Modeling the Climate Under Global Warming

While computer models churn out bleak forecasts for the planet's future, we also have a more conceptual understanding of what is happening as humans pump carbon dioxide into the air. But the conceptual understanding of carbon dioxide wrapping the planet in a blanket that traps more heat is not quite right.

A new study from the University of Washington and the Massachusetts Institute of Technology hopes to complete the understanding of what happens to the planet under climate change. Instead of carbon dioxide, or CO2, creating a blanket to slowly warm the planet, a paper this week in the Proceedings of the National Academy of Sciences shows the story is a little more complicated - though the ending is, unfortunately, the same.

"This is a neat study in that it changes the way we think about the climate system," said lead author Aaron Donohoe, a postdoctoral researcher at MIT who is now beginning as a research associate at the UW's Applied Physics Laboratory. "We looked at processes that are well captured in the models, but the conceptual understanding of how they work hasn't been fleshed out before."

When CO2 is first added, it does act as a blanket, trapping long-wave infrared energy coming off the Earth. The atmosphere then emits less of this long-wave radiation to space because the upper atmosphere is cooler than the Earth's surface, just as the top of your blanket is cooler than your body. But the Earth gradually heats up under this blanket, and hotter objects emit more long-wave radiation, so within about a decade the effect of adding the thicker blanket has been canceled by the warmer body emitting more energy.

So what keeps the planet warming after the first decade? In the longer term, the study shows that the Earth begins to absorb more shortwave radiation - the high-energy rays coming directly from the sun.

Previously people had shied away from talking about shortwave radiation because clouds can reflect this visible light back to space, and clouds remain one of the big unknowns under climate change.

Regardless of what happens to clouds, these researchers say, the planet is likely to have less ice and the air will become more humid under climate change, both of which will act to absorb more shortwave radiation from the sun. Those effects will be like putting tanning oil on the planet, letting it absorb more of the sun's incoming rays.

link.