Potential evapotranspiration and the likelihood of future drought

The likelihood of future drought is studied on the basis of two drought indices calculated from the Goddard Institute for Space Studies general circulation model (GISS GCM) transient and doubled CO2 climate changes. We use the Palmer drought severity index (PDSI) and a new supply-demand index (SDDI), the latter being the difference between the precipitation and potential evapotranspiration Ep, i.e., the difference between atmospheric supply of and demand for moisture. Both indices show increasing drought for the United States during the next century, with effects becoming apparent in the 1990s. If greenhouse gas emissions continue to increase rapidly, the model results suggest that severe drought (5% frequency today) will occur about 50% of the time by the 2050s. The results are driven by the large increase in Ep, associated with the simulated climate warming. Ee increases most where the temperature is highest, at low to mid-latitudes, while precipitation increases most where the air is coolest and easiest to saturate by the additional moisture, at higher latitudes. Thus atmospheric demand becomes greater than supply for most of the globe by the latter half of next century. We show that large Ee changes can lead to soil moisture deficits, as in the PDSI hydrologic budget, and vegetation desiccation, as is implied by vegetation and climate models. We suggest that drought intensification has been understated in most GCM simulations, including the GISS GCM, because of their lack of realistic land surface models. Paleoclimate analogues in the Mesozoic and Cenozoic are reviewed which imply that arid conditions can result from either increased temperatures or decreased precipitation, consistent with our use of the SDDI. The results depend primarily on the temperature increase, in particular the model sensitivity of 4oC warming for doubled CO2. Global precipitation cannot keep pace with increased demand over land because the land surface warms more than the ocean surface; this effect, along with greater atmospheric opacity, reduces low level gradients and evaporation from the ocean. If the forecast temperature changes come to pass, these conclusions suggest that drought conditions will increase dramatically. The anticipated increase in temperature associated with accumulating greenhouse gases has led to the concern that future water availability might be threatened [Smith and Tirpak, 1989], as warmer temperatures will likely lead to increased evaporation. Several approaches have been employed to evaluate this possibility. Stockton and Boggess [1979] used an empirical relationship derived by Langbein [1949] to assess the effect of a 2oC temperature increase and a 10% precipitation decrease (values consistent with some general circulation model (GCM) projections). They found on average a 50% reduction in water supply for seven western United States water regions. Karl and Riebsame [1989] criticized the Langbein relationship and concluded from the climate record over the United States that the influence of temperature on water availability is much more muted. Revelle and Waggoner [1983] correlated past temperature changes with fiver flows of the Colorado River, and with a 2oC increase they deduced a 30% decrease in runoff. There are serious difficulties in using the observational record for this type of analysis. The temperature changes in the historical past are very small compared with projections; the average warming used in the Karl and Riebsame [ 1989] study was only 1/10 that being projected for doubled CO2, an important consideration given that the water-holding capacity of the atmosphere and hence evaporation varies strongly