Context

According to new research by scientists from NASA and Chinese Academy of Sciences, soot deposited on Tibetan glaciers has contributed significantly to retreat of the world’s largest non-polar ice masses – the Himalayan glaciers.

About

• According to research, black carbon deposits on Himalayan ice threaten earth’s "third pole". Tibet's glaciers are retreating at an alarming rate.
• The study: To better understand the role that black soot has on glaciers, researchers trekked high into the Himalayas to collect ice cores that contain a record of soot deposition that spans back to the 1950s.

• Researchers collected ice cores at five locations on the Tibetan Plateau.
• The amount of soot at site number five, Zuoqiupu glacier, increased by 30 percent between 1990 and 2003.
• Rise in soot levels at Zuoqiupu follows a dip that followed the enacting of clean air regulations in Europe in the 1970s.
• According to another study, black carbon concentrations near the Gangotri glacier rose 400 times in summer due to forest fires and stubble burning from agricultural waste and triggered glacial melting. 

• Techniques used in the study: Scientists use satellite instruments such as the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the NASA satellites Terra and Aqua to enhance understanding of the region's albedo.
  • A new NASA climate satellite called Glory will carry a new type of aerosol sensor that should be able to distinguish between aerosol types more accurately than previous instruments.

Black soot/Black carbon – details

• Black soot is generated from industrial pollution, traffic, outdoor fires, and household burning of coal and biomass fuels. 
• Soot absorbs incoming solar radiation and can speed glacial melting when deposited on snow in sufficient quantities.
• Soot includes black carbon, as well as organic carbon.
• Many industrial processes produce both black carbon and organic carbon, but often in different proportions.
• Burning diesel fuel produces mainly black carbon.
• Burning wood produces mainly organic carbon.
• Since black carbon is darker and absorbs more radiation, it’s thought to have a stronger warming effect than organic carbon.
• The fine particles absorb light and about a million times more energy than carbon dioxide.
• Black carbon results from incomplete combustion of fossil fuels and biomass.
• It is said to be the second-largest contributor to climate change after CO2. 
• Unlike CO2, which can stay in the atmosphere for years together, black carbon is short-lived and remains in the atmosphere only for days to weeks before it descends as rain or snow.
• India is the second-largest emitter of black carbon in the world, with emissions expected to increase dramatically in the coming decades. Indo Gangetic plains are said to be the largest contributor.

Warming of Tibetan Plateau

• Temperature increase: Temperatures on the Tibetan Plateau – sometimes called Earth's "third pole" – have warmed by 0.3°C (0.5°F) per decade over the past 30 years, about twice the rate of observed global temperature increases.
• The retreat of glaciers: Fifty percent of the glaciers were retreating from 1950 to 1980 in the Tibetan region; that rose to 95 percent in the early 21st century. 

• Some glaciers are retreating so quickly that they could disappear by mid-century if current trends continue.
• General plateau-type glaciers – mostly found in Tibet – are retreating faster than valley types.
• The number of glacial lakes at high altitudes is increasing. If this burst, they pose a danger.

• Black soot is the cause: Black soot is responsible for as much as half of the glacial melt, and greenhouse gases are responsible for the rest.
  • During the last 20 years, the black soot concentration increased two- to three-fold relative to its concentration in 1975.
• Reason for black carbon increase: Black carbon, which is caused by incomplete combustion, is especially prevalent in India and China. The main reason for the increase in black carbon in the region is accelerated economic activity in India and China over the last 20 years
  • Most soot in the region comes from shipping, diesel engines, coal-fired power plants, and outdoor cooking stoves.
  • According to one study,  black carbon emitted in India increased by 46 percent from 1990 to 2000 and by another 51 percent from 2000 to 2010.
• Mechanism of black soot related warming: Atmospheric aerosols are tiny particles containing nitrates, sulfates, carbon and other matter, and can influence the climate. Unlike other aerosols, black carbon absorbs sunlight, similar to greenhouse gases.
  • But unlike greenhouse gases, black carbon does not heat the surface; it warms only the atmosphere.
  • This warming is one of two ways in which black carbon melts snow and ice.
  • The second effect results from the deposition of the black carbon on a white surface, which produces an albedo effect that accelerates melting.   Dirty snow absorbs far more sunlight—and gets warmer faster—than pure white snow.
  • Black carbon can have a powerful effect on local atmospheric temperature. A small amount of black carbon tends to be more potent than the same mass of sulfate or other aerosols.

Importance of Himalayan glaciers

• Replenish Himalayan rivers: Himalayan glaciers help replenish many of Asia’s most important rivers – including the Indus, Ganges, Yellow, and the Brahmaputra – such losses can haven profound impact on billions of people who rely on these rivers for freshwater.

• One-quarter of the population of China is in western regions where glacial melt provides the main dry season water source.

• Seasonal freshwater supply: While rain and snow would still help replenish Asian rivers in the absence of glaciers, the change can hamper efforts to manage seasonal water resources by altering when freshwater supplies available in areas already prone to water shortages.
  • Reduced black soot emissions, in addition to reduced greenhouse gases, is required to avoid the demise of Himalayan glaciers and retain the benefits of glaciers for seasonal freshwater supplies.

Other issues caused by black carbon increase

• Affects precipitation:  Black carbon affects precipitation and is a major factor in triggering extreme weather in eastern India and Bangladesh, where cyclones, hurricanes and flooding are common. There is also a case of black carbon led droughts in northern China and extreme floods in southern China. Similarly, almost all glaciers in Nepal are in retreat.

• It contributes to the decrease in rainfall over central India.
• Because black carbon heats the atmosphere, it changes the local heating profile, which increases convection, one of the primary causes of precipitation
• While this results in more intense rainfall in some regions, it leads to less in other regions.