James Hansen is also getting back into the climate sensitivity fray, posting up an Arxiv preprint that (surprise, surprise) comes up with a much more alarming figure than Lewis or Masters. The estimate is based on paleoclimate data, specifically δ18O data for foraminifera (a class of microscopic animals that got a mention in the Hockey Stick Illusion). However, as has often been noted in the past, these paleoestimates of climate sensitivity are fraught with difficulty as the quality of data on temperatures and forcings in the distant past is shaky indeed. Hansen alludes to these difficulties in his abstract, although his position seems to be that his new approach represents an improvement:
Cenozoic temperature, sea level and CO2 co-variations provide insights into climate sensitivity to external forcings and sea level sensitivity to climate change. Climate sensitivity depends on the initial climate state, but potentially can be accurately inferred from precise paleoclimate data. Pleistocene climate oscillations yield a fast-feedback climate sensitivity 3 ± 1°C for 4 W/m2 CO2 forcing if Holocene warming relative to the Last Glacial Maximum (LGM) is used as calibration, but the error (uncertainty) is substantial and partly subjective because of poorly defined LGM global temperature and possible human influences in the Holocene. Glacial-to-interglacial climate change leading to the prior (Eemian) interglacial is less ambiguous and implies a sensitivity in the upper part of the above range, i.e., 3-4°C for 4 W/m2 CO2 forcing. Slow feedbacks, especially change of ice sheet size and atmospheric CO2, amplify total Earth system sensitivity by an amount that depends on the time scale considered. Ice sheet response time is poorly defined, but we show that the slow response and hysteresis in prevailing ice sheet models are exaggerated. We use a global model, simplified to essential processes, to investigate state-dependence of climate sensitivity, finding an increased sensitivity towards warmer climates, as low cloud cover is diminished and increased water vapor elevates the tropopause. Burning all fossil fuels, we conclude, would make much of the planet uninhabitable by humans, thus calling into question strategies that emphasize adaptation to climate change.
You get a flavour of the difficulties from this summary of the methodology:
We hypothesize that the global climate variations of the Cenozoic...can be understood and analyzed via slow temporal changes of Earth's energy balance, which is a function of solar irradiance, atmospheric composition (specifically long-lived GHGs), and planetary surface albedo. Using measured amounts of GHGs during the past 800,000 years of glacial-interglacial climate oscillations and surface albedo inferred from sea level data, we show that a single empirical "fast-feedback" climate sensitivity can account well for global temperature change over that range of climate states.
And then when you read the detail it starts to hit you:
To clarify our calculations, let us first assume that fast-feedback climate sensitivity is a constant (state-independent) 3°C for doubled CO2 (0.75°C per W/m2). It is then trivial to convert our global temperature for the Cenozoic (Fig. 4a) to the total climate forcing throughout the Cenozoic, which is shown in Fig. S4a, as are results of subsequent steps. Next we subtract the solar forcing, a linear increase of 1 W/m2 over the Cenozoic era due to the Sun's 0.4% irradiance increase (Sackmann et al., 1993), and the surface albedo forcing due to changing ice sheet size, which we take as linear at 5 W/m2 for 180 m sea level change from 35 Myr BP to the LGM. These top-of-the-atmosphere and surface forcings are moderate in size, compared to the total forcing over the Cenozoic, and partially offsetting, as shown in Fig. S4b. The residual forcing, which has a maximum ~17 W/m2 just prior to 50 Myr BP, is the atmospheric forcing due to GHGs. Non-CO2 GHGs contribute 25% of the total GHG forcing in the period of ice core measurements. Atmospheric chemistry simulations (Beerling et al., 2011) reveal continued growth of non-CO2 gases (N2O, CH4 and tropospheric O3) in warmer climates, at only a slightly lower rate (1.7-2.3 W/m2 for 4×CO2, which itself is ~8 W/m2 ). Thus we take the CO2 forcing as 75% of the GHG forcing throughout the Cenozoic in our standard case, but we also consider the extreme case in which non-CO2 gases are fixed and thus contribute no climate forcing.
You need to know temperatures in the distant past, ice sheet sizes in the distant past, solar irradiance in the distant past, and greenhouse gas levels in the distant past. You wouldn't want to bet the house on the results would you?