Future prospects - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 336

content in the atmosphere. Lindzen provided a detailed discussion of several
aspects of the regional distribution of water vapor in the atmosphere and its rela-
tionship to global warming induced by increased CO 2 . Most climate models make
the assumption that relative humidity does not change with global warming and,
since warm air can hold more water vapor than cool air, a constant relative
humidity implies an increase in absolute humidity as the Earth warms. The basis
for the assumption that relative humidity does not change with global warming
lies in some rather old radiosonde data that indicate that the average distribution
of relative humidity (when plotted on altitude vs. latitude axes) does not change
much from winter to summer. The argument then goes that relative humidity
would also not change over the smaller temperature change characteristic of
global warming. However, Lindzen raised serious questions about the accuracy of
the radiosonde data. Clearly, the assumption of constant relative humidity rests on
a weak foundation, and that assumption is critical to the alarmist position that the
doubling of CO 2 produces unacceptable global warming due to increased absolute
humidity.
But Lindzen went further than this. He emphasized that the degree of water
vapor feedback as a heating force in any region depends on absolute humidity. In
desert regions with very low absolute humidity, an increase in humidity provides a
significant heating force. However, in regions with high absolute humidity, an
increase in humidity provides a very modest heating force (e.g., an increase in
relative humidity from 10 to 20% produces a forcing of 1.5W/m 2 , whereas an
increase in relative humidity from 50 to 60% produces a forcing of only
0.15W/m 2 ). Tropical regions that already have high humidity do not gain much
additional heating from an increase in humidity. Moreover, as Lindzen pointed
out:

''Given the nonlinearity of the radiative effect of water vapor, the average
radiative response to water vapor is not equal to the response of the average
water vapor.''

As already noted, a doubling of CO 2 implies forcing at the tropopause of
about 3.7W/m 2 . The question of climate sensitivity amounts to asking how much
must the Earth's surface warm to compensate for this forcing. This requires glob-
ally integrated total radiative flux at tropopause levels to be estimated. A global
change in the distribution of moist and dry regions can lead to a change in out-
going long-wavelength radiation (OLR) even in the absence of change in mean
temperature. Changes in circulation and changes in temperature can both play a
role in the moisture budget. Lindzen suggested:

the interesting possibility that the primary feedback process might
consist in the change in areal coverage of the very dry regions. Presumably,
natural variations include a full range of such possibilities so that observed ratios
of average temperature variations to variations in total OLR would show a
significant scatter.''

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