???Global dimming??? is caused mostly by urban pollution such as vehicle emissions, soot and burning of wood and crops. Experts say that rice harvests would have been up to 10.6 per cent higher if the brown haze had not been there.

The haze or an atmospheric brown cloud hanging over major Asian cities, including Delhi and Mumbai, essentially affects local conditions and can be swept away by a new weather system or winter rains as is often the case in the Capital. The process, known as ‘global dimming’ is completely opposite to that of global warming where outgoing radiation is absorbed and not allowed to escape.


De facto, we often hear and read a great deal about climate change and in particular, ‘global warming’. But it is only in recent years that we have been alerted to another man-made feature of climate change – ‘global dimming’.

It is caused mostly by urban pollution such as vehicle emissions, soot and burning of wood and crops.
Experts say that rice harvests would have been up to 10.6 per cent higher if the brown haze had not been there. With less sunlight coming through the clouds, some of the biggest cities in Asia are getting darker by 10-25 per cent. The Hindu Kush-Himalaya, Tibetan glaciers are melting, causing water shortage across Asia. Besides, if the cloud mixes with greenhouse gases, it will change weather patterns and destroy crops all over the world.


Thus, in India too, its effect is clear and perceptible. In fact, atmospheric brown clouds are not new to scientists in India, who have been researching their occurrence for some years and recent efforts have pointed to dust storms and local conditions as being responsible for the phenomenon. Indian summer monsoon is not only weakening but also shrinking in early and late season. Many areas have been experiencing extreme weather conditions; rice which needs a lot of water and other crops are suffering due to the brown haze and cities like Delhi are facing a sharp rise in lung-related problems.


India is getting about five per cent less sunlight than it did 20 years ago, according to a study by Padma Kumari and colleagues at the Indian Institute of Tropical Meteorology in Pune.


India is losing out on sunshine because a cloud of tiny air-borne particles released by the nation’s industries hovers above the subcontinent, blocking light from reaching the earth.


The effect of ‘global dimming’ varies by location, but worldwide it has been estimated to be of the order of a 4 per cent reduction over the three decades from 1960–1990. However, since 1990, the trend has reversed.


It is thought to have been caused by an increase in particulates such as sulphate aerosols in the atmosphere due to human action. The switch from a ‘global dimming” trend to a “brightening” trend in 1990 happened just as global aerosol levels started to decline.


‘Global dimming’ has interfered with the hydrological cycle by reducing evaporation and may have reduced rainfall in some areas. ‘Global dimming’ also creates a cooling effect that may have partially masked the effect of greenhouse gases on global warming.


It is thought that ‘global dimming’ was probably due to the increased presence of aerosol particles in the atmosphere caused by human action. Aerosols and other particulates absorb solar energy and reflect sunlight back into space. The pollutants can also become nuclei for cloud droplets. Water droplets in clouds coalesce around the particles. Increased pollution causes more particulates and thereby creates clouds consisting of a greater number of smaller droplets. The smaller droplets make clouds more reflective, so that more incoming sunlight is reflected back into space and less reaches the earth’s surface.


Clouds intercept both heat from the sun and heat radiated from the earth. Their effects are complex and vary in time, location and altitude. Usually during the daytime the interception of sunlight predominates, giving a cooling effect; however, at night the re-radiation of heat to the Earth slows the earth’s heat loss.


Some scientists now consider that the effects of ‘global dimming’ have masked the effect of global warming to some extent and that resolving ‘global dimming’ may, therefore, lead to increase in predictions of future temperature rise. The magnitude of this masking effect is one of the central problems in climate change with significant implications for future climate changes and policy responses to global warming.


Interactions between the two theories for climate modification have also been studied, as global warming and ‘global dimming’ are not mutually exclusive or contradictory.


In a paper published in the American Geophysical Union’s Geophysical Research Letters, a research team led by Anastasia Romanou of Columbia University’s Department of Applied Physics and Mathematics, New York, also showed that the apparently opposing forces of global warming and ‘global dimming’ can occur at the same time. ‘Global dimming’ interacts with global warming by blocking sunlight that would otherwise cause evaporation and the particulates bind to water droplets. Water vapour is one of the greenhouse gases. On the other hand, global dimming is affected by evaporation and rain. Rain has the effect of clearing out polluted skies.


Brown clouds have been found to amplify global warming according to V. Ramanathan, an atmospheric chemist at the Scripps Institution of Oceanography in La Jolla, CA. Interestingly, while the West sees atmospheric brown clouds as a major climate change factor in global warming, India sees the charges that its ‘traditional’ biofuels are the primary reason for the toxic haze as an attempt to put the developing world on back foot over climate change.


According to the study the dimming effect which is caused when incoming solar radiation is absorbed by soot and dust particles in the atmosphere is especially large in Asian monsoon regions owing to heavy pollution and frequent occurrence of dust storms. The situation is so alarming that we need to awaken ourselves and take corrective measures as early as possible.