FRONTIER Newsletter No.10 Mar.2000 


Delectability of Summer Dryness Caused by Greenhouse Warming


 This study investigates the temporal and special variation of soil moisture associated with global warming as simulated by long-term integrations of a coupled ocean-atmosphere model conducted earlier. Starting from year 1765 ,integrations of the coupled model for 300 years were performed for three scenarios:increasing greenhouse gases only,increasing sulfate-aerosol loading only and the combination of both radiative forcings.The integration with the combined radiative forcings reproduces approximately the observed increases of global mean surface air temperature during the 20 th century.

 Analysis of this integration indicates that both summer dryness and winter wetness occur in middle- to- high latitudes of North America and southern Europe.These features were identified in the earlier studies.In this study,we have also found that the percentage reduction of soil moisture is particularly large in some semi-arid regions of the subtropical to middle latitudes,such as the southwestern part of North America,central Asia and areas around the Mediterranean Sea (Fig.2 ).In these regions,soil moisture is reduced during most of the year,in contrast to the mid- continental region of middle to higher latitudes where soil moisture is reduced during summer but increased in winter.In southern Africa and Australia of the Southern Hemisphere,the percentage reduction of soil moisture is pronounced in winter.In response to the increase in the downward flux of terrestrial radiation associated with global warming,surface temperature increases markedly,enhancing evaporation.On the other hand,the precipitation rate hardly increases or even decreases slightly in these regions as the major fraction of the increased radiative energy absorbed by the land surface is removed as sensible heat flux rather than through evaporation.Both of these factors contribute to the reduction of soil moisture during most of the annual cycle.In sharp contrast to the situation in these semi- arid regions of the world,annual mean runoff increases greatly in high latitudes because of increased poleward transport of moisture in warmer atmosphere.
 An analysis of the central North American and southern European regions indicates that the time when the change of soil moisture exceeds one standard deviation about the control integration occurs considerably later than that of surface air temperature for a given experiment because the ratio of forced change to natural variability is much smaller for soil moisture compared with temperature.The corresponding lag time for runoff change is even greater than that of either precipitation or soil moisture for the same reason.Also according to the above criterion,the inclusion of the effect of sulfate aerosols in the greenhouse warming experiment delays the noticeable change of soil moisture by several decades.It appears that observed surface air temperature is a better indicator of greenhouse warming than hydrologic quantities such as precipitation, runoff and soil moisture.

Nevertheless,the reduction of soil moisture will become more notable as we proceed into the 21 st century.

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