ABCs start as indoor and outdoor air pollution consisting of particles and pollutant gases, such as nitrogen oxides (NO_x), carbon monoxide (CO), sulfur dioxide (SO₂), ammonia (NH₃), and hundreds of organic gases and acids. Widespread ABC plumes resulting from the combustion of biofuels from indoors; biomass burning outdoors; and fossil fuels are found in all densely inhabited regions and oceanic regions downwind of populated continents.

There are five identified regional ABC hotspots around the world:

• East Asia
• Indo-Gangetic Plain in South Asia
• Southeast Asia
• Southern Africa
• Amazon Basin

ABC hotspots are defined as regions where the annual mean anthropogenic aerosol optical depth (AOD) exceeds 0.3 and the percentage of contribution by absorbing aerosols exceeds 10% (absorbing AOD > 0.03).

Although substantial loadings of ABCs over Eastern USA and Europe have also been observed, in these extra-tropical regions the atmospheric concentrations of ABCs are large mainly during the summer season since precipitation removes the aerosols efficiently during other seasons. Furthermore, the soot concentrations are lower and hence these extra tropical regions are not included in the hotspots category.

In Asia, new aircraft and satellite data have revealed that ABC plumes, measuring 1-3km thick, surround the Hindu Kush-Himalayan-Tibetan glaciers, both from the South Asian and the East Asian sides. Between 1950 and 2002, soot emissions increased three-fold in India and five-fold in China, while sulfur emissions have increased ten-fold in China and seven-fold in India.

The brown clouds are in some cases and regions aggravating the impacts of greenhouse gas-induced climate change, says the report. Particles like black carbon and soot in ABCs absorb sunlight and heat the air and gases such as ozone enhance the greenhouse effect of CO₂.

Globally, however, brown clouds may be masking the warming impacts of climate change by between 20 and up to 80%, the researchers suggest, due to other particles in the ABCs such as sulfates and some organics which reflect sunlight and cool the surface.

Another effect of the aerosols in the brown clouds is to make hot spots dimmer. Dimming of between 10-25% is occurring over cities such as Karachi, Beijing, Shanghai and New Delhi. Guangzhou is among several cities that have recorded a 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—more than doubling between 1980 and 2004. In China, the observed dimming trend from the 1950s to the 1990s was about 3-4% per decade, with the larger trends after the 1970s.

Regions with large concentrations of ABCs may be getting cloudier which can also contribute to dimming but data are not sufficient to quantify this effect. Particles and aerosols in the ABCs may act to inhibit the formation of rain drops and rainfall.

The cloud is also having impacts on air quality and agriculture in Asia increasing risks to human health and food production for three billion people.

ABCs and Global Warming. The increase in global temperature linked with greenhouse gas emissions may currently be between 20-80% less as a result of brown clouds around the world, says the report.

As a result, if brown clouds were eliminated overnight, this could trigger a rapid global temperature rise of as much as 2° C. Added to the 0.75° C rise of the 20^th century, this could push global temperatures well above the +2° C threshold considered by many scientists to the point for dangerous climate change. Thus, simply tackling the pollution linked with brown cloud formation without simultaneously delivering big cuts in greenhouse gases could have a potentially disastrous effect.

Complex Regional Impacts on Temperature. The science of ABCs, woven with the science of greenhouse gases, is not simple and may be behind some highly complex warming and cooling patterns witnessed on continents and in different regions of specific countries.

• The masking of greenhouse warming by ABCs may in part be the explanation for the lack of a strong warming trend over India since the 1950s during the dry season which runs from January to May.

• ABCs may explain in part why the warming trend in India’s nighttime temperatures is much larger than the trend in daytime temperatures.

• Annual mean temperatures in mainland China have risen by more than one degree C in the past half century. However the trends have not been uniform with the Tibetan Plateau and the North, Northeast and Northwest of China experiencing the highest temperature rises. Conversely Southwest and central Eastern China has experienced a strong cooling trend of between 0.1 to 0.3 degrees C per decade.

The authors note that the combined effects of greenhouse gases, ABCs and rapid urbanization are required to explain the 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 respectively in the atmosphere and at the surface, combined with the impacts of greenhouse gases, may be also triggering sharp shifts in weather patterns.

This is being aggravated by dimming over the Northern Indian Ocean versus the relatively clean Southern Indian Ocean setting up new gradients in surface sea temperatures and surface sea evaporation rates.

ABCs, along with the global warming, may thus be acting to trigger significant drying in northern China and increased risk of flooding in southern China while in part also triggering other environmental and economic effects.

• Overall decrease in monsoon precipitation over India and Southeast Asia by between 5-7% since the 1950s.

• Since the 1950s the Indian summer monsoon is not only weakening but shrinking with a decrease in early and late season rainfall and a decline in the number of rainy days.

• In both China and India extreme rain events of more than 100 mm a day have increased.

• In both India and China very heavy rainfall of more than 150 mm a day have nearly doubled.

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 more than 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 meters a year. The retreat has accelerated in the past three and-a-half decades. The Gangotri glacier alone provides up to 70% of the water in the Ganges.

• The report suggests that 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 emerging as another major contributor to the melting of ice and snow.

• Elevated regions of the Himalayas within 100 km of Mount Everest experience large black carbon concentrations ranging from a few hundred to a few thousand nano grams per cubic metre.

Impacts on Agriculture. A great deal more research is required, the authors say, to determine crops at risk and the precise role various ABC-linked effects—separately or in combination with those of greenhouse gases—may or may not be having. However, possible effects may include:

• Damage to crops as a result of increased ground level ozone. In Europe a threshold concentration at which damage can occur is deemed to be 40 parts per billion The report says that in parts of Asia ground level ozone can reach 50 parts per billion during February to June and peaking again between September and November at 40 parts per billion.

• The studies suggest that growing season mean ozone concentrations in the range 30-45 parts per billion could see crop yield losses in the region of 10-40% for sensitive cultivars of important Asian crops such as wheat rice and legumes.

• A recent study translated such impacts on yield into annual economic losses estimating that for four key crops—wheat, rice, corn and soybean—these may amount to around $5 billion a year across China, the Republic of Korea and Japan.

• Other effects may include damage linked with the various acidic and toxic particles from brown clouds depositing on plants from the atmosphere.

• Reduced levels of photosynthesis and thus crop production due to dimming.

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 a variety of health effects from respiratory disease and cardio-vascular problems.

• Increases in concentrations of PM 2.5 of 20 micrograms per cubic meter in the air could lead to about 340,000 excess deaths per year in China and India.

• The World Health Organization estimates that more than 780,000 deaths in the two countries can be linked to solid fuel use in the home.

• Economic losses due to outdoor exposure to ABC-related PM 2.5 has been crudely estimated at 3.6% of GDP in China and 2.2% of GDP in India.

Project Atmospheric Brown Cloud was established by the United Nations Environment Programme (UNEP) in 2002 following the documentation of brown clouds and haze by the Indian Ocean Experiment (INDOEX). The science secretariat of ABC is located at the Center for Clouds, Chemistry and Climate, Scripps Institution of Oceanography, UCSD.

We believe today’s report brings ever more clarity to the ABC phenomena and in doing so must trigger an international response—one that tackles the twin threats of greenhouse gases and brown clouds and the unsustainable development that underpins both.

One of the most serious problems highlighted in the report is the documented retreat of the Hind Kush-Himalayan-Tibetan glaciers, which provide the head-waters for most Asian rivers, and thus have serious implications for the water and food security of Asia. The new research, by identifying some of the causal factors, offers hope for taking actions to slow down this disturbing phenomenon; it should be cautioned that significant uncertainty remains in our understanding of the complexity of the regional effects of ABCs and more surprises may await us.

—Professor Veerabhadran Ramanathan, head of the UNEP scientific panel carrying out the ABC research

The current project is funded by UNEP with support from the governments of Italy, Sweden and the United States.

Resources

• Atmospheric Brown Clouds Regional Assessment with Focus on Asia (Summary)

• Project Atmospheric Brown Clouds