7 December 2011
It’s an environmentalist’s nightmare: What would happen if the amount of carbon dioxide doubled overnight?
Yutian Wu asked just that question. Using computer models, she injected a virtual world with a uniformly distributed layer of carbon dioxide, and then on January 1 of the model – digital doomsday – doubled the amount of the gas.
Wu and her fellow researchers of the Lamont-Doherty Earth Observatory at Columbia University in New York, NY, didn’t do this to capriciously cook a simulated Earth – they did it to tease out details of the complex interactions between CO2, heat, and winds.
The researchers are particularly interested in a phenomenon predicted by many Intergovernmental Panel on Climate Change (IPCC) models as a result of climate change: the shifting of certain atmospheric winds, called the tropospheric zonal jets, toward the poles. Various studies propose explanations for those zonal jets, such as increased wind speed and the stretching of eddies, which rely on warming above the subtropics in the upper troposphere, Wu said. The troposphere is the layer of the atmosphere closest to the ground.
“What causes the subtropical upper tropospheric warming in the first place?” she asked during a talk Monday at an afternoon atmospheric science session at the American Geophysical Union’s Fall Meeting. “Is it driven by the increase of latent heating in the atmosphere, which is usually assumed in the (literature)? Or is the upper tropospheric warming a consequence of the (atmospheric) circulation change?”
To find out, she used a virtual model since a larger disturbance makes it easier to see the dynamics.
“If you increase CO2 forcing gradually, like 1 percent per year,” she said in a follow-up interview, “It’s hard to see. It’s a small signal.”
“If you give it an instantaneous CO2 doubling, it has a large forcing. So we would like to see how the atmospheric circulation gradually adjusts to that forcing.”
In the first month of the virtual CO2-rich world, the tropical air in the upper troposphere warmed. In the following months, eddies converged and push the air to the subtropics, as well as down to lower altitudes.
“We showed that the subtropical upper tropospheric warming is a consequence of the circulation change, rather than the other way around,” she said in her talk. “So back to the question of what causes subtropical upper tropospheric warming, the answer is eddies.”
And even though the model may seem abrupt, Wu said that after it settled down to equilibrium, it was consistent with findings of an earlier, more gradual simulation by the IPCC.
And although they mainly focused on the troposphere, Wu’s team found surprising results in the layer above it, called the stratosphere. That layer cooled very fast in their model, dropping one to two degrees centigrade in a matter of weeks. Then the cool air circulated down to the troposphere.
“We didn’t expect anything to happen in the stratosphere,” she said. “It turns out to be very important.”
-Stephen Tung is a science communication graduate student at UC Santa Cruz