A recent study conducted by researchers at the Chinese Academy of Sciences and published in the journal Science of the Total Environment suggests that black carbon and dust play a crucial role in the melting of Tibetan Plateau glaciers—and the researchers think they know the sources of that troublesome sediment.
“We believe that black carbon, dust, and other light-absorbing impurities must be important factors in accelerating … ice melting worldwide,” Yang Li, a coauthor of the study, told GlacierHub. And, according to the study, East and South Asia are the largest sources of black carbon emissions that are transported to the Tibetan Plateau.
Black carbon, also called soot, is a byproduct of the partial combustion of organic matter and fossil fuels.
Susan Kaspari is an associate professor at Central Washington University and worked previously with Shichang Kang, another one of the study’s authors. “When you see emissions coming off the back of a truck that’s really black, you’re seeing the black carbon,” Kaspari told GlacierHub.
Along with fossil fuels, an important source of black carbon is the burning of biofuels, such as wood or animal waste, she added.
“[Black carbon] doesn’t stay in the atmosphere a really long time,” Kaspari said. “Usually it will stay in the atmosphere on the scale of a few days to at the most, maybe two weeks.”
Gravity and precipitation eventually pull the black carbon back to earth. And that’s where the trouble comes in for glaciers.
Scientists describe black carbon, along with dust, as a light-absorbing particle, meaning that due to its dark color it absorbs more energy from the sun compared to other light-colored materials—especially the typically bright-colored surfaces of glaciers. When black carbon settles on snow and ice, “It absorbs more energy from the sun, and then that warms the snowpack or ice, and leads to accelerated melt,” Kaspari said.
Kaspari and Kang, among others, published a study in 2011 that detailed how black carbon concentrations in the Tibetan Plateau have increased dramatically. “We documented a three-fold increase from preindustrial to industrial periods, starting around the 1970s, relative to, prior to that period of time,” Kaspari said.
Various anthropogenic activities contributed to this increase, including the Kuwait oil fires set by Iraqi forces during the 1991 Gulf War.
Li’s new study focused on the Laohugou Basin on the northern slope of the western Qilian Mountains, which lie on the Tibetan Plateau. These mountains lost 20.9 percent of their glacial area—about 22 cubic kilometers of ice—in the past 50 years, according to a study conducted last year.
Li and his co-researchers sampled the ice, snow, and nearby topsoil of the Laohugou Basin glacier during the summer and winter of 2016 and measured concentrations of black carbon and dust. To determine the effect of the black carbon and dust on the amount of energy absorbed by the glacier, they used SNICAR, a model for determining the albedo of snow and ice surface.
They found large spaciotemporal variability in the concentrations of black carbon and dust. Still, they concluded that the concentrations of black carbon and dust on the glacier were “comparable to or higher than” concentrations on most other Third Pole glaciers. The concentrations, though, were lower than those of some select glaciers of the Tibetan Plateau, specifically, including the Baishui No. 1 and Xiao Dongkemadi glaciers, which indicated, according to the study, “discrepancies in the deposition, enrichment, and re-exposure of [black carbon] over the Tibetan Plateau.”
Li and his coauthors found, however, that dust plays a more important role than black carbon in accelerating melting.
Susan Kaspari found a similar result in her 2014 study that measured black carbon and dust on the glacier ice and snow of Solukhumbu, Nepal.
“Let’s say you had a hundred parts per billion black carbon, which would be certainly enough black carbon to cause a change in how much energy is being absorbed,” she said. “If you put that on a snow pack that was quite clean, that black carbon could have a really large impact.”
“If you took that same amount of black carbon and it was deposited upon a snow pack that already had a lot of dust,” she added, “the efficacy, or how effective that black carbon would be in absorbing energy, would be a lot less because the dust is already absorbing some of that solar radiation that could otherwise be absorbed by the black carbon.”
The Tibetan Plateau is a region that is “naturally dusty already,” said Kaspari, who added that the rising temperatures brought about by climate change exacerbate the situation. “As the glaciers are retreating,” she said, “you’re exposing more and more area that used to be covered with glacier that has a lot of dust.”
And that dust, she added, gets blown onto glaciers.
Li and his coauthors found local topsoil to be a likely source of not only the glacier’s dust, but also its black carbon. Urban activities, such as automobile exhaust and industrial pollution, release black carbon that pollutes the soil, according to the study.
To reduce the amount of black carbon released into the environment, Kaspari suggested more efficient combustion methods, more efficient engines, and the elimination of coal-fired power plants.
Natural sources of black carbon, such as wildfires, are more difficult to mitigate. And there’s no feasible way to remove black carbon that’s already settled across the surface of the world’s glaciers.
Li told GlacierHub that the results of his study do not speak to the possible concentrations of black carbon in other glaciated regions of the Tibetan Plateau. “The concentrations of black carbon and dust in the Tibetan Plateau glaciers must vary broadly, because of the spatiotemporal variability in wet, dry, and post depositional conditions,” he said.
Still, along with other studies that research black carbon concentrations in other glaciers of the Tibetan Plateau, the work of Li and his coauthors adds to our evidence that human activity accelerates the melting of glaciers in Tibet and worldwide.
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