Climate Change Processes - Climate Dynamics - page 171

One example is a feedback involving the naturally occurring CCN dimethyl
sulfide, or DMS, which is released into the atmosphere at the ocean surface by
phytoplankton. When phytoplankton growth responds to changes in climate
conditions, DMS emissions and cloud amounts or distributions could be ef-
fected. This feedback is the kind of process sometimes discussed in connection
with the Gaia hypothesis (section 2.5).
The indirect effect of aerosols also includes changes in cloud albedo through
modifications of cloud droplet size distributions, liquid water content, cloud
height, and cloud lifetimes. The variety of aerosol types associated with human
activity, as well as their complicated distributions in space and time, makes it
difficult to evaluate these indirect effects on climate.
11.3 EXTREME HYDROLOGIC EVENTS
Many projections of future climate due to increases in greenhouse gases sug-
gest that the global hydrologic cycle will intensify in the warmer climate and
result in increased incidence of severe rainfall events, flooding, and drought.
These projections are fairly robust in the sense that many different models
under various assumptions about future emissions are in general agreement,
but the projections may not apply regionally. Some models also predict that
decreases in global- and annual-mean rainfall frequency will accompany the
increases in the intensity of precipitation events.
Our physical understanding of how and why rainfall intensity may increase
as climate warms derives from a scaling argument based on the Clausius-
Clapeyron equation (Eq. 2.8). Recall that this equation expresses the relation-
ship between the temperature of an isothermal, closed system consisting of
moist air overlying a flat, quiescent water surface and the amount of water
vapor in the overlying air. As the temperature of the system increases, evapora-
tion increases, and more water vapor enters the air. When the rate of conden-
sation onto the surface equals the evaporation rate, the saturated atmosphere
will contain more water vapor at the new equilibrium.
Although the Clausius-Clapeyron equation is strictly applicable only to the
closed system for which it is derived, it is often applied less formally to climate.
For temperatures typical of the earth's surface, the saturation partial pressure
of water vapor, e s , increases exponentially with temperature (see Fig. 2.32) —
about 7% for every 1 K warming. Because relative humidity is predicted to
remain constant as climate warms, we can expect that the partial pressure of
water vapor will increase exponentially, as will the specific humidity, q . Ac-
cording to Eq. 9.8, an increase in q leads to an increase in the column moisture
convergence if the circulation remains unchanged. Of course, the circulation
will not remain unchanged as the global climate warms, and this fact intro-
duces uncertainty in projections of intensified precipitation and also suggests
that changes in precipitation intensity will be regional.
Projections of increases in the occurrence of drought can be confusing
because these events are often not precisely defined. There are four general
types of drought, namely, meteorological, hydrological, agricultural, and so-
cioeconomic. Meteorological drought refers to extended periods with negative