Wednesday, March 30, 2005

Global Dimming

It appears that we're not as bright as we used to be. I don't mean that we're getting dumber, although that's certainly possible (and it would explain how you-know-who ended up in the White House). The world is simply not receiving as much sunlight as it used to and this "global dimming" may be temporarily mitigating the effects of global warming. This, at least, is the contention of climatologists whose studies were presented on the BBC science program Horizon. (The transcript of the program is available here.)

According to Dr. David Travis, an American climatologist, when air travel was suspended in the United States for three days after 9/11, the temperature range across the country increased one degree centigrade. In climate terms, this was a surprisingly large and sudden increase. Dr. Travis attributes the increase to the rapid decrease in the amount of water vapor (contrails) deposited by aircraft into the atmosphere.

What is the relationship between global dimming and global warming? Dr. Peter Cox addresses this question in the BBC program:

We've got two competing effects really, . . . the greenhouse effect, which has tended to warm up the climate. But then we've got this other effect that's much stronger than we thought, which is a cooling effect that comes from particles in the atmosphere. And they're competing with one another. And we know the climate's moved to a warmer state by about 0.6 of a degree over the last hundred years. So the whole thing's moved this way. If it turns out that the cooling is stronger than we thought then the warming also is a lot stronger than we thought, and that means the climate's more sensitive to carbon dioxide than we originally thought, and it means our models may be under sensitive to carbon dioxide.

In other words, we may be underestimating the true impact of global warming because of global dimming's cooling effect. If you're thinking this means we don't have to worry about global warming, then perhaps we should discuss the possibility that we're not as bright as we used to be.

(Via Kevin Drum.)