Should you be worried about global dimming?

Illuminating answers to environmental questions.
Dec. 26 2007 7:50 AM

Hey, Who Turned Out the Lights?

What you need to know about global dimming.

As if I wasn't already freaked out enough about the planet's future, my sister just mentioned something called "global dimming." I'm not too sure what this is, but it sounds ominous. Is this dimming phenomenon worth losing sleep over? Or is this just environmental fearmongering?

The Lantern's hunch is that your sister recently caught this scary BBC documentary, which first aired in 2005. Replete with images of darkened skies and impoverished children, the film got lots of viewers panicked about global dimming, or the decline in sunlight. Though generally informative, the BBC production overplayed the doomsday angle. Not only is the Earth not in imminent danger of being cast into eternal darkness, but the planet has actually gotten a bit brighter over the past 15 years. But the uptick in sunlight may not be as positive a development as it seems.

Advertisement

Technically, the term "global dimming" refers to the reduction of solar radiation hitting the planet's surface, a phenomenon caused by the proliferation of aerosols in the atmosphere. These aerosols can be both anthropogenic and natural in origin: Though industrial soot certainly plays a huge role in dimming, nothing affects sunlight's Earthward journey like an erupting volcano spewing sulfate particles. When these aerosols reach the clouds, they cause the formation of smaller-than-normal water droplets; according to David Sington, the producer of the BBC documentary, an aerosol-affected cloud will contain six times as many droplets as an unaffected one. Because a bunch of smaller droplets have more aggregate surface area than fewer, larger droplets, the clouds polluted with aerosols are more reflective, and thus more effective at repelling sunlight.

Between 1958 and 1988, the amount of solar radiation reaching the Earth's surface declined by 10 percent, an estimate that scientists arrived at by studying readings from more than 1,600 pyranometers worldwide. But since 1991, when the eruption of Mount Pinatubo caused the Earth to get much dimmer for about two years, there has been an overall brightening trend. In fact, according to NASA's Global Aerosol Climatology Project, worldwide aerosol levels in 2005 were 20 percent off their late 1980s peak (though we're not quite to pre-1958 levels yet). It's not entirely clear if this decline is due to mankind's efforts, particularly the campaign against the emissions responsible for acid rain. Next year's Glory satellite mission should clear things up; the spacecraft is equipped with a sensor that can differentiate between anthropogenic and natural aerosols. If it turns out that we've been somewhat effective at controlling aerosols, hey, let's call it a minor environmental victory.

Yet the recent brightening trend isn't necessarily cause for celebration. Though dimming can severely harm crops—less sunlight means less evaporation, which in turn means less rain—it may also mask the most noticeable effects of global warming. Some scientists argue that by reducing the amount of solar radiation hitting the Earth's surface, the long postwar dimming trend helped keep the planet cooler than it should have been, given the simultaneous escalation in greenhouse gas emissions. Indeed, since the world has gotten brighter in recent years, temperatures have risen; the six warmest years on record since 1850 have all occurred since 1997.

While the world may be getting brighter overall, two rising economic superpowers are actually getting darker. Over the last half-century, the amount of sunlight hitting each square yard of Chinese soil has declined by 3.7 watts; India has experienced a similar decrease in solar radiation. This could be attributable to the two nations' surge in aerosol-producing economic activity, which hasn't yet been accompanied by meaningful regulations meant to control emissions. If China and India crack down on their soot output, however, the attendant brightening could result in even higher average temperatures.

Still, getting rid of that nasty Asian brown cloud, a huge mass of aerosols that currently hovers over a chunk of the continent, has to be a good thing—not only because it's allegedly wreaking lethal havoc on weather patterns, but also because it actually causes warming at high altitudes. Though aerosols may mask warming at the planet's surface, recent research shows that the brown cloud is heating up the lower atmosphere from approximately 6,500 to 16,500 feet. That's bad news for the Himalayan glaciers, which are melting at a distressingly rapid clip.

Brendan I. Koerner is a contributing editor at Wired and a columnist for Gizmodo. His first book, Now the Hell Will Start, is out now.

  Slate Plus
Slate Picks
Nov. 21 2014 1:38 PM What Happened at Slate This Week? See if you can keep pace with the copy desk, Slate’s most comprehensive reading team.