Millions of hectares of forest have been hit by some of the worst wildfires in Australian history. The rains that began helped to curb the elements. Scientists believe that clouds of smoke from burning forests reach the stratosphere, spread across the globe and can cause global climate change.
Hot summer 2019
Since the beginning of the 20th century, the air temperature in Australia has increased by about one degree Celsius. It gets warmer only in the Arctic. But the southern continent, in principle, has a hot climate, droughts often occur there, and annual forest fires are the norm.
Simulation of the situation shows that the hotter the climate, the more frequent and stronger natural fires around the world. Australia is at risk. The current disaster came after a record hot year, prolonged heat waves, droughts, and a lack of rainfall.
This time everything was much worse than in 2009, when the air in Melbourne heated up to 46.4 degrees. That Sunday - February 7 (in the Southern Hemisphere at this time of summer) - was called black: the city was enveloped in dense smog.
The current fires have destroyed hundreds of households, destroyed entire populations of wild animals, koalas have lost part of their original habitats, and there are human casualties. The economic damage from the disaster is enormous, especially when you consider the indirect consequences for human health and the environment.
According to scientists at Monash University in Melbourne, during forest fires, the risk of exacerbations of lung diseases, asthma, and respiratory tract infections increases. Perhaps there is a connection with an increase in the number of cardiovascular and eye diseases, mental disorders, complications during childbirth. The direct danger comes from inhaling ultrafine soot particles of 2, 5 and ten micrometers in size, which have not been effectively protected against.
Fire in the Bunyip Sate forest in Australia. 7 February 2009
Heated stratosphere
Air from the lower atmosphere does not penetrate into the stratosphere due to the tropopause, where the temperature gradient is too steep. Only during strong volcanic eruptions, the ash column is able to overcome the barrier and reach the lower stratosphere at an altitude of 11 to 25 kilometers. There, the particles are mixed and carried around the globe.
For a long time, it was believed that soot from forest fires does not pass through the tropopause and circulates somewhere low, near the source of the emission. At the end of the 20th century, enough facts have accumulated to refute this point of view. One of them is the 2009 Australia fires.
Scientists from the University of La Trobe (Melbourne), analyzing data from the Swedish satellite "One", showed that the plume of smoke rose to a height of 17-19 kilometers a week after the start of the fires. Combustion products circulated around the planet at the latitude of the tropics for six weeks.
Scientists explain this phenomenon by the fact that forest fires are an open fire, from where, like from a fire, a huge mass of smoke rises upward. There it cools down and, rising even higher, forms a cloud - it is called pyrocumulative (PyroCb). It continues to gain height due to internal convective currents and turns into a thundercloud. At the same time, it may not rain, and lightning causes new fires.
Particularly powerful pyrocumulative clouds can penetrate the stratosphere and travel throughout the hemisphere. For example, scientists from Tomsk discovered over Siberia traces of forest fires that raged in Canada in June 1991. In this they were helped by data from lidars - laser devices that analyze the composition of air. It is curious that the soot clouds from these fires reached the stratosphere of Europe even before the aerosols from the eruption of Mount Pinatubo in the Philippines. By the way, its eruptive column (a column of pyroclastic material) reached 21 kilometers.
In 2002, the authors of the article write, 17 pyrocumulative clouds from fires were observed in the United States, some of which reached the lower stratosphere.
The transformation of a pyrocumulative cloud from a forest fire into a thundercloud
The legacy of a nuclear war
Pyrocumulative clouds introduce an element of uncertainty into modeling the planet's climate. In this regard, scientists from the United States propose to resort to computer simulations designed to describe a nuclear war.
For example, they took the forest fires on August 12, 2017 in North America. According to satellite data, on that day, a plume of smoke rose 12 kilometers, and in a couple of weeks an aerosol layer of combustion products was found already at an altitude of 23 kilometers. In the following months, smoke spread across the stratosphere throughout the Northern Hemisphere.
Soot is composed of organic and black carbon - residues from incomplete combustion of biomass. These particles are capable of absorbing solar radiation and heating the air. The more black carbon in the cloud, the higher and faster it rises, the slower it is removed from the stratosphere.
Simulations of urban fires caused by nuclear explosions show that black carbon heats the air, carries with it large amounts of water vapor, and destroys ozone in the stratosphere. Severe forest fires seem to trigger the same mechanism. In any case, in August 2017, local anomalies of ozone and water vapor were found in the stratosphere. Scientists associate this with burning forests.
It is possible that soot from forest fires, being in the stratosphere for eight or more months, globally affects the climate. On the one hand, it can cool the air by scattering solar radiation, as is the case with volcanic eruptions. On the other hand, a huge amount of CO2, the strongest greenhouse gas, enters the atmosphere. Of course, it enters the natural carbon cycle on the planet and over time will be absorbed again by the biomass from which it was taken. However, there are indications that this natural balance may be wobbling, and too many fires, droughts, and lack of rain around the world will mean that forests will not be able to grow as quickly as they used to. And the excess of greenhouse gases in the atmosphere increases its heating.
Fires destroy forests in all parts of the world every year. It has now been proven that the strongest of them are capable of delivering soot clouds to the lower part of the stratosphere at an altitude of more than 11 kilometers. This could affect the climate of the entire Earth.