Around 13 megacities have so far been identified as ABC hotpots. Bangkok, Beijing, Cairo, Dhaka, Karachi, Kolkata, Lagos, Mumbai, New Delhi, Seoul, Shanghai, Shenzhen and Tehran where soot levels are 10 per cent of the total mass of all human-made particles. ABCs can reduce sunlight hitting the Earth's surface in two ways. Some of the particles such as sulphates, linked with burning coal and other fossil fuels, reflect and scatter rays back into space. Others, also linked with fossil fuel and biomass burning, in particular black carbon in soot, absorb sunlight before it reaches the ground. The overall effect is to make 'hot spot' cities darker or dimmer. 'Dimming'of between 10-25 per cent is occurring over cities such as Karachi, Beijing, Shanghai and New Delhi Guangzhou is among several cities that have recorded a more than 20 per cent reduction in sunlight since the 1970s For India as a whole, the dimming trend has been running at about two per cent 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 cent per decade, with the larger trends after the 1970s,' says the report. Impact on Cloud Formation and a Further Dimming Effect 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 net effect is an extension of cloud life-times,' says the report. 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 especially during dry seasons. The pollution can also be transported around the world via winds in the upper troposphere (above 5 km in altitude). As a result global temperature rises-linked with greenhouse gas emissions-may currently be between 20 per cent and 80 per cent less as a result of brown clouds around the world says the report. If brown clouds were eliminated overnight, this could trigger a rapid global temperature rise of as much as to 2 degrees C. Added to the 0.75 degrees C rise of the 20th century, this could push global temperatures well above 2 degrees C-considered by many scientists to be a crucial and dangerous threshold. Thus simply tackling the pollution linked with brown cloud formation without simultaneously delivering big cuts in greenhouse gases could have a potentially disastrous effect. 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 over 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 combined effects of greenhouse gases, ABCs and rapid urbanization are required to explain the complex pattern of warming and cooling trends in China,' says the report. 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 five and seven per cent 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 has nearly doubled. The Hindu Kush-Himalaya-Tibetan glaciers provide the headwaters 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 per cent of India's irrigated croplands. The Chinese Academy of Sciences estimates that the glaciers have shrunk 5 per cent 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 three and-a-half decades. The Gangotri glacier alone provides up to 70 per cent of the water in the Ganges. ABC solar heating of the atmosphere, due to the absorption of soot and black carbon pollution 'is suggested to be 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 says the report. 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 nanogrammes per cubic metre. Impacts of ABCs on food production and farmers'livelihoods may be many. However there remains a great deal more research to undertake in terms of 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. 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 per cent 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 soya bean-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' 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. Outdoor exposure: Increases in concentrations of PM 2.5 of 20 microgrammes per cubic metre could lead to about 340,000 excess deaths per year in China and India Indoor exposure: the World Health Organization estimates that over 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 PM2.5 has been crudely estimated at 3.6 per cent of GDP in China and 2.2 per cent of GDP in India.