Environmental Engineering Reference
In-Depth Information
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 CO
2
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 ap-
plied to the CO
2
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 sedi-
mentary 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.
