What's Global Dimming?

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.

This entry was posted in News. Bookmark the permalink. Both comments and trackbacks are currently closed.


  1. Posted May 11, 2007 at 10:46 am | Permalink

    A great article about global dimming was posted on the Celcias blog the other day:


  2. stateoffear
    Posted January 30, 2008 at 6:51 pm | Permalink

    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

  3. Posted January 31, 2008 at 12:09 pm | Permalink

    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“).