About that AGW

BLUF:  The climate is not nearly as sensitive to CO2 levels as believed by the AGW TRUE BELIEVERS.

One of the key measures of the impact of atmospheric carbon dioxide is called the climate sensitivity, which provides an estimate of how much the planet will warm in response to a doubling of the CO₂ concentration. This figure has been estimated using a variety of methods, producing a range of values; the Intergovernmental Panel on Climate Change estimates that the most likely value is 3 Kelvin, but recognizes there's a reasonable chance it could range anywhere from 2.4-4.5K. A new study that uses a climate model to evaluate the peak of the last glacial period, however, suggests that the IPCC's figure might be a bit high, and that very high values are overwhelmingly unlikely.

To look at climate sensitivity, the authors short-circuited the actual role of carbon dioxide, and simply changed its impact by adjusting the amount of infrared radiation that escapes through the atmosphere (carbon dioxide acts by trapping this radiation). Each of the 47 different models has a different value for this escaping radiation, and so models different levels of greenhouse gas impact.

This approach let them set a number of limits on the climate sensitivity. For example, model runs where it was too low keep the planet warmer than it was at the LGM. In other words, if the contribution of reduced CO₂levels is too small, the changes in the remaining forcings aren't enough to trigger a deep glaciation. In the same way, high climate sensitivities produce an extremely cold planet, far colder than the LGM. In fact, climate sensitivities above 6K trigger a global glaciation, or snowball Earth—something that has happened in the past, but not for over half a billion years. "Our model thus suggests that large climate sensitivities cannot be reconciled with paleoclimatic and geologic evidence, and hence should be assigned near-zero probability," they conclude.

http://arstechnica.com/science/news/2011/11/study-of-the-last-glacial-maximum-suggests-lower-climate-sensitivity.ars