantly more due to the soot they contain, unlike volcanic aerosol, which mainly reflected sunlight.

This absorption effect was intensively investigated in a previous TROPOS study and a direct warming of the entire Southern Hemisphere of up to +0.5 watts per square meter was estimated. Even this direct warming effect, which seems relatively simple, has been the subject of controversial debate among experts. However, it becomes much more complex when you look at the chain of adjustment mechanisms that our Earth’s atmosphere has subsequently undergone as a result of the effects of Australian smoke.

To investigate this mechanism in more detail, the current TROPOS study built on the previous year’s work and used the global aerosol climate model ECHAM-HAM to comprehensively quantify the effects of forest fire particles. Using computer simulations, it could be shown that the absorption of sunlight by the Australian smoke led to a relevant temperature increase of the upper troposphere and the lower stratosphere by several degrees Celsius.

Excitingly, this not only happened locally in the Southern Hemisphere, but the positive temperature anomaly also extended across the tropics towards the Northern Hemisphere over the course of a few months. After analyzing the simulation data, this interhemispheric coupling could be attributed to changes in the global circulation, in particular to the weakening of the Southern Hemisphere upwelling circulation branch.

In the current study, interesting evidence was found for a feedback effect of the described stratospheric changes on our weather systems. For example, the warming of the upper troposphere layers in the model led to a decrease in relative humidity and thus to a decrease in cirrus clouds. In addition, interrelationships were identified that suggest that a decrease in global precipitation of approximately 0.2% is within the realm of possibility due to the effect of Australian smoke.

“The exact consequences of extreme fires on our weather and climate are difficult to estimate due to the existing uncertainties. Our study is also affected by these uncertainties. Nevertheless, we were able to find that the Australian fires, which were a minor source in relation to effects is extremely uncertain. ” says Dr. Fabian Senf from TROPOS.

The TROPOS research has shown that smoke from large forest fires can lead to changes in global circulation. It therefore joins several international studies that show evidence of large-scale changes. For example, in 2022, British researchers led by Prof. Jim M. Haywood of the University of Exeter warned about the risks of artificially introducing absorbing particles into the stratosphere.

This could strengthen the positive phase of the North Atlantic Oscillation in winter, which is associated with floods in northern Europe and droughts in southern Europe. The far-reaching consequences of smoke in the stratosphere and its potential impact on the troposphere are not yet clear.

“As rapid climate change increases the risk and intensity of forest fires, there is an urgent need to improve the description of extreme fires and their impacts in global climate models,” says Prof. Ina Tegen from TROPOS. To this end, it is necessary to better understand the atmospheric conditions that lead to extreme pyro-convective smoke clouds and the characteristics of the emitted smoke particles in order to adequately account for these possible interactions in climate projections.