The implied climate sensitivity, 3/4°C per W/m2 (equivalent to 3°C for doubled CO2), is consistent with climate model estimates, but of greater precision. We can never be certain that climate models accurately include all relevant processes. But we know that the real world included all changes of clouds, water vapor, sea ice and any other such fast "feedbacks" that exist.

It must be recognized that the specific climate sensitivity derived in this way includes only "fast" feedbacks. We call this the Charney climate sensitivity, because it is essentially the case considered by Charney (1979), in which water vapor, clouds and sea ice were allowed to change in response to climate change, but GHG (greenhouse gas) amounts, ice sheet area, sea level and vegetation distributions were taken as specified boundary conditions. We would expect the Charney climate sensitivity to be most relevant on decadal time scales. On longer time scales as the quantities assumed to be fixed can change in response to climate change, thus becoming powerful climate feedback mechanisms.

Important insight emerges from close examination of the temperature and forcing curves: the temperature change leads the forcings by several hundred years. Thus the greenhouse gas and ice sheet changes, although they are the principal direct mechanisms for the climate change, are changing as feedbacks. The pacemaker and instigator of the changes is cyclic variation of the Earth's orbit (Hays et al. 1976), which alters the seasonal and geographical distribution of solar radiation. Insolation changes by a significant amount over several thousand years.

Variations of atmospheric CO2 occurring as a climate feedback on the time scale of the ice ages (Figure 3) can be ~100 ppm in 5000 years, or 0.02 ppm/year. This atmospheric change is due to a shifting of carbon among the atmosphere, ocean, soil and biosphere compartments within the surface carbon pool, a warmer climate driving more CO2 into the air. This natural glacial-interglacial variation of atmospheric CO2 is quite rapid in comparison with the geologic cycling of carbon between the Earth's crust and the surface carbon pool, which amounts to ~10**(-4) ppm/year of CO2, as discussed above.

These natural rates of atmospheric CO2 change must be compared with the human-caused growth of atmospheric CO2, which is now ~2 ppm/year (see below). Humans, indeed, are now in control of long-lived atmospheric GHGs. As a result it is important to investigate climate sensitivity for the case in which GHGs are specified as the forcing. The Charney climate sensitivity applies to this case under the assumption that slow feedbacks such as ice sheet area, vegetation distribution, and climate-induced GHG changes are not allowed to operate.

As a complement to the Charney climate sensitivity, let us derive the climate sensitivity that applies if these slow feedbacks are allowed to operate: we call this the "long-term" climate sensitivity. We can obtain this "long-term" climate sensitivity from paleoclimate data by finding the scale factor that causes the GHG forcing to match the paleoclimate temperature change as accurately as possible. Figure 4 shows that multiplying the climate forcing due to long-lived GHGs (CO2 + CH4 + N2O) by 3.02°C per W/m2 yields remarkably good agreement with Antarctic temperature. Given that glacial-interglacial global temperature change is about half of Antarctic temperature change, this implies a "long-term" climate sensitivity of ~1.5 W/m2 or about 6°C for doubled CO2.

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.

This post was created for ClimateProgress.org, a project of the Center for American Progress Action Fund.