James Wright at Skeptical Science has constructed a summary of some recent work done by James Hansen and colleagues. Climate sensitivity is the temperature change caused by a doubling of CO2. In this post I referred to a paper by Hansen et al, Earth’s energy imbalance and implications. Wright is working from a paper by Hansen and Sato entitled Paleoclimate implications for human-made climate change which contains much of the same material, including Table 1 on p16.
The bottom line goes like this:
The exact value of climate sensitivity depends on which feedbacks you include, the climate state you start with, and what timescale you’re interested in. While the Earth has ice sheets the total climate sensitivity to CO2 is up to 8°C: 1.2°C direct warming, 1.8°C from fast feedbacks, 1°C from greenhouse gas feedbacks, and nearly 4°C from ice albedo feedbacks. The slow feedbacks have historically occurred over centuries to millennia, but could become significant this century. Including CO2 itself as a feedback would make climate sensitivity even higher, except for the weathering feedback which operates on a geologic timescale. (Emphasis mine)
CO2 alone, not CO2e, of 450ppm is likely to give us an ice-free planet – eventually.
I take it that the stability we’ve had during the Holocene is unusual. Upset the balance with a bit of the trace gas CO2 and the system can go wild. Continue reading Climate clippings 42