What's Global Dimming?
April 3, 2007 | Posted by Bill Chameides in Science - General
Last week, Vic asked about the phenomenon of "global dimming" and its effect on global warming. Good question. Here's the answer.
"Dimming of the sun" or "global dimming" is not due to anything happening to the sun. It's due to something happening on Earth that blocks the sun's rays. Global dimming is important from a climate standpoint because less sunlight reaching the Earth creates a cooling effect.
What causes global dimming? Most likely air pollution, in the form of tiny particles produced when we burn fossil fuels. Pollution particles block sunlight in two ways. First, they cause more water droplets to condense in the air, forming darker clouds that block light (see Columbia University article). Second, some airborne particles - particularly aerosols - reflect sunlight on their own.
The cooling effect of global dimming has significantly slowed global warming - ironic, since air pollution is by no means a good thing. From the Nova program, "Dimming of the Sun" on PBS: "… more than half the warming effect of our greenhouse emissions has been masked by the cooling effect of particle pollution."
The damage to health from air pollution prompted Congress to pass the Clean Air Act of 1970 (see history), and other countries, especially in Europe, have followed suit. As a result, air pollution has gone down, global dimming has decreased, and global warming has noticeably accelerated - particularly since 1990. (See "Impact of global dimming and brightening on global warming".)
The recent IPCC reports (see description of IPCC) say global warming is accelerating at a faster rate than many models predict. The reduction in air pollution, and therefore global dimming, may well be the reason. That said, many models now do account for global dimming, as noted in the RealClimate article "Global Dimming and climate models".
The phenomenon of global dimming has given some scientists an intriguing idea. If adding reflective particles can cool the planet, why not do it deliberately to counteract global warming? This strategy, called "geo-engineering", will be the topic of tomorrow's post.
4 Responses
Comment from scanter
May 11th, 2007 at 10:46 am
A great article about global dimming was posted on the Celcias blog the other day:
http://www.celsias.com/blog/2007/05/09/veiling-our-true-predicament-part-i-global-dimming/
Comment from stateoffear
January 30th, 2008 at 6:51 pm
I suppose "global dimming" is why environmentalists in the 1970s wanted to put ash on the poles so the ice would melt and the earth would warm up?
–The Vampiric State of Fear
Comment from Sheryl Canter
January 31st, 2008 at 12:09 pm
Actually, that's exactly right. There was a brief period when dimming of the Sun from coal particles had a stronger effect than the warming from CO2 emissions. Then the warming effect overtook the cooling effect (see our article on the global cooling scare).
And today, ash really is melting the ice on the North Pole (see our article "Global Warming from Soot").
Pingback from Climate 411 » Global Warming from Soot - Blogs & Podcasts - Environmental Defense Fund
April 22nd, 2008 at 11:25 am
[...] reduces the Earth’s reflectivity, or "albedo". Atmospheric soot also contributes to global dimming, which cools the Earth by screening out the Sun, but its heat-retaining effect is [...]
Leave a Reply
You must log in to post comments | Login | Register an account
User comments reflect the opinions of the responsible contributor only, and do not reflect the viewpoint of Environmental Defense Fund. We reserve the right to delete comments that may be considered offensive, illegal or inappropriate. We also reserve the right to delete duplicate comments, or comments that have no relationship to the original post.