Atmospheric Brown Cloud Impacts
on Glaciers, Agriculture & the Monsoon
Source: United Nations Environment Program
Cities from Beijing to New Delhi are getting darker, glaciers in ranges like the Himalayas are melting faster and weather systems becoming more extreme, due to the combined effects of man-made Atmospheric Brown Clouds (ABCs) and greenhouse gases in the atmosphere. These are among the conclusions of UNEP scientists studying the 3 km-thick atmospheric layer of soot and other man-made particles that stretches from the Arabian Peninsula to China and the western Pacific Ocean.
Brown clouds result from burning of fossil fuels and biomass. They are aggravating the impacts of greenhouse gas-induced climate change because they lead to the formation of black carbon and soot particles that absorb sunlight and heat the air—as well as ozone that enhances the greenhouse effect of carbon dioxide. Paradoxically, brown clouds also happen to mask warming impacts of climate change in some cases by 20%-80%, because they contain sulphates and some organics which reflect sunlight and cool the surface. ABCs impact both air quality and agriculture in Asia, increasing risks to health and food production for three billion people. This is in part because of Asia’s already highly variable climate—the formation of an annual Monsoon, the region’s massive growth and because it is home to around half the world's population.
UNEP Regional Assessment Report, with Focus on Asia
Five regional hotspots for ABCs have been identified. These are: East Asia (eastern China); the Indo-Gangetic plains in South Asia; Southeast Asia (Cambodia, Indonesia, Thailand, and Vietnam); From sub-Saharan Africa into Angola, Zambia and Zimbabwe; the Amazon Basin of South America.
Cities and 'Dimming'
Thirteen megacities have so far been identified as ABC hotpots. Bangkok, Beijing, Cairo, Dhaka, Karachi, Kolkata, Lagos, Mumbai, New Delhi, Seoul, Shanghai, Shenzhen and Tehran all have soot levels constitute 10% of the total mass of all human-made particles.
ABCs reduce sunlight hitting the Earth's surface in two ways. Sulphates, linked with burning fossil fuels, reflect and scatter rays back into space. Others like blck carbon originate from either fossil fuel or biomass burning, and absorb sunlight before it reaches the ground. The overall effect is to make 'hot spot' cities darker or dimmer.
'Dimming' of between 10-25% is occurring over Karachi, Beijing, Shanghai and New Delhi. Guangzhou is among several cities that have recorded more than 20% reduction in sunlight since the 1970s. For India as a whole, the dimming trend has been running at about 2% per decade between 1960 and 2000. In China the observed dimming trend from the 1950s to the 1990s was about 4% per decade.
Masking the Impacts of Climate Change
ABCs shield the surface from sunlight by reflecting solar radiation back to space and by absorbing heat in the atmosphere. These two dimming phenomena can act to artificially cool the Earth's surface during dry seasons. The pollution is also transported around the world via winds in the upper troposphere.
Global temperature rises may currently be between 20 – 80% less as a result of brown clouds around the world. Thus simply tackling ABC pollution without simultaneously delivering big cuts in greenhouse gases could have a potentially disastrous effect.
Complex Regional Impacts on Temperature
The masking of greenhouse warming by ABCs may partly explain the lack of strong warming over India since the 1950s during the dry season, which runs from January to May. Similarly, ABCs may explain why the warming trend in India's night temperatures is much larger than the trend in daytime temperatures.
Annual mean temperatures in mainland China have risen by over one degree C in the past half century. However the Tibetan Plateau and the North, Northeast and Northwest of China are experiencing the highest temperature rises. Conversely SW and central Eastern China have experienced a strong cooling trend.Combined effects of greenhouse gases, ABCs and rapid urbanization are required to explain this complex pattern of warming and cooling trends in China..
Impacts on Weather Patterns Including the East Asian Monsoon
The large heating and cooling effects of ABCs in the atmosphere versus at the surface, combined with the impacts of greenhouse gases, are likely triggering sharp shifts in weather patterns. This is aggravated by dimming over the Northern Indian Ocean versus the relatively clean Southern Indian Ocean, setting up new gradients in surface sea temperatures and evaporation rates.
ABCs along with the global warming appear to be triggering significant drying in northern China alongside increased risk of flooding in southern China. ABCs have partly triggered the decrease in monsoon precipitation over India and Southeast Asia by 5-7% since the 1950s; weakening and shrinking of the Indian summer monsoon since the 1950s; and more frequent extreme rain events in both China and India.
Impact on Glaciers
The Hindu Kush-Himalaya-Tibetan glaciers provide the head-waters for the major river systems including the Ganges, Brahmaputra, Mekong and Yangtze rivers. The Ganges basin is home to over 400 million people and holds 40% of India's irrigated croplands.
The Chinese Academy of Sciences estimates that the glaciers have shrunk 5% since the 1950s and the volume of China's nearly 47,000 glaciers has fallen by 3,000 square km over the past quarter century. Glaciers in India such as the Siachen, Gangotri and Chhota Shigiri glaciers are retreating at rates of between 10 and 25 metres a year. The retreat has accelerated in the past 35 years.The Gangotri glacier alone provides up to 70% of the water in the Ganges.
ABC solar heating of the atmosphere, due to the absorption of soot and black carbon pollution is as important as greenhouse gas warming in accounting for the anomalously large warming trend observed in the elevated regions such as the Himalayan-Tibetan region. Decreased reflection of solar radiation by snow and ice due to increasing deposits of black carbon is another major contributor to the melting of ice and snow. Elevated regions of the Himalayas within 100 km of Mount Everest now have large black carbon concentrations ranging from a few hundred to a few thousand nanograms per cubic metre.
Impacts on Agriculture
There are multiple impacts of ABCs on food production and farmers' livelihoods. However there is a lot more research to be done with respect to crops at risk and the precise role of various ABC-linked effects in combination with greenhouse gases.
Damage to crops occurs from increased ground level ozone, beyond a threshold concentration of 40 parts per billion. In parts of Asia it reaches 50 parts per billion from February to June. Studies suggest that growing season mean ozone concentrations in the range 30 - 45 parts per billion could cause crop yield losses if up to 40% for important Asian crops such as wheat, rice and legumes. This implies annual economic losses for wheat, rice, corn and soya bean of about $5 billion a year across China, Korea and Japan. Other damaging effects include damage caused by various acidic and toxic particles from brown clouds deposited on plants and reduced photosynthesis due to 'dimming'.
Direct Human Health Impacts of ABCs
Brown clouds contain a variety of toxic aerosols, carcinogens and particles including particulate matter (PM) of less than 2.5 microns in width. These have been linked with health effects from respiratory disease and cardio-vascular problems. Increases in outdoor concentrations of PM 2.5 to 20 micrograms per cubic metre would lead to about 340,000 excess deaths per year in China and India. Economic losses due to outdoor exposure are estimated at 3.6% of GDP in China and 2.2% of GDP in India. In addition, the World Health Organization estimates that at least 780,000 deaths in the two countries are linked to indoor exposure through solid fuel use in the home.