Korey Carter
Physics today
February 2015
New field research suggests that on the Tibetan plateau the effect of black soot on glacial melting rivals, and may even surpass, the effect of greenhouse gas emissions.
The Tibetan plateau is the world’s highest and largest plateau covering an area about five times the size of Spain. Often referred to as the “Earth’s third pole,” the area includes the headwaters of multiple Asian rivers and also contains the world’s third largest repository of permanent ice.
Meltwater from both seasonal snowpack and the glacial fields of the Tibetan plateau replenishes more than half of the supply of the Indus, Yellow, and Brahmaputra rivers and up to two thirds of the summer flow of the Ganges.
Collectively, the waterways are the predominant fresh water source for more than a billion people.
Because its elevation exceeds 4500 m, the Tibetan plateau determines much of Asia’s weather—including the steering of westerly wind currents from the Mediterranean Sea and the brewing of summer monsoonal rains.
Over the past decade, scientists have observed drastic changes in the climate on the Tibetan plateau. In 2009 researchers noted that temperatures in the area had warmed by 0.3°C per decade, which is twice the rate observed elsewhere in Asia. Rising temperatures represent a particular threat to the nearly 46–000 glaciers on the Tibetan plateau because of the distinct possibility that without a change in the rate of global temperature increase many of the glaciers could vanish by the middle of the century.
Part of a glacier-capped mountain chain about 110 km west-northwest of the Tibetan capital, Lhasa. Taken in 2007 by the ASTER instrument aboard NASA’s Terra orbiter, this false-color image displays snow-free ground as reddish-brown, snow and ice as white, and water as navy blue and aquamarine. CREDIT: NASA Earth Observatory
As glaciers recede and eventually disappear, they release stored water, which provides a short-term increase to many of the major waterways on the Asian subcontinent. Over the long term, though, the results are far less encouraging—the loss of glacial headwater from the Tibetan plateau would mean a dramatic decline in dry season water availability (as the seasonal snowpack would have already melted) for nearly 20% of the world’s population.
Glacial retreat on the Tibetan plateau is driven to some extent by global warming, which results from increasing greenhouse gas emissions, but other factors are most likely involved, too. Black soot is an aerosol pollutant that warms the troposphere, and there is increasing evidence that black soot is one of the additional agents contributing to surface melting.
The soot is comprised of black carbon (BC) and organic carbon (OC), with the BC absorbing incoming solar radiation (across the visible spectrum) and the OC absorbing primarily in the UV portion of the spectrum. When black soot is deposited on snow in sufficient quantities, the effect is similar to what happens with dark surfaces such as pavement or black roofs in the summertime: The absorption of more sunlight makes the surface hotter than it would if it were exclusively a lighter color.
The sources of black soot on the Tibetan plateau include diesel engines, coal-fired power plants, and outdoor cooking stoves. As the plateau is located near regions in South and East Asia that have been (and are predicted to remain) the largest sources of black soot in the world, glacial melting that results from increased black soot concentrations is expected to remain a persistent problem.
Glaciers and snowpacks absorb solar radiation, yet their highly reflective surfaces allows for the snow to stay cool, thereby inhibiting melting. The glaciers and snowpacks of the Tibetan plateau are characterized by their high albedo—the fraction of solar energy reflected from the Earth back into space. Snowpack albedo decreases as surface temperatures increase, altering the morphology of the snow and making it coarser.
The upshot is that melting takes place more readily when the coarser snow crystals are exposed to the Sun. Black soot pollution also decreases albedo and researchers have found that only a very small concentration of soot (10 nanograms of soot per gram of snow) may significantly alter the albedo of a thick snow layer.
On the Tibetan plateau aerosol pollution is significant, especially at the lower elevations, where the snow is more likely (by one to two orders of magnitude) to be polluted than is the case for snow sampled from higher elevations. From ice core samples from locations across the plateau have it is clear that pollution is becoming an issue even at the higher elevations, where it has integrated into the snowpack.
Changes in albedo due to black soot deposition leads to the effect known as snow darkening. Researchers have found that snow darkening on the Tibetan plateau has decreased snow cover in both the western and eastern Himalayas by 1–8 days per year.
Once the snow cover begins to melt, a positive feedback loop occurs. Melting snow retains a portion of the black soot (and other pollutants). As more snow and ice melt, the concentration of black soot increases, since there is less snow in the layer. As a result, the altered composition of the snowpack raises melting rate on the plateau.
Thus far, modeling soot’s impact remains challenging, not least because absorption of solar radiation depends on many factors, including size and shape of the snow crystals and whether the soot is integrated within the snow crystals or externally mixed. Theoretical results based on the global climate are not in complete agreement with local observations.
The discrepancy is particularly wide for the Tibetan plateau due to its complex topography, which features high spatial variability and multiple atmospheric circulation and surface processes. Scientists have therefore begun initial analyses on the closely related question of black soot’s effect on regional climate in areas with extensive snow and sea ice. Results thus far suggest that a substantial fraction of regional warming in the past century is a result of black soot pollution, and this fraction is much larger than that of CO2 pollution.
Scenarios for the future climate are often dramatic and are usually made with the assumption that fossil fuels will continue to be burned for the foreseeable future. Reducing human-made climate-forcing agents that have a net warming effect on the environment, including black soot, is a necessity if we want to prevent the deleterious effect of glacier loss on the Tibetan plateau. A significant reduction in fresh water supplies for more than a billion people would represent a global climate and human health catastrophe.
Yet alternative scenarios do exist where coal emissions are phased out (over the next 2–3 decades) and unconventional fossil fuels, such as tar sands and shale oil, are not fully developed. Thus we face a choice: Permanently alter the environment with substantial consequences or adopt a more enlightened approach that reduces the human footprint on the Tibetan plateau and preserves these resources for the people who need them.
Korey Carter is a graduate student in the department of chemistry at the George Washington University in Washington, DC.
