Don: Hey Yael, interested in a stock tip?

Yael: Uh, I guess . . .

D: Invest in companies that make sunscreen.

Y: Okay. Why?

D: Because atmospheric aerosol levels are decreasing.

Y: What's "atmospheric aerosol"?

D: Well, an aerosol is any small particle floating in the atmosphere, like water particles,      dust, and pollutants.

Y: And they have something to do with sunscreen . . .?

D: I was getting to that. See, aerosols in the atmosphere act as a sort of global      sunscreen. They scatter sunlight, reflecting some of it back into space. The less      sunlight reaching Earth, the less the earth warms up.

Y: So you're saying that atmospheric aerosol acts as a sort of counterbalance to global      warming created by greenhouse gasses?

D: Sort of. But as I said, scientists believe that, for various reasons, global aerosol levels      are going down. This could be due to legislation in Europe and other places to curb      air pollution. It's probably also partly due to natural causes. In any case, the point is      that, overall, more sunlight is coming through the atmosphere. And so it's brighter and      hotter.

Y: Hence investing in sunscreen. Okay, but wait . . . if atmospheric aerosol reflects      sunlight, why not put more of it back into the atmosphere as a way to decrease      global warming?

D: I thought of that, too, but the problem is that manmade aerosols tend to pollute the      atmosphere and cause asthma, allergies, and other health problems. The lesson, I      think, is that if we can significantly reduce the amount of aerosols in the atmosphere,      we can also reduce atmospheric greenhouse gases . . . which is the only real way to      curb global warming.  

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