FUTURE CLIMATE CHANGE - The Climate Casino - page 44

temperature increases predicted by the models is a little above 3°C,
which is almost double the transient response. The transition to equilib-
rium proceeds slowly, over many centuries. 9 The long-run warming is so
gradual because the deep oceans warm slowly. This great inertia contrib-
utes to the diffi culty of projecting temperature increases and climate
change. As with cigarette smoking, it may take a long time to see the
effects. Luckily, this slow response has a happy side. If today's rising con-
centrations of CO 2 are reversed relatively quickly, then temperature will
also come down because the deep oceans have not yet warmed.
Many nonscientists look at the divergence among climate models
and wonder why these uncertainties cannot be resolved. There is a joke
about economists: “If you ask fi ve economists, you will get six an-
swers.” This is actually true here because some climate models get dif-
ferent answers as they are refi ned over time.
There are valid reasons for these differences. The basic greenhouse
effect described above is well understood and has relatively little uncer-
tainty. The major uncertainties about the magnitude of the tempera-
ture increase come when modelers include additional factors that can
dampen or amplify the basic effect. For example, if a warmer earth melts
snow and ice, this will expose more land and ocean surface, making
the earth darker. The darker surface absorbs more sunlight, which then
warms the surface and amplifi es the greenhouse effect. This is known
as the “albedo effect” and is exactly like the effect of your car becoming
darker.
The most important amplifying factor is the greater evaporation of
water with higher temperatures, which leads to increased water vapor
in the atmosphere. Remember that water vapor is a powerful GHG.
Clouds are another important contributor to uncertainty. Clouds are a
headache for modelers because they can both cool and warm: Clouds can
cool the globe when they refl ect sunlight back into space or warm it
when they trap heat radiating from the earth's surface. Modeling cloud
formation is extremely challenging and produces a substantial amount
of the difference among models.
Climatologists have estimated that if there were no feedback effects,
the global warming from doubling CO 2 would be relatively small, about