Surprisingly counterproductive
Fire in the forest is not bad per se, it is often an important part of the ecosystem. And sometimes the heated event turns into a surprisingly cool late episode
In the mid-1980s, US and Canadian researchers started some experiments to test a very special horror scenario. They intentionally set fires in some coniferous forests to simulate the consequences of a nuclear attack. The question: Does the nuclear apocalypse actually cause a nuclear winter – triggered by dust, soot residue and other aerosols that are being thrown up? According to the hypothesis, they darken the atmosphere across continents and block out sunlight, temperatures plummet, crop failures follow, the surviving part of humanity has to starve or freeze to death. As a result, the number of casu alties among the population should be even higher than through the war itself.
In fact, in the experiment, the daytime temperatures in the regions over which the ash clouds passed cooled by an average of between 1.7 and 7 degrees Celsius, while remaining stable at night. The scientists interpreted this result as partial confirmation of the nuclear winter thesis, after all they were only allowed to burn smaller areas for their measurements, while a war would affect a significantly larger area.
It is now old hat that aerosols in the atmosphere usually act like sun protection, reducing the radiation and thus cooling. They can come from volcanic eruptions like that of Mount Tambora in Indonesia, which caused the "Year Without a Summer" in North America and Europe in 1815, or from huge forest fires in Siberia's taiga or Canadian coniferous forests. However, if the soot settles over glaciers or covers other light-colored surfaces, it reduces the emissivity of these surfaces - the so-called albedo - and thus promotes their heating up like that of the surrounding air layers. At the same time, such large fires also release enormous amounts of carbon dioxide from the charring trees, causing climatologists and environmentalists to fear that they are further promoting climate change. In any case, average temperatures in arctic and boreal latitudes are rising faster than in other regions of the world, which is lengthening the fire season and encouraging additional fires.
The overall balance of this destruction for the climate is difficult to calculate, which probably spurred on scientists around James Randerson from the University of California in Irvine. They worked on the area of the Donnelly Flats fire in Alaska in June 1999, which consumed 7,600 hectares of black spruce forest at the time. Over the years, the group studied incoming and outgoing radiation, CO2 absorbed or emitted by the plants, calculated how much carbon dioxide and methane was released as a direct result of the fire, how temperatures changed and much more. They then compared the data with the results from a burn area twelve and eighty years older.
In the first few months after the event, the environment reacted as feared: Despite the initially dense concentration of aerosols, the environment warmed up, because in addition to the greenhouse gases released into the air in considerable quantities, there was also ground-level ozone, which retained heat. Soot and dark ashes soon settled on the ground, causing further heating during the summer. Even in distant polar regions, they polluted sea and glacier ice, reduced albedo, and stored more solar energy.
The situation reversed the following year, however, as fall and winter precipitation washed away or covered the dark particles. And it wasn't just the snow that brightened up the area in early spring, but also the absence of the trees and their needles. Now the reflection from the surface into space increased, which noticeably cooled the atmosphere near the ground - an effect that persisted even as the scorched area began to be reforested. Because the recovery of the vegetation begins in these latitudes with aspen and birch, whose lighter, broad foliage also has a higher albedo than the dark needles of the black spruce. They also shed their leaves in the fall, which in turn promotes snow cover on the ground.
A large part of the carbon dioxide originally released is now also absorbed again by the emerging trees, including spruce soon again, so that this balance balances out more and more over the years. However, according to the scientists' calculations, it will take around sixty years for a negative greenhouse gas value to become positive. But when, after about eighty years, a new, pure and dark black spruce stand takes over the area, the ecosystem stores a similar amount of carbon as before the last fire - the region then experiences a periodic, slight net cooling.
Do concrete instructions for action arise from these findings? Not necessarily: A return, for example, to the total fire suppression of the past so that less CO2 is released cannot be justified with these results. And it would have fatal consequences anyway, as the past has already shown. Foresters dreaded fires for a variety of reasons, so they fought them as best they could. To the detriment of the ecosystem, which depends on this disruption as the elixir of life: the heat converts poorly decomposable plant waste such as needles and makes the nutrients in them generally available again. In addition, many species have adapted so well to fire that they are only able to germinate after it appears – processes that have been prevented by the avoidance strategy. In their wake, fuel accumulated in the forests, resulting in even more devastating fire events.
But even the opposite policy would not necessarily bring the desired results. More frequent and widespread fires might cool the northern hemisphere in the future, but they would certainly not be in the interests of the timber industry. After all, poplar and birch wood does not have the quality of spruce, which usually grows straight – not to mention the questionable consequences for the ecosystem as a whole. Randerson and his colleagues are therefore proposing the usual way to curb climate change: save energy.
