Wednesday, January 19, 2005

Global Dimming 

"I can sum my intuition up in five words: we're in for climatic mayhem."
--Bruce Sterling, Wired

Zemblan patriot T.C. calls to our attention the transcript of a recent, disquieting report from the BBC program Horizon suggesting that scientists may have grossly underestimated the potency of the greenhouse effect. The good news is that global warming appears to have been partially offset, for the time being, by a concomitant cooling effect; the bad news is that the cooling effect is probably caused by "visible pollutants" in the atmosphere, particulate matter that not only blocks sunlight, but increases the reflectivity of the earth's cloud cover, bouncing sunbeams back into space. Cutting back on those toxic pollutants is an obvious necessity, but it may result in a sudden acceleration of the warming process -- leading to catastrophic climate change.

The theory of "global dimming" originated with scientist Gerry Stanhill, a specialist in irrigation design. He compared current Israeli sunlight readings with records from the fifties -- and was stunned to discover that over that period, there had been an inexplicable 22% drop in the level of solar radiation. Subsequent research showed there had been drops of 9% in Antarctica, 10% in the USA, almost 30% in Russia, and 16% in the British Isles. His problem was that, at first, no one believed his findings:
NARRATOR: Of course, there was a good reason for the scepticism. Less energy from the Sun should be making the world cooler. Yet scientists knew the Earth was getting hotter. As the carbon dioxide and other greenhouse gases we emit trap ever more heat in the Earth's atmosphere and cause Global Warming.

DR BEATE LIEPERT: My friends' reaction actually to Gerry's and to my work at the same time too was, oh my God, this is really extreme, you are, um, contradicting global warming. Er, do you know how many billions of dollars was spent on global warming research, and you and this old guy are contradicting us?
Then scientists in Australia noticed that, despite a general rise in temperatures worldwide, the "pan evaporation rate" had fallen throughout the nineties:
PROF GRAHAM FARQUHAR (Australian National University): It's called pan evaporation rate because it's evaporation rate from a pan. Every day all over the world people come out in the morning and see how much water they've got to add to a pan to bring it back to the level it was the same time the morning before. It's that simple.

DR MICHAEL RODERICK: And then one day, just by accident, I had to go to the library to get an article out Nature. As you do, I couldn't find it. And I just glanced at a, through the thing, and there was an article called Evaporation Losing Its Strength. Which reported a decline in pan evaporation over Russia, United States and Eastern Europe. And there in the, in the measurements, they said that the, the pans had on average, evaporated about a hundred millimetres less of water in the last thirty years.

NARRATOR: Mike knew how much sunlight was needed to evaporate a millimetre of water. So he put the two sets of figures together - the drop in evaporation with the drop in sunlight . . . . It was the same with Europe and the USA. The drop in evaporation rate matched exactly the drop in sunlight reported by Beate Liepert and Gerry Stanhill. Two completely independent sets of observations had come to the same conclusion.
Experiments in the Maldives demonstrated that visible pollutants were the likely culprits. There is virtually no air pollution in the Maldives. The southern tip of the chain catches the pure airstream from Antarctica; the northern tip is blanketed by dirty air from industrialized India. That layer of pollutants, and the clouds whose composition it changed, resulted in a solar-energy disparity of 10% from south to north. Scientists who incorporated the Maldives data into their atmospheric models realized that the deadly Ethiopian drought and famine of the mid-eighties might have been a direct consequence of global dimming: when the sun did not warm the northern oceans as expected, the tropical rain bands, which are normally drawn by the heat, moved southward instead of northward. They never reached the sub-Saharan Sahel. A million people died as a result.

Bizarrely, 9/11 supplied more confirmation of global dimming. After the attacks, all planes were grounded for a three-day stretch, and for the first time in decades there were no jet contrails in the skies above America. In those three days the average temperature range -- that is, the difference between the daily high and the nightly low -- jumped by over one degree C., the largest swing in the last thirty years.
DR DAVID TRAVIS: The 9/11 study showed that if you remove a contributor to Global Dimming, jet contrails, just for a three day period, we see an immediate response of the surface of temperature. Do the same thing globally, we might see a large scale increase in global warming.

NARRATOR: This is the real sting in the tail. Solve the problem of Global Dimming and the world could get considerably hotter. And this is not just theory, it may already be happening. In Western Europe the steps we have taken to cut air pollution have started to bear fruit in a noticeable improvement in air quality and even a slight reduction in Global Dimming over the last few years. Yet at the same time, after decades in which they held steady, European temperatures have started rapidly to rise culminating in the savage summer of 2003.

Forest fires devastated Portugal. Glaciers melted in the Alps. And in France people died by the thousand. Could this be the penalty of reducing Global Dimming without tackling the root cause of global warming?
A good Q&A on global dimming is here.

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