How the Australian fires turned the stratosphere upside down in 2020

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The injection of smoke into the stratosphere from the 2019-2020 Australian fires ranks as the largest ever observed by satellite. Also, researchers have shown that in addition to affecting the ozone layer, this phenomenon had induced a significant redistribution of energy between the stratosphere and the earth’s surface. Results recently published in the journal Geophysical research letters.

The worst fires in history

At an estimated cost of more than $ 1.3 billion, the wildfires that hit Australia between December 2019 and January 2020 were the most destructive in the country’s history. Visible from the International Space Station, they destroyed vast areas – including homes – and fed impressive plumes of smoke. The latter being sufficiently massive to inject dust into the stratosphere. The quantity involved amounts to nearly 1 million tonnes.

According to recent work, these injections caused a warming of the stratosphere by 1 to 2 ° C in the mid-latitudes of the southern hemisphere. A rise in temperature which persisted during the first six months of the year 2020. Indeed, the great stability of the stratosphere means that the injected dust is not easily evacuated. Also, their influence lasts over time, unlike those emitted in the troposphere – very quickly leached.

Pyrocumulonimbus captured in South New Wales on January 4, 2020. It is through these convective towers that the smoke penetrates the stratosphere. Credits: Michael thompson.

The determining role of carbon black

But why a warming? The answer lies in the composition of the plumes produced by forest fires. Observations show that their smoke is mainly composed of organic carbon and carbon black. However, the latter absorbs solar radiation very efficiently and thus causes a rise in temperature of the surrounding air. The counterpart of this increased absorption is surface cooling, resulting from the decrease in solar energy reaching the ground. With an estimated carbon black fraction of 2.5%, the model used by the researchers reports a radiative forcing of -0.03 W / m² at the top of the atmosphere. In other words, the two effects described above tend to offset each other with a resultant close to zero.

In addition, scientists show that dust has promoted the destruction of stratospheric ozone at the high latitudes of the southern hemisphere. A loss resulting from the enrichment of the fumes in sulfuric acid and their concomitant transport to the south pole. Thus, the dust acted in the manner of sulphated aerosols, that is to say as catalysts for the destruction of ozone. The calculations put the associated deficit between 4 and 6%, the equivalent of 10 to 20 Dobsonian units.


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