Environmental Engineering Reference
In-Depth Information
FIGURE 4.7 A schematic of the precipitation P (black) and evapotranspiration E (red) and P-E (blue) av-
eraged around latitude circles. Solid lines indicate the unperturbed climate, and dotted lines a warmer
climate. E increases slightly but is overwhelmed, on average, by the increase in magnitude of P-E, which is
controlled by the magnitude of the horizontal water vapor fluxes.
The increase in precipitation in subpolar latitudes, for example, is due to
an increase in the flux of vapor into these latitudes and not primarily to an
increase in evaporation.
This picture is conservative, in that it assumes that changes in the at-
mospheric circulation are small so that the fluxes of vapor are dominated
by the increases in the vapor itself and not in the winds carrying the vapor.
This assumption has to be modified to understand model projections in more
detail (see Section 4.4).
In the tropics the circulation is primarily driven by the heat released
where water vapor condenses, so one cannot assume that the tropical winds
will remain the same as water vapor increases. Partly due to the complexity
of the interactions between winds and rainfall in the tropics, model projec-
tions for changes in rainfall are not as robust as in other areas. Some models
predict dramatic drying of the Amazon, with important consequences for the
forest and the global carbon cycle, but other models do not. Some models
