Acceleration of the Global Hydrologic Cycle

The global hydrologic cycle, which can be conceptually described as evaporation of water vapor from the ocean, transport of water vapor by atmospheric winds to land regions, condensation, and precipitation of atmospheric water back to the surface, and subsequent transport, by streams, of this water back to the ocean, may intensify or accelerate in the future, as the planet warms owing to an increasing greenhouse resulting from increasing emissions of anthropogenic gases. As in many previous studies, increased hydrologic fluxes are found in a 100-year integration of the NCAR Coupled Climate System Model when forced with increasing CO[[inf]]2[[/inf]] levels. That is, as the earth warms, the evaporation increases, which not only increases the precipitation rate, but also the subsequent moisture convergence to land regions. However, if this increase in the cycling rate by various processes is measured with respect to the changing atmospheric and surface water reservoirs, then the atmospheric hydrologic cycle may appear to be actually deaccelerating since the atmospheric reservoir in this model is influenced more by the temperature changes than the actual transformation processes are. By contrast the land surface water reservoirs are less affected and thus there does appear to be an accelerated land surface hydrologic cycle, especially at high latitudes, and a coupled land-atmosphere acceleration, at least over land regions. There are also important regional differences.