PBS' Nova is running a new episode on a phenomenon they've dubbed, "Global Dimming"—the reduction of the sun's intensity due to atmospheric pollution. On its surface this might seem like a perfectly obvious consequence of pollution, but, as the documentary demonstrates, scientists are making astonishing discoveries about its true scope.
Via a clever experiment in the Maldives, researchers were able to measure the actual reduction caused by human pollution. They expected to find something on the order of a half of one percent. Instead, they discovered man-made pollution caused as much as a ten percent reduction in the intensity of the sun's rays reaching the ground.
How, you ask? First, plainly, simply putting particulates in the air blocks sunlight. But—and here's the tricky part—pollution particulates tend to be much smaller than ordinary dust in our atmosphere. Consequently, they create clouds (as water vapor condenses around the particulates) that are vastly more reflective than ordinary clouds. These pollution-enhanced clouds have a profoundly greater cooling effect on the Earth below.
Now wait. I know what Fox News is going to say: (please imitate Brit Hume's gravely voice) 'Scientists have discovered that man-made pollution may actually be making the world cooler, casting further doubt upon the alarmist hypotheses espoused by Global Warming enthusiasts.'
And now, let us return to reality. Yes, Global Dimming is indeed acting to cool the Earth. But (and this is the scary part) it is in a losing tug-of-war with the warming effects of Greenhouse Gases, and it has not been factored into the climate models that are currently being used to predict future warming.
In other words, take away Global Dimming, and we would be much warmer than we already are. This has profound implications for the future, especially as nations work to reduce particulate pollution, which is especially deadly to humans. North America and Europe, for example, have made significant gains in battling particulate pollution.
By studying the atmosphere in the days following 911, when nearly all commercial air flights were grounded, scientists got a preview of what Global Warming might look like without the mitigating effect of Global Dimming. Air planes create contrails: clouds produced by condensation around pollution particulates. Without these contrails criss-crossing the sky scientists observed the greatest 3-day spike in temperature gradients across the U.S. in 30 years.
Based on these observations, some scientists believe that Global Dimming is currently masking fully 50% of Global Warming's true potential. In other words, as we succeed in reducing particulate pollution, we may unleash the full potential of Global Warming: a future shockingly warmer (up to 15 degrees Celsius) than any currently predicted.
That possibility would lead to an Earth vastly different from the one we know today. The entire state of Florida would be underwater, as would all of New York City. The world's forests would vanish. Farmlands would become deserts. Ice Sheets would melt, as would methane fields, further reinforcing the warming effect. If you want a doomsday scenario, this is it.
I expect we'll see considerable debate about these new findings in the coming years. If the climate models are currently undervaluing the effects of greenhouse gases in our atmosphere, it would mean our window of opportunity to change course is much smaller than we imagine. There is much more to this subject than I've touched on here. For more information, see pbs.org's page on Global Dimming.