SO2 only helps north America---causes differential warming which is more destructive

But opposite in effect. Is the aerosol umbrella, then, a mandate to do nothing about global warming? Or to do nothing about reducing sulfur emissions? In a word, Charlson says, no. To him, the notion that humanity could fine-tune a system as big and complex as the climate is laughable. "There's always this temptation to tell ourselves we can handle it, that we're bigger than it is," he says. "Personally, I find that attitude very arrogant. It assumes that we understand climate well enough to engineer it, and we don't." Some of Charlson's findings about the parasol effect suggest that it won't help at all with some serious aspects of the global warming problem, such as rising sea levels. Sulfate aerosols may even make some warming effects worse, Charlson says. The reasons lie in the fundamental difference between greenhouse gases--which rise to the stratosphere and cover the globe--and sulfates, which travel only a few hundred miles. Because sulfates have such a limited range, almost all man-made aerosols are floating above the Northern Hemisphere, where 90 percent of industrial activity is still concentrated. By contrast, the Southern Hemisphere gets almost no such "protection" from man-made sulfates. Even in the relatively clean air of Seattle, Charlson says, "the amount of light scattered by haze is probably 10 to 100 times higher than it is in the Southern Hemisphere." With one hemisphere bearing the full brunt of global warming while the other is protected by an umbrella of pollution, he says, seas would still rise uniformly all over the globe, as the warmer southern waters expand. In other words, sulfates can't save the Maldives, the low-lying island nation in the Indian Ocean. But a rise in sea levels, Charlson says, might not be the biggest effect to worry about. Much more important, he points out, could be the increased difference in temperature between the two hemispheres. That's likely to affect the large-scale weather systems on which people depend. "More frequent occurrence of drought is a possibility," Charlson says. "Or of violent storms. Or the opposite--less frequent storms. I'd give either chance equal billing. The thing people need to understand is that a slight regional shift in any direction is a big concern. Last year in the mountains around Seattle we had more precipitation as rain and less as snow than normal. And the snowpack is our reserve of water that fills the reservoirs in late spring. So just because the balance of snow to rain changed, we had a drought here."