CO2 doubling could lead to 6°C climate change

It has been commonly thought that CO2 doubling could lead to a 3°C increase. Hansen’s recent (2007) article looks at both current climate forcings and the paleoclimate record to conclude that "long-term" sensitivity is 6°C for doubled CO2. http://www.columbia.edu/%7Ejeh1/East-West_070925.pdf From the report: Which climate sensitivity is more relevant to humanity: the Charney 3°C for doubled CO2 or the ‘long-term’ 6°C for doubled CO2? Both. The net human-made climate forcing, including negative forcing by tropospheric aerosols, has been substantially positive only for the past three decades. On that time scale the Charney sensitivity is a good approximation, as little contribution from slow feedbacks would be expected. Thus climate models with 3°C sensitivity for doubled CO2, incorporating only the fast feedbacks, are able to achieve good agreement with observed warming of the past century. We suggest, however, that these models provide only a lower limit on the expected warming on century time scales due to the assumed forcings. The real world will be aiming on the longer run at a warming corresponding to the higher climate sensitivity. Note that the 6°C sensitivity for doubled CO2 applies to the Pleistocene. About half of that sensitivity is from the ice sheet albedo feedback. At earlier times in the Cenozoic, between 65 and 35 My BP when there was little ice on the planet, the sensitivity should have been closer to the Charney 3°C sensitivity. Elsewhere (Hansen et al. 2007a) we have described evidence that slower feedbacks, such as poleward expansion of forests, darkening and shrinking of ice sheets, and release of methane from melting tundra, are likely to be significant on decade-century time scales. This realization increases the urgency of estimating the level of climate change that would have dangerous consequences for humanity and other creatures on the planet, and the urgency of defining a realistic path that could avoid these dangerous consequence.