is hampered by imprecision in our knowledge of past climate and past greenhouse gas concentrations. The principal known processes involved in Earth System Sensitivity are:
The carbon cycle, including ocean carbon uptake and release, terrestrial carbon uptake or release, and release of methane by destabilization of clathrates stored in permafrost or in sea-floor sediments
Major land ice sheets (such as those of Greenland and Antarctica)
Vegetation changes affecting albedo and the hydrological cycle
Changes in atmospheric chemistry that may affect aerosol formation and methane concentration
Changes in atmospheric dust loading
Because the climate system is being pushed into uncharted territory without any precise past analogue, it is possible that the Earth system is subject to additional as-yet unidentified feedbacks. Although all of the above feedbacks have been implicated in past climate changes (as reviewed, e.g., in Lunt et al., 2010), the following discussion will focus on the first two.
As net cumulative CO2 emissions increase, the amount by which the global temperature exceeds the peaks of the past 2 million years increases. Moreover, the length of time over which the climate is substantially warmer than previous interglacials becomes longer, allowing more time for slow components of the climate system to respond. The very long-term human imprint on climate can be assessed by computing the warming remaining after many centuries, taking into account only the climate sensitivity applied to the CO2 remaining after allowing for uptake of carbon emissions by land and ocean. The resulting warming would be affected further by the additional feedbacks involved in Earth System Sensitivity, but examining the basic long-term warming gives an indication of the magnitude of climate change upon which these feedbacks act.
The uncertainty in the future course of climate is affected both by uncertainties in climate sensitivity and uncertainties in the carbon cycle. The joint effects of these uncertainties are presented in Figure 6.1. Some of the carbon cycle models included in the calculation sequester a moderate amount of carbon in land ecosystems during the early centuries, but none produces a significant long-term carbon release from land or marine sedimentary carbon pools. The effect of such a release would need to be taken into account by explicitly adding it in to the cumulative emissions directly produced by fossil fuel burning and land-use changes.