The Water Cycle and Global Warming |
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Figure 8. The vapor pressure of water (in millibars) as a function of temperature (in degrees C).
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| One aspect of concern for the future is how the water cycle will
respond to global warming. This is dictated by the physics
of water vapor in the air. A warm atmosphere can hold much more
water vapor than a cold atmosphere (something we all experience with
seasonal humidity changes). The water carrying capacity of the
air is governed by the vapor pressure dependence on temperature,
plotted here in Figure 8. We see a strong non-linear dependence
of vapor pressure on temperature. At the present mean global
surface air temperature of 14 degrees C, the vapor pressure increases
about 7% for each one degree increase in temperature. Other
things being equal (winds, cloudiness etc) we may expect an
intensification of the water cycle of 7% per degree of global
warming. This means more intense droughts in some regions,
stronger rainfall in others. In many ways, society is more
vulnerable to changes in the water cycle than to temperature changes
alone. Since most of the water cycle is between atmosphere and
ocean, we expect that the oceanic salinity field will be an excellent
indicator of a changing water cycle. We thus view it
as an especially important climate variable to monitor.
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