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In August 2014, a Lufthansa flight from Munich to San Francisco took off with an unusual cargo in tow.
During 22 passenger flights over the following 14 months, delicate sensors strapped to the bottom of the modified Airbus A340 aircraft took and analysed air samples from the upper atmosphere, looking for the presence of soot (also known as black carbon) from burning wildfires. The results, published in the journal PNAS, are alarming, particularly in light of the devastating blazes that have swept through northern California in recent weeks.
“Black carbon is the most strongly light-absorbing material from fires, and is regarded as one of the most important individual climate-warming agents,” says Yafang Cheng, one of the authors of the paper, from the Max Planck Institute for Chemistry in Mainz, Germany.
It has a dual effect, explains Thomas Smith, an assistant professor in environmental geography at LSE, who was not involved in the research. “Soot in the atmosphere is dark and will absorb both incoming sunlight and outgoing infrared energy emitted by the Earth,” he says. “This absorption will lead to warming.”
The flights helped researchers understand how black carbon particles released by wildfires move through the atmosphere, and the news isn’t good. Cheng and colleagues from China, the United States and Saudi Arabia discovered that black carbon particles from wildfires were being carried higher into the atmosphere than previously thought, even reaching the lower stratosphere, which starts at 10,000 metres above the Earth’s surface.
At that height, they have a much stronger warming effect – up to ten times stronger than at lower altitudes, according to Cheng. These particles can persist in the upper atmosphere for months, causing regional warming in the area of the wildfire, and potentially beyond.
“Wildfire-produced pollution can be transported thousands of miles away from the source, when weather patterns are favourable,” says Apostolos Voulgarakis, a senior lecturer in global climate and environmental change at Imperial College London, who was not involved in the research. “This is especially true for fires that produce emissions that are very buoyant and therefore travel in the upper parts of the troposphere or even the stratosphere, where lifetimes of pollutants are longer.”
It’s a complex picture that we don’t fully understand yet, and more research is required to figure out the exact contribution black carbon makes to global warming at higher altitudes, and whether these emissions can have a global impact. “To assess whether wildfires are ‘making climate change worse’ we would need to know whether the amount of black carbon released by wildfires has changed over time,” says Smith. “This is not the aim of the paper, and currently there is no global assessment of wildfire activity history.”
But black carbon isn’t the only way wildfires can influence climate change. As well as soot, burning plant matter also releases carbon dioxide. “It has been estimated that annual wildfire emissions of carbon are equivalent to around 25 per cent of emissions from fossil fuel combustion,” says Voulgarakis.
Large-scale fires also change what scientists call ‘albedo’ - the reflectiveness of the Earth’s surface. The blackened stumps of a post-fire forest absorb more light than the trees that were there before, and therefore contribute to warming. Fewer plants also means less carbon dioxide being drawn from the air by photosynthesis, so wildfires are bad news all round when it comes to climate change.
Individual events, even ones as large as the Camp Fire in California, are unlikely to contribute much on their own, however. “At any one time, there are many fires of a similar size burning across the planet; it’s just that this one has been close to populated places,” says Smith.
But, there’s increasing evidence that wildfires could be caused by climate change. A recent US report found that climate change could triple the frequency of large wildfires in the west of the country, while a study published in Nature concluded that a three degree rise in global temperature could cause a 100 per cent increase in wildfires in Mediterranean Europe.
“Climate change is certainly leading to conditions that favour more extreme fire behaviour,” says Smith. “In California, this climate change influence is manifested by earlier snowmelt, an earlier dry season, and later summers.” That means drier soils, drier vegetation, and a longer fire season with more intense burning and a faster spread of flames.
A grim picture is emerging. “Fires are part of a positive feedback loop,” Smith continues. “Warming should lead to more fires, which will in turn lead to more greenhouse gas emissions and black carbon emissions.”
Climate change can make wildfires worse, leading to more climate change, and more wildfires. It’s a vicious cycle, and according to Voulgarakis, there’s only one way out. “The only solution to this problem is to drastically reduce the human footprint on the global climate system.”
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This article was originally published by WIRED UK
