Global Dimming

GLOBAL DIMMING….This isn’t new, but it’s new to me and maybe to you too. Did you know that in the three days following 9/11 the average temperature range across the United States (the difference between the daytime high and the nighttime low) rose one degree Centigrade? That’s the biggest, fastest climate change ever observed.

The reason, it turns out, is that American airspace was shut down, and no airplanes means no contrails. Since contrails absorb sunlight, getting rid of them allows more sunlight to reach the ground and causes a rise in surface temperature. When planes started flying again temperatures went back down.

As this BBC report says, this is a dramatic example of an effect called Global Dimming, something that scientists have recently concluded is far larger than they previously thought: since 1950, increased amounts of soot and ash have reduced the amount of sunlight reaching the surface of the earth by an astonishing 10-30% in various parts of the world.

So shouldn’t this cause global surface temperatures to decline considerably? Normally yes, but we’ve avoided this problem because during the same period greenhouse gases have been trapping ever more heat than before. These two effects cancel each other out, but greenhouse gases have been winning the race: overall surface temperatures have risen about .6 degrees Centigrade in the past century.

But here’s the bad news: this means that the effect of greenhouse gases on global warming is probably stronger than we’ve previously thought. The only reason global temperature increases have been fairly modest so far is because of the cooling effect of global dimming.

So what happens if particle pollution is brought under control ? via cleaner burning coal technologies, for example? It means that suddenly greenhouse gases will have no competition, and instead of temperatures rising only moderately, they’ll start skyrocketing.

In other words, our current models, which assume that climate has only a moderate sensitivity to greenhouse gases, might have been fooled by the countervailing effect of particle pollution. Once particle pollution levels flatten out or decline, we may find that climate is far more responsive to greenhouses gases than we thought.

Cleaner burning cars, anyone?