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.

Good News: I was Wrong About Global Dimming

Yippee! There’s great news. The TV show NOVA scared me a few weeks ago by describing the phenomenon called “global dimming.” I blogged about it basing my story on the NOVA presentation. Three smart readers, Manuel Rozental, Gary Boyd, and "Anonymous" entered useful comments on my blog. I’m not sure who “Anonymous” is, but he pointed out that some recent discoveries have disconfirmed much of the NOVA thesis, and he directed me to his web site, http://www.scientificblogging.com/the_soot_files. (This seems to have been written by one Lee Rodgers (AKA “Leebert”) His argument, which I presume is well-founded, though I have not sought further evidence, is that the atmospheric pollutants are of two types: (a) regular water-vapor clouds and (b) soot – and that contrary to NOVA, the soot element does not contribute to global cooling but rather to global warming. Such soot emanates from industrial particulates, and is especially conspicuous in Asia, where it is described as the “Asian brown cloud” (see photo).

Whereas according to NOVA, clouds generally shield the earth from the sun’s rays and offset some of the global warming effects of the greenhouse gases, Rodgers’s web site argues the contrary: that the soot element actually accounts for a large part of the global warming effect.

This will be very welcome news if it is established, since soot is easier to remove from the atmosphere than greenhouse gases. Whereas NOVA implied that we should avoid removing particulates too rapidly, since that would even intensify global warming, Rodgers’s web site calls for the opposite, arguing that a quick reduction in soot will slow down the GHG effects and give us longer to fix that problem.

Rodgers argues that soot from Asia is the main factor behind the melting of glaciers, for soot impairs the albedo (reflectivity) of the white surface. Citing Professor V. Ramanathan (one of the main researchers cited by the NOVA program too), Rogers claims that the deglaciation of the Himalayas largely results from the effects of airborne soot in the region. The sooty brown clouds seem to inhibit the formation of low-altitude cumulus clouds that provide a true net cooling benefit. Much of the soot comes from Asian and Pacific-region coal-powered industries and cooking fires, possibly accounting for up to 12 percent of global warming, worldwide. Add to that the melt-off of the Arctic,

“we might readily account for 33 percent of all human-caused global warming being due to soot.

“Were another 10-15 percent of all global warming found to result from other sources of soot in the industrialized West and in the tropics and subtropics (wood burning, slash and burn agriculture), we might be looking at a net 45-48 percent of global warming thus far being due to readily-mitigated soot....

[If the soot is eliminated] “We could observe tangible, meaningful results in addressing climate change, with the Arctic and Greenland ice sheets regenerating and the monsoon cycles in Asia resuming their normal patterns.“

So where does this soot originate? I found a brief article and map in the September 2007 issue of New Scientist (page 15). The Blacksmith Institute in New York, has released its annual list of the world’s most toxic places. They created a database of 600 polluted sites, then pared it down to the worst ten places on earth, in terms of the number of people harmed by the emissions. Six of these were in Asia, India, and Russia. The list includes Linfen, the heart of China’s coal industry; Sukinda, in India, which affects 2.6 million people with its chromite mines; and Dzerzhinsk, Russia, where people are exposed to the toxic by-products of chemical weapons. This list of 400 sites would provide a good list of the major sources of soot in the world. It will give us an excellent starting point for solving a big part of Earth’s problems.

Thanks to you, Lee Rodgers.