| Abstract: A synthesis of available data for
the Mississippi River basin (area 3x106
km2) reveals an upward trend in
evaporation during recent decades, driven primarily by increases in
precipitation and secondarily by human water use. A cloud-related
decrease in surface net radiation appears to have accompanied the
precipitation trend. Resultant evaporative and radiative cooling
of the land and lower atmosphere quantitatively explains downward trends
in observed pan evaporation. These cooling tendencies also
reconcile the observed regional atmospheric cooling with the anticipated
regional "greenhouse warming." If recent high levels of
precipitation (which correlate with the North Atlantic Oscillation) are
mainly caused by an internal climatic fluctuation, an eventual return to
normal precipitation could reveal heretofore-unrealized warming in the
basin. If, instead, they are caused by some unidentified forcing
that will continue to grow in the future, then continued intensification
of water cycling and suppression of warming in the basin could result. |