For the past decade there has been growing concern that increasing atmospheric carbon dioxide concentrations could result in significant changes in the distribution of temperature and precipitation, and in the frequency and severity of floods and droughts. There is some evidence of change in precipitation and temperature patterns based on trends in the historic data. In some areas, precipitation intensities are changing, and average temperatures, particularly at night, are increasing at mid and high latitudes. Another cause of concern are the trends indicated in scenarios generated by global climate models under doubled and quadrupled atmospheric carbon dioxide concentrations.

This talk will review the evidence for trends in important water cycle variables such as precipitation, temperature and streamflow. It will assess the results of modeling studies and compare them to historical data. Finally, it will consider the consequences of possible changes in climate on future flood and drought regimes. In addition, these projections will be considered in the context of other agents of change, such as land use change, that could also affect flood potential and the availability of water. The paper concludes with a summary of some new scientific programs in the USA and internationally that will help reduce the uncertainties associated with projected changes in flood potential and water supply and demand.