Global warming can be cool

These days, many people who hear the term "global warming" tend to wrinkle their noses and launch a lecture on responsibility. Of course, rapid human-caused climate change would be bad news for billions of people. But generally, global warming saves our butt every day. And, between 800 and 600 million years ago, runaway warming has cut short a particularly nasty sort of climate change: Snowball Earth.

Greenhouse gasses in the atmosphere - water vapor, carbon dioxide, ozone and methane - let sunlight pass on down to heat up the surface, but when the same amount of heat is reflected back at infrared frequencies, the gasses gobble it up and send much of it right back down to Earth. Be glad of this. Without that heat, the Earth would be too cold for liquid water to exist.

There is good geological evidence of several episodes of Snowball Earth - a time of cold and runaway glaciation that covered not only the continents with ice, but may well have frozen the entire surface of the oceans. Such episodes haven't happened since about 600 million years ago, perhaps because life forms like worms in marine sediments have churned up the seafloor, preventing the squestering of carbon and keeping it free to do its greenouse thing. (It is no surprise that fossiles of multicellular creatures don't exist until after this period.)

But how come the planet didn't just stay frozen? One leading theory is that as ice piled up, ocean levels dropped significantly. Seafloor, much of which lies atop huge deposits of methane, was exposed to erosion. Before long, methane (21 times as efficient, greenhousewise, as carbon dioxide) was streaming into the atmosphere. Runaway warming counteracted the runaway freezing.

This is not to say that humans ought to be messing with a planetary feedback system that's worked for billions of years. But think of global warming every time you enjoy a cool drink of water.