Fiddling with the dimmer switch.

Days were brighter when you were a kid. You haven't seen the difference, but you might have felt it. In recent years, data analysis by scientists in Israel, Australia, and the United States has shown that sunlight intensity, averaged across hundreds of locations on all continents, decreased by 1.3 to 3% per decade from the 1950s to 1990s. When reported a few years ago [1], these findings were controversial, but subsequent research has helped confirm the occurrence if not the precise magnitude of so-called "global dimming."

The reduction in sunlight can be traced largely to the burning of fuels, which releases "aerosols"--black carbon, sulfates, nitrates, and other substances--that cause clouds to intercept more sunlight or reflect it back to the sky. Probably the most important likely result of dimming has been to mask, in part, rising temperatures brought on by human production of greenhouse gases. But anti-pollution efforts in the industrialized West, along with the 1990s economic crash in the former Soviet Union and Eastern Europe, curbed the release of aerosols in the northern temperate zone, and that appears to have led to overall global brightening in the last decade or so. Some analysts now say the sudden onslaught of hot years since 1990 actually represents a longer, more gradual warming trend that was masked in the 60s, 70s, and 80s by the dimming trend.

Were it spread evenly over the globe in a 24-hour period, the intensity of sunlight would be about 185 watts on each square meter (185 W/[m.sup.2]). That average includes nighttime, so you can think of typical daytime illumination as what would be supplied, more or less, by a 300-watt light bulb hanging low over each square meter. Of course, the amount of sunlight varies drastically from place to place; it can hit an intensity of 1000 W/[m.sup.2] at noon on a cloudless day in the tropics. And in any given place, the intensity can drop and rise by hundreds of watts in a few moments, with the passing of a cloud. So when it is estimated that from 1950 to 1990, average sunlight intensity fell worldwide by 5 W/m2 per decade [1], it doesn't sound like much. But, depending on where you are, the consequences of dimming may be severe.

To paraphrase that old saying about politics, all climate change is local. Indeed, the term "global dimming" obscures more than it enlightens. The deepest dimming during the past half-century occurred in the Northern Hemisphere, most intensely in the most heavily populated regions [2], and especially in the United States, with its voracious energy consumption. Plotting both sunlight reduction and fossil fuel emissions against latitude in the global North, researchers have seen the graphs align almost perfectly with each other [1, Fig. 2]. Dimming in the Southern Hemisphere began only around 1980 but appears to have been seeing just as sharp a decline [1, Fig. 1].

Photosynthesis in plants, including food crops, depends on sunlight, but human-caused dimming hasn't reduced sunlight intensity enough to depress agricultural productivity directly [1]. Crop growth is almost always limited by a deficiency of nutrients or water, or attacks by disease or other stresses before it suffers from insufficient light intensity. But crop production can fall victim to recurring droughts spawned by dimming pollution. Sulfates from industrial Europe and North America are now blamed for having weakened atmospheric circulation in the intertropical convergence zone, causing the Sahel region south of the Sahara desert to become drier [3]. That Sahelian drought of the 1970s and 80s killed more than a million people.

Now, food production in South Asia is imperiled by phenomena known as "Atmospheric Brown Clouds." While the North has been brightening in recent years, rapidly industrializing areas of Asia and the surrounding oceans have seen continued dimming, associated with the emergence of extensive, dark clouds with lifetimes measured in weeks. Generated by combustion of biomass and fossil fuels, brown clouds have been forming each winter since the 1970s, with increasing size and frequency, over South Asia and the northern Indian Ocean [3]. The extensive clouds reflect more sunlight back to the sky than do normal clouds--because of the aerosol pollutants they contain--and also absorb more sunlight because of the darker color conferred on them by particulate matter.

The pollutants have combined to cut the amount of sunlight reaching South Asia and the northern Indian Ocean by approximately 8% between 1930 and 2000, canceling out as much as 50% of the warming caused by greenhouse gases during that period [4]. Over the Indian Ocean, as much as a thousand kilometers from the power plants and vehicles that produce them, concentrations of black carbon, sulfates, fly ash, and other materials have been detected at concentrations comparable to those in suburban air pollution in North America [5].

While cooling the surface, the brown clouds absorb heat and warm the atmospheric layer in which they hover. That stabilizes the lower atmosphere, and the warmer-than-normal clouds have a greater capacity to hold moisture. The result is to weaken the engine that drives the monsoon rains. With almost 2 billion people depending on those rains to provide their food and water supply, further disruption of the monsoon cycle could precipitate a massive ecological and humanitarian crisis.

China, too, is dimming. US Department of Energy scientists have found that despite a decrease in cloud cover during the past half-century, the Chinese land surface is receiving less sunlight, because fossil fuel combustion has "entrenched China in a foggy haze." [6]. And the country's pollution can now be detected drifting across the Pacific to US shores [7]. It's a kind of poetic justice. The ever-increasing exportation of manufacturing capacity, accompanied by importation of most consumer goods, largely from China, has helped the US achieve clearer skies. But pollution generated in part by producing those goods in China is finding its way home.

Meanwhile, humanity, led by the US and other Western nations, continues to pump greenhouse gases into the atmosphere at a brisk rate. The tug-of-war between warming and dimming, until recently, seems to have raised the overall temperature of the planet only gradually. Now, it appears, warming is getting the upper hand in places like the US that have reduced aerosol pollution.

In monsoon-dependent India, the future is less certain. The leader of the Atmospheric Brown Clouds Project, Dr. V. Ramanathan of the Scripps Institution of Oceanography, has said, "Some years the aerosols might win and in some years the greenhouse effect may win. So we are concerned that in coming decades the variability between the two will become large and it will be difficult to cope with rapid changes from year to year." [8]

Brown cloud research has shown that the impact of dimming can extend well beyond urban and industrial areas--that massive polluted clouds can even drift from continent to continent. And humanity's continuing hunger for dirty energy is likely to smother any global brightening trend. With China's coal extraction having doubled in just the past 5 years, to 2.1 billion metric ton in 2005, with the US still producing more than 50% of its electricity with coal, with energy demand accelerating, and with world coal reserves estimated at almost 4.5 trillion tons, we're faced with centuries worth of dirty fuel that's crying out to be burned as fast as possible.

A sudden, human-made climatic change in a random direction does not, as one might expect, have equal chances of being harmful of beneficial. Because life on Earth evolves toward equilibrium with its current environment--and on a long time scale--and because industrial civilization has become so complicated, fragile, and vulnerable, any rapid climatic change, including dimming, is almost guaranteed to prove a disaster with no silver lining.

But that never stops anti-ecological hawks from looking for one. In July, 2006, Energy Tribune commentator Mac Johnson blamed environmentalists for tearing away the "protective layer of soot" that the Industrial Revolution had provided, leading, perhaps, to recent, more rapid temperature increases: "So now we can perceive why things have gotten so out of control on the global warming front: clean air laws. We're practically naked now." [9] He concluded his essay, apparently with his tongue only partly lodged in his cheek, "Ah, soot ... Nature's sunscreen."

A scientist who should know better, Paul Crutzen at the Max Planck Chemistry Institute in Germany, wants to dim the planet intentionally as a way of curbing global warming. Crutzen, a Nobel laureate, published a paper this year [10] suggesting that we shoot payloads of sulfur into the stratosphere to increase "the Earth's reflective powers" and have a cooling effect.

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Aside from the general rule that any global climatic manipulation will have unforeseen and almost surely negative consequences, there is the temporary nature of Crutzen's solution. What goes up must come down: The sulfur would remain in the stratosphere for only two years, and would eventually reach the planet's surface again, mostly as acid rain. Or it might create droughts in some regions. It can also damage the ozone layer. On the positive side, it has been suggested, the sulfur might create beautiful sunsets.

Other so-called "geoengineers" have put forward even more audacious proposals, including the construction of a 2000-kilometer-diameter mirror between the Sun and Earth [11]. They calculated that, properly placed, the mirror's shading/cooling effect could compensate for a doubling of atmospheric carbon dioxide concentration. The resource consumption, ecological hazards, centralized decision-making, and general risk of disaster involved in such geoengineering proposals have, so far, saved us from their actually being implemented.

The straightforward approach to both global warming and global dimming is, of course, to make deep cuts in energy consumption planet-wide. The rapidly industrializing nations of the South will have to find their own ways to get the energy they need without ecological devastation. But with the average American using 10 times as much energy as the average person in China, and 24 times as much as the average Indian, it's the clear duty of the United States to take the lead in slashing consumption.

But instead of that, scientists and policymakers are focusing on technological fixes like carbon sequestration, stratospheric seeding, and colossal mirrors. New products, techniques, or regulations have been used with some success by rich countries in dealing with simpler problems like ozone depletion, acid rain, and global dimming. But global warming can never be curbed without wholesale conversion to non-fossil energy sources, and those will never provide the power bonanza that coal, oil, and natural gas have given us for decades.

Growth-dependent economies were built around that power bonanza and have no way to deal with necessary reductions in energy consumption that could go as deep as 80%. Global capitalism will not--indeed, cannot--give up the easy exploitation of concentrated energy that need only be mined or pumped. And there's no cloud, mirror, or umbrella big enough to shield us from the consequences of that greed for energy.

Stan Cox is a plant breeder and writer in Salina, Kansas.

References

1. Stanhill, G. and S. Cohen. 2001. Agricultural and Forest Meteorology 107:255

2. Alpert, P., P. Kishcha, Y.J. Kaufman, and R. Schwartz-bard. Geophysical Research Letters 32: L17802, DOI: 10.1029/2005GL023320

3. Rotstayn, L.D. and U. Lohmann. 2002. Journal of Climate 15:2103.

4. Ramanathan, V., C. Chung, D. Kim, T. Bettge, L. Buja, J.T. Kiehl, W.M. Washington, Q. Fu, D.R. Sikka, and M. Wild. 2005. Proceedings of the National Academy of Sciences 102:5326

5. United Nations Environment Program, "The atmospheric brown cloud: climate and other environmental impacts" (http://www.rrcap.unep.org/issues/air/impactstudy/index.cfm)

6. National Atmospheric and Space Administration press release: http://earthobservatory.nasa.gov/Newsroom/MediaAlerts/2006/2006011721484.html

7. Chea, T. "Pollution from China drifting east." Honolulu Advertiser, July 30, 2006. (http://the.honoluluadvertiser.com/article/2006/Jul/30/bz/FP607300313.html)

8. Scripps Institution of Oceanography press release (http://scrippsnews.ucsd.edu/article_detail.cfm?article_num=731)

9. Johnson, M. "Soot: Nature's sunscreen?" Energy Tribune, July 11, 2006. (http://www.energytribune.com/articles.cfm?aid=165)

10. Crutzen, P. 2006. Climatic Change: DOI 10.1007/s10584-006-9101-y

11. Govindasamy, B., and K. Caldeira. 2000. Geophysical Research Letters 27:2141.