FUTURE CLIMATE CHANGE - The Climate Casino - page 41

we can actually calculate temperatures on the moon, and these fi t the
lunar reality reasonably well.
The “enhanced greenhouse effect” is what happens when humans
enter the picture and add more GHGs. The current stock of atmospheric
GHGs absorbs some but not all of the outgoing long-wave radiation.
As more and more gases are added, an increasing fraction of the outgoing
long-wave radiation is absorbed by the atmosphere, and this in turn
pushes the planetary temperature equilibrium higher. The process of
CO 2 -induced global warming means that humans are adding more “blan-
kets” to the atmosphere in the form of additional CO 2 , thereby increasing
the average temperature on the earth's surface. Increasing the atmo-
spheric concentration of CO 2 by what seems a tiny fraction (from about
280 to 560 ppm) is projected to increase average surface temperature of
the planet by around 3°C (5 1 ⁄ 2 °F).
The enhanced greenhouse effect has diminishing returns, however.
As CO 2 blocks more and more of the outgoing radiation, adding yet
more CO 2 has a smaller impact. The capacity to absorb outgoing radia-
tion gradually becomes saturated. Hence, doubling the amount of CO 2
in the atmosphere might raise the temperature by 3°C, but adding the
same quantity again might lead to an increase of only 1.8°C.
The exact pace and extent of future CO 2 -induced warming are
highly uncertain, particularly beyond the next few decades. However,
there is little scientifi c doubt that humans are causing major geo-
physical changes that are unprecedented for the last few thousand
years. Scientists have detected the results of the changes in several
areas. Emissions and atmospheric concentrations of GHGs are rising,
as we just saw. The average surface temperature is also rising. Other
“fi ngerprints” are also evident, including warming oceans, melting
glaciers and ice sheets, enhanced polar warming, stratospheric cool-
ing, and a shrinking ice cap in the Arctic Ocean. 5 Most of these are
consistent with a warming that is induced by GHGs rather than natu-
ral variability.
The idea of sitting in a black car or a white car on a hot summer day
makes a useful analogy to CO 2 warming. The white car will refl ect
more of the sunlight and remain relatively cool, while the black car will