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).

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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Last updated December 12, 2007
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