A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

We know about global warming, but what about global dimming?

April 20, 2007

Dear Cecil:

It seems like everyone is worried about global warming, but you (or at least I) never hear anything about the lesser-known but possibly more important phenomenon of global dimming. Could you give us the straight dope on this, since no one else seems to know anything about it?

Too hot! Too cold! Surely if we're patient all the bad stuff will cancel out and we can go back to driving Hummers with a clear conscience, right? Sorry. Though I hesitate to make bold pronouncements, global dimming could mean we're in more trouble from global warming than we thought.

First some vocabulary. At any given moment earth receives about 174 petawatts (174 billion megawatts) of solar energy, about 30 percent of which is reflected immediately back into space, mainly by clouds. Though the remainder drifts off into the void eventually, most of it sticks around long enough to warm the planet and keep us alive. What concerns scientists is that over the past 50 years the average amount of sunlight hitting the earth's surface has markedly decreased, a phenomenon English scientist Gerry Stanhill dubbed global dimming in 2001. How much sunlight has dimmed depends on where you are, but overall it's something like 1 to 3 percent per decade.

The problem isn't that the sun is running out of gas, but rather that air pollution prevents its rays from reaching the ground. Some will object: I thought pollution was causing global warming. Different pollutants. The ones contributing to dimming, called anthropogenic aerosols, include tiny particles of sulfur, soot, and dust. In the atmosphere these particles do two things: they reflect some solar radiation back into space and absorb other radiation before it can reach the surface; they also increase cloud formation and make clouds more reflective, meaning still more sun gets blocked. Net effect: lower temperatures at the earth's surface.

Cooling due to airborne crud is nothing new. In contrast to the long-term effects of global warming, which are harder to demonstrate, we've seen short-term global dimming before due to volcanic eruptions. The enormous amounts of sulfur dioxide released by volcanoes form clouds of particles that can stay airborne for years. (Volcanic ash blocks the sun too, but according to the U.S. Geological Survey sulfur aerosols have the most prolonged impact.) Examples from history abound, one of the most dramatic being the eruption of Indonesia's Mount Tambora in 1815. The powerful blast took off the top mile of the mountain, killed tens of thousands of people, and released so much sulfur and ash into the air that 1816 was known as the "year without a summer." Crop failures, famine, bitter winter cold and record snows, and a strange dry summer fog made 1816 a bad year for most of North America and Europe - New England got heavy snowfall in June and Virginia allegedly had frost on the Fourth of July. More recently, the 1991 eruption of Mount Pinatubo in the Philippines, which sent about 20 million tons of sulfur dioxide aloft (about twice the amount released annually by all U.S. power plants), caused an average temperature drop in the northern hemisphere of one degree Fahrenheit. That may not seem like much, but some volcano and climate researchers believe Pinatubo is partly responsible for the cool, wet summer of 1992 and the floods of 1993.

Pollutants released at ground level may not be the only contributor to global dimming. A 2002 study published in Nature analyzed the impact of the reduction of jet contrails after the mass grounding of U.S. flights following the 9/11 attacks. Reviewing weather data from some 4,000 reporting stations, researchers found temperatures from September 11 to 14, 2001 were two degrees higher than the 30-year average, and three degrees higher than during the three-day periods before and after the grounding.

The good news is that global dimming seems to be reversing itself, mainly because we've been cleaning up the environment. Measurements show that since 1990 or so, more sunlight has been reaching the earth's surface, with about a 4 percent increase in the last decade. The bad news is that, because dimming threw off the measurements, global warming may have been underestimated and projections of long-term temperature increases may be too low. This has provoked at least one brainiac to propose an antiwarming strategy using artificial volcanoes to send clouds of sulfur into the air.

One region that hasn't seen an increase in sunlight lies beneath the "Asian brown cloud," a two-mile-thick haze of pollutants hanging over much of south and east Asia. Blamed for everything from changing rainfall patterns and fiercer Pacific storms to crop losses and health problems, the cloud is a curse that needs to be cleaned up. The irony is that once it and other reservoirs of aerosol soot and sulfur are eliminated, the greenhouse gases behind global warming might end up hurting us more.

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