Climatic changes that result from increasing concentrations of atmospheric carbon dioxide may affect the availability of water for vegetation, groundwater recharge, runoff, and human consumption. Most studies of the effects of climatic change on water resources focus on changes in mean characteristics of hydrologic variables and do not consider the effects of these changes amid natural climatic variability. In this study, the Thornthwaite moisture index, an index of the supply of water in an area (precipitation) relative to the climatic demand for water (potential evapotranspiration), was used to examine the effects of a hypothetical increase in air temperature on moisture conditions in the United States. The effects of a gradual increase in air temperature at the rate of 4??C per 100 years, with no accompanying change in precipitation, was used to induce a change in Thornthwaite moisture index values for the United States in order to: (i) determine the relation between natural variability in climate and the time needed for significant trends in the moisture index to occur in response to hypothetical warming; (ii) identify the characteristics of areas (e.g. wet/cool, hot/dry etc.) that are most likely to be the first to experience significant changes in the moisture index given the hypothetical temperature increase. The increased temperature resulted in increased potential evapotranspiration and a decrease in the moisture index across the United States. Decreases in the moisture index were greatest in cool/wet regions and least in hot/dry regions. The time required to detect significant trends in the moisture index was a function of both the magnitude of change in the moisture index and the natural year-to-year variability of the moisture index. In general, when the ratio of the magnitude of change in the moisture index to the magnitude of variability was large, the time required to detect significant trends was short. This ratio was largest in cool/wet regions resulting in the shortest detection times. ?? 1991.