Sensitivity of Soil Moisture to Doubling of Carbon Dioxide in Climate Model Experiments. Part I: North America

Abstract

In anticipation of a global warming in the decades ahead due to an increased greenhouse effect from infrared-absorbing trace gases, it is not too early to enquire what this may mean in terms of changes in patterns of precipitation and soil moisture. These patterns are a secondary result of the temperature distribution and are therefore difficult to specify, but the practical importance attached to any regional changes in soil moisture is obviously enormous. In this study we have attempted to see how experiments with climate models reveal regional soil moisture changes over North America when carbon dioxide is doubled (or the equivalent radiation perturbation due to an increase in all the ?greenhouse gases? takes place). Results from five of the current state of the art climate models are compared with each other and with existing estimates of soil moisture distribution, with emphasis on how they respond to the carbon dioxide perturbation. (The models are those of GFDL, GISS, NCAR, OSU, and UKMO.) There is considerable difference between the soil moisture formulations and the resulting outputs of these five models in terms of soil moisture distribution changes, but some general conclusions can be drawn nevertheless: (i) The agreement between models is considerably better in the wintertime than in the summertime; (ii) one subset of the models agrees remarkably well in winter, while a different subset shows better agreement in summer; (iii) we can tentatively conclude on the basis of the model experiments that in winter there may be an increase in soil moisture in North America at high latitudes and an onset of drier conditions in the southern states and Mexico; and (iv) that in summer there may be a tendency toward drier conditions in the middle of the continent together with wetter conditions along the Gulf Coast and the West Coast of the United States and Canada. These conclusions must, of course, be taken with great caution until they can be checked with greatly improved climate models, but it is reassuring to note that most of these features are consistent with the results of studies of past warmer periods.