In other words, the sun's energy reaching the earth's surface has been gradually diminishing. The effect varies from place to place, but globally it has been observed that this reduction amounted to as much as 2 per cent to 5 per cent between 1950 and 1990. Dimming has a cooling effect, and thus all calculations and models based on warming by greenhouse gases may have been distorted by this effect.

Controversy abounds as to who should be credited with discovering global dimming. In a landmark paper published in the Tellus in 1966, M Budyko clearly points out this reduction in solar radiation. His paper attracted very little attention, as the word "dimming" had not yet been coined then. A flurry of papers on atmospheric transparency, cloudiness, and on trends and assessments on solar radiation were published by Atsumo Ohmura, Vivii Russak, and Beate Liepert during the 80s and the early 90s. All these papers pointed to the gradual reduction in solar radiation.

Gerry Stanhill, an English scientist studied these declines worldwide and introduced the term "global dimming." He found that sunlight was falling by 10 per cent over the US, as much as 30 per cent in parts of the former Soviet Union, and by almost 16 per cent in parts of Great Britain. His findings, published in 2001, however, met with skepticism, and critics simply refused to believe these numbers.

It is interesting to note that the observed effect is from measurements within the earth's atmosphere, and the value of solar radiation at the top of the atmosphere has not changed by more than a fraction of the global average. It was only recently that climatologists at the Australian National University discovered corroborating evidence in the global evaporation record. They declared that declining evaporation rates could be explained by declining levels of solar radiation. Further evidence of the declining solar radiation emerged from the Geophysical Union Meeting in 2004, held at Montreal, Canada.

The phenomenon that had puzzled the scientific community for more than half a century was finally established. Dimming appears to be related to particle pollution. Early in the 2005 it was discovered that the dimming trends had reversed since around 1990. Analysis of recent data also reveals that the surface of the planet has brightened by about 4 per cent in the past decades. The brightening trends have also been corroborated by analysis of satellite data. It is likely that at least parts of this change are due to decreases in particle pollution.

It is currently thought that increased presence of aerosols particles is responsible for global dimming. Burning coal, oil and wood produces not only carbon dioxide, the principle greenhouse gas responsible for green house warming, but also tiny airborne particles of soot, ash, sulfur compounds and other particles. Aerosols and other pollutants absorb energy and reflect sunlight back into space.

The pollutants can also become nuclei of water droplets. These water droplets in clouds coalesce around the particles. Increased pollution, resulting in more particulates, creates clouds consisting of a greater number of smaller droplets, which in turn makes them more reflective, thus bouncing more sun's rays back into space.

Some climatologists suggested that aircraft contrails (known as vapour trails) also contribute to global dimming. The constant flow of aircrafts meant that the hypothesis could never be tested. The near total shut-down of air-traffic following 9/11 attacks on the twin towers, provided a rare opportunity to observe the dimming effects at least in the US. Measurements during that period produced variations of one degree celsius in some parts of the US. Aircraft contrails may have been raising nighttime temperatures and lowering daytime temperatures much more than previously thought.

As things stand, scientists believe that global dimming may have caused large-scale changes in weather systems. Climate modelers suggest that dimming, by shielding the oceans from the full power of the sun, may be disrupting the pattern of rainfall the world over. They go so far as to suggest that this reduction in sunshine may have led to the failure of monsoon in sub-Saharan Africa, together with the associated famines which claimed hundreds of thousands of lives during the 1970s and 80s.

Similar sentiments are also expressed by Professor V Ramanathan, of the University of California, San Diego, when he says: "My main concern is global dimming is also having a detrimental impact on Asian monsoon. We are talking about billions of people." Ohmura believes that during the 70s and 80s global dimming caused by air pollution, buffered the climate against global warming, caused by greenhouse gasses. As the increasing amounts of gases warmed the earth, the increasing amounts of air pollution reduced the sunlight that reached the earth's surface, thereby cooling the planet. In other words, one form of pollution counteracted the other.

The implications of the above are that the impact of both the particle pollution and the greenhouse gases on the world's climate have been underestimated by the scientific community. Does it also imply that the Kyoto Protocol underestimated the dangers of global warming? It now appears that the greenhouse gases are expected to rise in the coming decades and that particle pollutants are being brought under control.

The situation has been aptly summed up by Dr Peter Cox, a leading climate modeler: "We are going to be in a situation, unless we act, where the cooling pollutant is dropping off while the warming pollutant is going up." That means we'll get falling temperatures and increased heating at the same time and that's a problem. That means all forecasts of global warming will now have to be revised.

Dr. Mohammed Muniruzzaman is Professor, Department of Physics, Jahangirnar University.