A recent proposal to control global warming is to release sulfur droplets into the atmosphere, which would hopefully block out some of the Sun's rays. Venus has been running an eons-long simulation of just that plan...and it doesn't look good.
Back in 2008, the Venus Express probe detected a layer of sulfur dioxide about 100 kilometers above the planet's surface. Its presence was shocking to scientists - although Venus has plenty of sulfuric acid clouds at 50 kilometers formed by the mixing of water vapor and sulfur dioxide, anything above that was thought to be ripped apart by the Sun's fierce radiation. So what was this layer doing there?
Computer simulations have now figured out the answer. At high enough altitudes, the sulfuric acid droplets can occasionally evaporate, after which they would drift upwards and be broken into its constituent parts by the Sun. This would release sulfur dioxide, accounting for the layer of sulfur dioxide at such a high altitude.
Here's where global warming enters the picture. Nobel Prize winner Paul Crutzen has recently argued we should inject large quantities of sulfur dioxide into Earth's atmosphere at about 20 kilometers above the planet's surface. He bases this idea on observation of the areas around volcanoes, where ejected sulfur dioxide forms a layer of haze that reflects some of the Sun's rays. Spread over the whole planet, sulfur dioxide could cool the entire Earth by about 0.5 degrees Celsius, not an insignificant difference.
The problem is that that assumes the sulfur dioxide remains hazy and Sun-deflecting for a long time. We don't know that for certain, however. If the sulfur dioxide reacts with water vapor too quickly, then it'll change into sulfuric acid droplets, which will be clear and so not block out any of the Sun's rays. It's also probably not a fantastic idea to have unnaturally large quantities of sulfuric acid in the atmosphere, either.
What's great is that we've been given an opportunity to see how the idea works before testing it out on our planet. It's an unusual case where another planet's chemistry actually lines up pretty much perfectly with something we want to do with our own planet. Of course, it's not exactly encouraging that we want to make our atmosphere more like Venus's, and the Venus's sulfur cycle suggests the gas might turn clear too quickly for the plan to work here. Still, at least we're finding this out now before potentially pumping lots of sulfuric acid into the atmosphere.