Geoscience Reference
In-Depth Information
“fast feedback processes”—those involving the direct effects of increas-
ing concentrations of GHGs and the associated rapid feedback, such as
changes in water vapor, clouds, and sea ice. These are slow by economists'
standards, as they occur over a few hundred years rather than in a few
minutes or months, but they are fast by the standards of earth scientists.
However, there are also likely to be “slow feedback processes” that
would amplify the effects of global warming. The slow processes in-
volve ice sheet disintegration, the migration of vegetation, and acceler-
ated releases of GHGs (such as the frozen methane just discussed) from
soils, tundra, and ocean sediments, as well as decomposing vegetation.
For example, as glaciers and ice sheets melt, or as the spring snows melt
earlier, the earth becomes darker. This leads to a lower albedo (refl ec-
tivity), which in turn further warms the earth.
Some model calculations suggest that, when the slow feedback pro-
cesses are included, climate sensitivity may be twice as large as that
calculated by the current suite of climate models. That is, the long-run
sensitivity to CO
2
doubling might be as high as 6°C instead of the stan-
dard 3°C found in most models today.
8
While this is a very frightening prospect, it has yet to be validated
by multiple models. Furthermore, it applies over a time span of hun-
dreds to thousands of years. We probably have time to understand and
react to these slow feedback processes, so they may be less alarming
than would appear at fi rst blush. Careful modeling of the economic,
emissions, and longer-term climate models will be necessary to deter-
mine how central these slow feedback processes are to decisions about
climate policy.
The four global-scale tipping points discussed above are easily visu-
alized and dramatized. Many marine scientists believe that a less dra-
matic but equally important tipping point has already been passed. The
combination of rising CO
2
concentrations and warming is likely to cause
catastrophic loss of coral reefs along with major impacts on the systems
that depend upon them.
Although coral reefs represent a small fraction of the oceans, they
are extremely productive in nourishing marine life. Scientists estimate
