Environmental Engineering Reference
In-Depth Information
Focusing on fixed seasons provides one snapshot into these projected
patterns. An alternative procedure, emphasizing the extent to which the
“dry get drier”, is to plot the precipitation changes in the driest 3 months
at each location, irrespective of calendar date (Solomon et al., 2009). This
alternative version is provided in the section, “Overview of Climate Changes
and Illustrative Impacts.” Local details differ, but the large-scale pattern is
unchanged.
Our confidence in this model-generated pattern is enhanced by an un-
derstanding of the simple underlying mechanism controlling its structure.
The starting point on which this understanding is based is the increase in
the saturation pressure for water vapor with increasing temperature. Most
of the water vapor in the atmosphere resides in the lowest 2-3 Km, where
the saturation vapor pressure increases at roughly 7% per 1°C of warming.
The ratio of water vapor in the atmosphere to this saturation value is re-
ferred to as the relative humidity. Especially near the surface, observational
trends (e.g, Trenberth et al., 2005) and simple physical arguments (Held and
Soden, 2006) consistently show that relative humidity cannot be expected
to change substantially on average, and certainly not enough to counterbal-
ance the increase in water vapor expected from the increase in saturation
vapor pressure. As a result, there is high confidence in the projection that
the total water vapor in the atmosphere will increase at roughly 7% per
1°C of warming.
If one averages around latitude circles, the hydrological cycle on Earth
can be pictured schematically as in Figure 4.7. Evapotranspiration is larg-
est in lower latitudes and decreases steadily as one moves poleward. Pre-
cipitation is larger than evapotranspiration in subpolar latitudes, and near
the equator, and less than evapotranspiration in the subtropics. Fluxes of
water vapor in the atmosphere are responsible for these local excesses and
deficits. The atmosphere is converging water into the regions of excess and
diverging water from the regions of deficit. As temperature increases and
the amount of water vapor increases, the divergence and convergence of
the water vapor flux increase proportionally, increasing the magnitudes of
these excesses and deficits This simple picture also explains the magnitude
of the increases and decreases in precipitation expected: because water
vapor increases by roughly 7% per 1°C, the atmospheric fluxes increase by
the same percentage, so that the pattern of precipitation minus evaporation
is also amplified by about the same factor.
The final element in the picture is that the changes in the global mean
hydrological cycle, the globally averaged precipitation and evapotranspira-
tion, increase more slowly than do the increase in these water vapor fluxes.
