The Hydrological Cycle

Regional evaporation rates are a central part of the hydrological cycle, and so the question as to whether decreases in pan evaporation indicate decreasing or increasing regional evaporation is of great importance. An increasing hydro-logical cycle with increased regional evaporation would lead to increased rainfall rates. However, it would also increase cloudiness whose feedback influence would cause a decrease in Eg#. As noted above, cloud changes have been relatively small.

Prior to the twenty-first century, it was assumed that global warming would enhance evaporation and lead to an enhancement (or spinning up) of the hydrological cycle. Ramanathan et al. [49] evaluated the influences of anthropogenic aerosols on solar and thermal radiation balances, atmospheric temperature profiles and climate. They found that 'aerosols enhance scattering and absorption of solar radiation and produce brighter clouds that are less efficient at releasing precipitation. These in turn lead to large reductions in the amount of solar irradiance reaching Earth's surface, a corresponding increase in solar heating of the atmosphere, changes in the atmospheric temperature structure, suppression of rainfall, and less efficient removal of pollutants. Thus, these aerosol effects can lead to a weaker hydrological cycle'. A case in point is the Indian sub-continent where anthropogenic aerosol 'brown clouds' can reduce Eg# by more than 10% and change the regional hydrological cycle. In particular, dark aerosols absorb solar radiation and cause enhanced atmospheric warming and decreased Eg#, which decreases surface temperatures and evaporation rates. Together, these enhance atmospheric stability and spin down the hydrological cycle [78].

Liepert et al. [79] and Wild et al. [80] also considered that a reduction of Eg# and related reductions in evaporation rates could be 'spinning down' the hydrological cycle. They argued that reductions in surface solar radiation were only partly offset by enhanced down-welling longwave radiation from the warmer and moister atmosphere and that the radiative imbalance at the surface leads to weaker latent and sensible heat fluxes and hence to reductions in evaporation and precipitation despite global warming. This is in line with experimental evidence of the influence of aerosols on climate [81].

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