Fires raged across the Amazon last summer, receiving worldwide media attention. In August 2019, there were 3 times as many active fires in the Brazilian Amazon as there were in August 2018—and more than in any August since 2010. This increase is largely attributed to land clearance for logging and farming, with almost 4 times more deforestation in July 2019 than the July average from 2016 to 2018.
As the biomass burned, it produced huge plumes of smoke that reached high into the atmosphere. In a new study, scientists show that black carbon particles from such smoke can fall on Andean glaciers to the west, causing them to melt faster. This sooty material, produced when biomass and other carbon-based fuels don’t fully combust, reduces the reflectiveness of ice and snow and increases the amount of heat they absorb from sunlight.
It is already known that black carbon from fossil fuel and biomass burning in the Northern Hemisphere has accelerated glacier melting in the Arctic, said Newton de Magalhães Neto of Rio de Janeiro State University in Brazil, who led the research. In South America, he explained, the “Amazon Basin, through biomass burning, is the biggest source of black carbon,” and it is close to the tropical Andean glaciers.
To assess the impact of Amazonian black carbon on the glaciers, the researchers studied Zongo Glacier in Bolivia, focusing on the years 2007 and 2010, when there were more fires than usual in the Amazon.
After identifying major sources of smoke plumes in the Amazon Basin during fires, the team modeled how particles like black carbon move through the atmosphere. The modeling showed that a predominant east-to-west air circulation in the tropical-equatorial region can transport aerosols from biomass burning in the Amazon to tropical Andean glaciers, mainly in Bolivia and Peru.
By comparing the atmospheric modeling with data from ice cores, the researchers estimated black carbon concentrations on Zongo Glacier. In agreement with the particle transport simulations, the ice cores showed a strong seasonality, with low black carbon levels occurring at Zongo during the wet season and high values during the Amazon fire season.
The researchers calculated that black carbon from Amazonian fires can increase glacier melting by 3%–4%. But their analyses also showed that the impact of black carbon depends on dust concentrations on the glacier because dust absorbs heat that otherwise would be absorbed by the black carbon. Using previously reported measurements, they estimated that low levels of dust (~10 parts per million) also increase melting by 3%–4%, whereas high dust concentrations (~100 parts per million) increase melting by about 11%. When both black carbon and dust are present, they can increase glacial melting by 6%–12%, depending on whether the dust levels are low or high.
“We found that for the tropical Andean glaciers, Amazon Basin biomass burning…contributes significantly to the melting process,” de Magalhães Neto said.
In 2010, for example, black carbon from Amazon fires increased the annual water runoff from the Zongo Glacier by 4.5%, the researchers found.
Societal Impacts for South America
The effect of black carbon on glacier melting might seem small, the authors noted in the study, but it is significant and should be accounted for in future models, particularly for years with high numbers of Amazon fires. They wrote that future climate change predictions point to a drier eastern Amazon, which would increase the risk of fires. And as global food demand is expected to increase deforestation in the Amazon, the use of slash-and-burn farming techniques that cause many fires in the region is also expected to increase.
Thorsten Seehaus, a glaciologist at the Friedrich-Alexander University Erlangen-Nürnberg in Germany, said that glaciers across the Andes are losing ice mass. “In Peru, for example, since 2000, about 30% of glacier surface area has been lost,” said Seehaus, who was not involved in the study. Because of the societal impact of Andean glacier melting, he said that analyzing the causes of glacial retreat is crucial. “It is important to see what the changes are and to improve predictions for the future, especially with regard to water resource management.”
In South America, glaciers are crucial water sources for millions of people, so their accelerated retreat is cause for concern. In this context, the study authors wrote that their results show that biomass burning in the Amazon increases the vulnerability of water supplies on a continental scale. The work was published in Scientific Reports.
This is “a much-needed study as we have long suspected that black carbon could play a significant role in driving melt on Andean glaciers,” said Mathias Vuille, a climate scientist at the University at Albany in New York who was not involved in the research. “We have seen this primarily in the Himalayas and to some extent also on Arctic glaciers and on Greenland,” Vuille said, “but there hasn’t been a lot of research done on whether there really is a link” between Amazon fires and Andean glacial melting.
Seehaus added that the work provides a good starting point for further research. He said that going forward, satellite data could be used to map seasonal changes in glacier albedo—a measure of how much light glaciers reflect—in different areas of Peru and Bolivia.
“One important next step for this work is expanding the analysis to other glaciers in the tropical Andes,” de Magalhães Neto said. “For this we will implement a complete monitoring program that accounts [for] glaciological, hydrological, meteorological, and atmospheric aerosol data.”
—Michael Allen (michael_h_allen@hotmail.com), Science Writer