What abrupt warming did, by one estimate

The graph @39 merits attention. It is based on Lindzen and Choi 2009 0.15 oC/W/m2. The Lindzen and Choi 2011 that AustinGreen linked later says 0.30 oC/W/m2, and we’ll return to that in a moment.

The graph design emphasizes the large IR absorption that occurs at low (increasing) CO2 concentrations, and so is not ideal for our purposes. We want to know how much T will increase starting from 280 ppm CO2. Those bars are all very small in the graph, so I did my best effort with Paint (an image bit editor) to measure them. Then a spreadsheet for each 20-ppm wide bar.

It indicates increasing from 280 ppm to current 400 ppm corresponds to 0.45 oC. That does not agree well with the instrumental records of T increase during the corresponding time. However, doubled, according to Lindzen and Choi 2011, it does agree well.

So we might feel confident to usethat information to predict T increase associated with 280 -> 600 ppm. That number, doubled according to Lindzen and Choi 2011, is 1.66 oC

Whether that is what we’ll actually get at 600 ppm depends on matters other than absorption of IR by CO2. The most prominent seem to be ocean heat dynamics, mobilization of part of the methane reservoirs in high-latitude soils and marine clathrates, and cloud responses. High or low, thick or thin clouds can reflect incoming solar short-wave energy, or trap outgoing long-wave (IR), or both. Uncertainties in all 3 make the current coupled climate models…er…less than ideal in my opinion.