Large-Scale Changes of Soil Wetness Induced by an Increase in Atmospheric Carbon Dioxide

Abstract

The change in soil wetness in response to an increase of atmospheric concentration of carbon dioxide is investigated by two versions of a climate model which consists of a general circulation model of the atmosphere and a static mixed layer ocean. In the first version of the model, the distribution of cloud cover is specified whereas it is computed in the second version incorporating the interaction among cloud cover, radiative transfer and the atmospheric circulation. The CO2-induced changes of climate and hydrology are evaluated based upon a comparison between two quasi-equilibrium climates of a model with a normal and an above normal concentration of atmospheric carbon dioxide. It is shown that, in response to a doubling (or quadrupling) of atmospheric carbon dioxide, soil moisture is reduced in summer over extensive midcontinental regions of both North America and Eurasia in middle and high latitudes. Based upon the budget analysis of heat and water, the physical mechanisms responsible for the CO2-induced changes of soil moisture are determined for the following four regions: northern Canada, northern Siberia, the Great Plains of North America and southern Europe. It is found that, over northern Canada and northern Siberia, the CO2-induced reduction of soil moisture in summer results from the earlier occurrence of the snowmelt season followed by a period of intense evaporation. Over the Great Plains of North America, the earlier termination of the snowmelt season also contributes to the reduction of soil moisture during the summer season. In addition, the rainy period of late spring ends earlier, thus enhancing the CO2-induced reduction of soil moisture in summer. In the model with variable cloud cover, the summer dryness over the Great Plains is enhanced further by a reduction of cloud amount and precipitation in the lower model atmosphere. This reduction of cloud amount increases the solar energy reaching the continental surface and the rate of potential evaporation. Both the decrease of precipitation and the increase of potential evaporation further reduce the soil moisture during early summer and help to maintain it at a low level throughout the summer. Over Southern Europe, the CO2-induced reduction of soil wetness occurs in a qualitatively similar manner, although the relative magnitude of the contribution from the change in snowmelt is smaller. During winter, soil moisture increases poleward of 30°N in response to an increase of atmospheric carbon dioxide. Because of the CO2-induced warming, a greater fraction of the total precipitation occurs as rainfall rather than snowfall. The warmer atmosphere also causes the accumulated snow cover to melt during winter. Both processes act to increase the soil moisture in all four regions during the winter season. The increase of soil moisture is enhanced further in high latitudes due to the increase of precipitation resulting from the penetration of warm, moisture-rich air into higher latitudes. The CO2-induced warming of the lower model troposphere increases with increasing latitude. The present analysis suggests that the changes of soil wetness described in this investigation are controlled by the latitudinal profile of the warming and are very broad scale, mid-continental phenomena.