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? –G.S., Chicago
Best of Chicago voting is live now. Vote for your favorites »
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.
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.
Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.
