NARRATOR: A 22% drop in solar energy was simply massive. If it was true surely Israelis should be freezing. There had to be something wrong. So when Gerry published his results they were ignored.

DR GERALD STANHILL: I must say the publications had almost no effect whatsoever on the scientific community. ...

PROF VEERABHADRAN RAMANATHAN: The stunning part of the experiment was this pollutant layer which was three kilometre thick, cut down the sunlight reaching the ocean by more than 10%.

NARRATOR: A 10% fall in sunlight meant that particle pollution was having a far bigger effect than anyone had thought possible.

PROF VEERABHADRAN RAMANATHAN: Our models led us to believe the human impact on the dimming was close to half to one per cent. So what we discovered was tenfold.

NARRATOR: INDOEX showed that the pollution particles were blocking some sunlight themselves; but far more significant was what they were doing to the clouds. They were turning them into giant mirrors. Clouds are made of droplets of water. These only form when water vapour in the atmosphere starts to condense on the surface of naturally occurring airborne particles, typically pollen or sea salt. As they grow, the water droplets eventually become so heavy they fall as rain. But Ramanathan found that polluted air contained far more particles than the unpolluted air, particles of ash, soot and sulphur dioxide.

"I see some datasets that are consistent and some that aren't," Dr. Ellsworth G. Dutton, who heads surface-radiation monitoring at the National Oceanic and Atmospheric Administration, said. "Certainly, the magnitude of the phenomenon is in considerable question."

Dr. Beate G. Liepert, a research scientist at the Lamont-Doherty Earth Observatory of Columbia University, has analyzed similar information and arrives at a smaller estimate of the dimming than Drs. Stanhill and Cohen. Dr. Liepert puts it at 4 percent from 1961 to 1990, or 1.3 percent a decade. "It's a little bit the way you do the statistics," she said.

A major set of measurements from the Indian Ocean in 1999 showed that air pollution did block significant sunlight. Following plumes of soot and other pollution, scientists measured sunlight under the plumes that was 10 percent less bright than in clear air.

NARRATOR: Once the Greenland ice cap begins to melt, nothing will stop it. Many of the world's major cities will be living on borrowed time. Decade by decade, the risk of catastrophic flooding would increase inexorably. But unless action is taken it won't stop there. Because after Greenland, the world's tropical rainforests will start to wither in the heat.

DR PETER COX: 2040 it could be four degrees warmer, the climate change could have led to big drying particularly in the Amazon Basin, that would make the forest unsustainable, we'd expect the forest to catch fire probably, turn into savannah and maybe ultimately even desert if it gets really really dry as our model suggests.