Global Dimming or Warming?

I’ve been perturbed lately by some apparent contradictions in conclusions about “global dimming” — a concept that I described with alarm several weeks ago. The PBS show NOVA had described its effects. It seems that water vapor, gases, contrails from airplanes, and particulates in the atmosphere often blanket south Asia during the dry season with “brown clouds.” These are largely the result of emissions from the rapidly industrializing countries and, as all aerosols do, they block out some of the sunshine and have an overall cooling effect on the planet, partially offsetting the warming effect of greenhouse gases. What that means is that the true heating effect of greenhouse gases would be much higher if there were not so much gunk in the air, masking some of the warming. Hence cleaning up the atmosphere would be a mixed blessing. Although the pollutants are bad for our health, their disappearance would hasten the terrible day when the polar icecaps all melt and the world succumbs to global warming.

But then I was informed that this theory was disproved after the NOVA program was produced. Global dimming, according to this new theory, is not real. Instead, the particulates in the atmosphere are exacerbating the heating effect, rather than masking it.

But how can this be? I have been puzzled ever since until I discovered two articles in the August 2 issue of Nature. I still don’t understand the full complexity of the situation, but the current version goes like this: Yes, there are cooling (“dimming”) effects. But these brown clouds vary in content and in their effects, especially according to the altitude. Presumably the water vapor does have a cooling effect. But the particulates have contradictory implications. Some of them reflect light in all directions, sending a good part of it back into space. But low-lying black soot particles heat the atmosphere where they are floating, thus intensifying the heating impact of greenhouse gases.

V. Ramanathan and his team studied these effects by sending three unmanned aerial vehicles flying through the clouds over the Indian Ocean, stacked vertically over each other, with a horizontal separation of tends of meters or less, and a temporal separation of less than ten seconds. This made it possible to measure the atmospheric solar heating rates directly. They found that atmospheric brown clouds enhanced lower atmospheric solar heating by about 50 percent.

Extrapolating these effects over time, the researchers concluded that the combined warming trend of the brown clouds comes to about 0.25K per decade – possibly enough to account for the observed retreat of the Himalayan glaciers.

The blessing of this discovery is this: It should be easier to scrub industrial soot out of the atmosphere than to halt the emissions of greenhouse gases. If this is done promptly, it could reduce the alarming heating effects that are now melting the glaciers of the Himalayas. Since the rivers of south Asia depend to a great extent on the glaciers, everything that can be done to conserve this ice will save the lives of countless human beings who will otherwise find themselves without drinking water in a few years.

Now Dr. Ramanathan and his colleagues are analyzing the content of the brown plumes of pollution that emanate from Asia but are blown across the Pacific to influence the climate of North America. This project is just getting started.

Another implication of this research seems to be that water vapor does have a “dimming” effect. Hence there may still be merit to the “geo-engineering” proposal to spray sea water into the atmosphere to increase the global cloud cover.