It is convenient to consider potential climate change in terms of
Earth's radiation balance. In round numbers, and averaged over the
whole surface of the globe, Earth intercepts 340 W/m
of
shortwave (visible) solar radiation. About 100 W/m
on average
is reflected back to space, having no further effect on
terrestrial climate. Much of this reflection is by clouds. The
energy budget is balanced by emission to space of 240 W/m
of longwave (infrared) radiation by ground, clouds, and
atmosphere. Measurements from space and radiative transfer
calculations [e.g., Kuhn et al., 1989; Raval and Ramanathan,
1989] have
shown that the relationship between outgoing longwave radiation and
surface temperature is approximately linear on Earth today, with
a sensitivity of 2 W/m
per degree C.
Calculations of the greenhouse contributions of gases in Earth's
atmosphere [e.g., Raval and Ramanathan, 1989] show that water
vapor contributes about 80 W/m
at Earth's average surface
temperature, and all other gases, mostly carbon dioxide, about
40 W/m
. The greenhouse effect of water vapor is more than twice
that of carbon dioxide. Water vapor warms the Earth by approximately
40
C. Paleoclimatologists have tended to assume that atmospheric
water vapor is controlled by temperature so that, for example,
relative humidity has remained constant. Zonally-averaged data for
the present [ Peixoto and Oort, 1992] show that relative humidity is
indeed more or less independent of latitude, but is a strong function
of altitude, decreasing from 75% near the surface to about 35% in the
upper troposphere. Cloud and precipitation processes cause the
decrease with altitude as well as the undersaturation of average air. We
do not understand these processes well enough to assert that
the distribution of relative humidity has not changed with time, so those
of us interested in past climates may be making a mistake by
concentrating on carbon dioxide to the exclusion of water vapor as a
cause of climate change.