Trotz Klimawandel: Zahl der weltweiten Landschaftsbrände drastisch gesunken

Im August 2016 versuchte Anne-Sophie Brändlin in einem Beitrag der Deutschen Welle einen Zusammenhang zwischen dem Klimawandel und der weltweiten Waldbrandgefahr herszustellen:

Wie der Klimawandel Flächenbrände anfacht
Haben Flächenbrände in den letzten Jahren weltweit zugenommen? Wenn ja, ist der Klimawandel schuld? Die Forschung bringt Licht ins Dunkel – und gibt auch Antworten darauf, welche Regionen am verwundbarsten sind. [...] Die Anzahl an Flächenbränden hat in den letzten Jahren anscheinend zugenommen. Oder kommt uns das nur so vor, weil uns solche schlechte Nachrichten heute öfter erreichen? [...] Wissenschaftler legen nahe, dass sich in den letzten Jahrzehnten die Zahl der Flächenbrände tatsächlich erhöht hat, vor allem im Westen der USA. Der “Union of Concerned Scientists” (UCS) zufolge haben die westlichen US-Bundesstaaten in den letzten Jahrzehnten einen Anstieg der durchschnittlichen Zahl großer Vegetationsbrände erfahren. Ausführliche Studien haben herausgefunden, dass solche Flächenbrände seit den 1970er und 80er Jahren fast fünf Mal häufiger auftraten. [...] Laut Funk sind nicht nur die Wälder der USA durch zunehmende Flächenbrände gefährdet – der Trend sei, dass Flächenbrände immer mehr Gebiete auf der ganzen Welt betreffen. “In den vergangenen Jahren gab es große Brände in Sibirien und an anderen Orten auf der ganzen Welt, wo es normalerweise keine großflächigen Brände gibt”, sagt er. [...] Wälder, die in Zukunft immer häufiger von Feuer und Klimawandel betroffen sein werden, und deswegen die verwundbarsten sind, befinden sich in der borealen Zone. Diese zieht sich über die nördliche Hemisphäre durch Alaska, Kanada, Skandinavien und Russland.

Eine Statistik der US-Waldbrände gibt es hier. Ein deutlicher Trend lässt sich nicht ausmachen. Überhaupt scheint der Klimawandel nur an einem sehr geringen Teil der Brände beteiligt zu sein, wenn überhaupt. Siehe “Neue Studie der George Washington University: Modelle zu kalifornischen Waldbränden überschätzen Rolle des Klimawandels“.

Spulen wir ein Jahr vor. Am 29. Juni 2017 veröffentlichte das Goddard Space Flight Center der NASA eine Pressemitteilung, die Anne-Sophie Brändlin sowie Anhänger der Klimaalarmlinie aufgeschreckt haben wird: Die Zahl der Landschaftsbrände weltweit hat doch tatsächlich – trotz Klimawandel – abgenommen!

NASA Detects Drop in Global Fires

Shifting livelihoods across the tropical forest frontiers of South America, the Eurasian Steppe, and the savannas of Africa are altering landscapes and leading to a significant decline in the amount of land burned by fire each year, a trend that NASA satellites have detected from space.

The ongoing transition from nomadic cultures to settled lifestyles and intensifying agriculture has led to a steep drop not only in the use of fire on local lands, but in the prevalence of fire worldwide, researchers at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and colleagues found.

Globally, the total acreage burned by fires each year declined by 24 percent between 1998 and 2015, according to a new paper in Science that analyzes NASA’s satellite data, as well as population and socioeconomic information. The decline in burned lands was largest in savannas and grasslands, where fires are essential for maintaining healthy ecosystems and habitat conservation.

Across Africa, fires typically burn an area about half the size of the continental United States every year, said Niels Andela, a research scientist at Goddard and lead author on the paper. In traditional savanna cultures with common lands, people often set fires to keep grazing lands productive and free of shrubs. As many of these communities have shifted to cultivate more permanent fields and to build more houses, roads and villages, the use of fire declines. As economic development continues, the landscape becomes more fragmented, communities often enact legislation to control fires and the burned area declines even more.

By 2015, savanna fires in Africa had declined by 270,000 square miles (700,000 square kilometers) — an area the size of Texas.

“When land use intensifies on savannas, fire is used less and less as a tool,” Andela said. “As soon as people invest in houses, crops and livestock, they don’t want these fires close by anymore. The way of doing agriculture changes, the practices change, and fire  slowly disappears from the grassland landscape.”

Andela and an international team of scientists analyzed the fire data, derived from the Moderate Resolution Imaging Spectrometer (MODIS) instruments on NASA’s Terra and Aqua satellites, as well as other sources. They compared these datasets with trends in population, agriculture, livestock density and gross domestic product. 

The scientists found a different pattern in the rainforests and other humid regions close to the equator. Natural fires are rare in tropical forests, but as people settle an area they often burn to clear land for cropland and pastures. After the land is first cleared, as more people move into the area and increase the investments in agriculture, they set fewer fires and the burned area declines again. 

The impact of human-caused changes in savannas, grasslands and tropical forests is so large that it offsets much of the increased risk of fire caused by warming global temperatures, said Doug Morton, a research scientist at Goddard and a co-author of the study. Still, the impact of a warming and drying climate is seen at higher latitudes, where fire has increased in parts of Canada and the American west. Regions of China, India, Brazil and southern Africa also show an increase in burned area. But the expansiveness of savannas and grasslands puts the global trend in decline.

“Climate change has increased fire risk in many regions, but satellite burned area data show that human activity has effectively counterbalanced that climate risk, especially across the global tropics,” Morton said. “We’ve seen a substantial global decline over the satellite record, and the loss of fire has some really important implications for the Earth system.”

Fewer and smaller fires on the savanna favors trees and shrubs instead of open grasslands, altering habitat for the region’s iconic mammals, like elephants, rhinoceroses and lions.

“Humans are interrupting the ancient, natural cycle of burning and regrowth in these areas,” senior author Jim Randerson, a professor of Earth system science at the University of California, Irvine, said of the African savannas. “Fire had been instrumental for millennia in maintaining healthy savannas, keeping shrubs and trees at bay and eliminating dead vegetation.”

There are benefits to fewer fires as well. Regions with less fire also saw a drop in carbon monoxide emissions and an improvement in air quality during the peak of the fire season, confirming the burned area trends using data from other NASA satellites. With less fire, the vegetation in savannas is also able to build up — taking up carbon dioxide from the atmosphere, instead of releasing it into the atmosphere during fires. The 24 percent decline in burned area may have contributed about 7 percent to the ability of global vegetation to absorb the increase in carbon dioxide emissions from burning fossil fuels and land use change.

The decline in burned area from human activity raises some difficult questions, Morton said: “For fire-dependent ecosystems like savannas, the challenge is to balance the need for frequent burning to maintain habitat for large mammals and biodiversity, while reducing fire on the landscape to improve air quality and protect people’s property and agriculture.”

As these savannas and grasslands continue to develop and agriculture intensifies, however, the researchers expect the global decline in fires to continue. It’s a trend that should be incorporated into computer models that forecast climate and carbon dynamics, Morton said.

“The loss of fire from agricultural landscapes has a big impact on communities and ecosystems. Looking ahead, models that account for changes in fire activity from human management will help us understand the feedbacks from fewer fires on vegetation, air quality and climate,” he said.

For more information and to explore the data:

https://www.globalfiredata.org

To read the paper, visit: 

http://science.sciencemag.org/content/356/6345/1356