As we enter the 21st century, it is possible to reach beyond the headlines to describe what is now known about climate change. The Intergovernmental Panel on Climate Change evaluated the scientific aspects of global climate change; the current consensus is described in a recent series of reports. Since the 19th century, concentrations of atmospheric carbon dioxide, methane, nitrous oxide and sulfate aerosol dust have increased significantly. While there is scientific agreement that warming is occurring, the controversy now concerns the extent of subsequent impacts in the future. In California, the impacts of global warming are likely to include reduced water availability and quality, poorer air quality, associated economic consequences, biodiversity shifts and health effects. The changes are expected to continue at an increasing pace well into the next century, perhaps outstripping our scientific, economic and social ability to cope with them.

This project implemented the initial phase of California-2100 (Cal21), which is aimed at making and evaluating high resolution estimates of climate change over California out to the year 2100. The initial WRC component of this project has been focused on evaluating how well regional climate models reproduce the variations of important components of the water budget for California, and estimating the effects of the increases over the past century in irrigation in California on regional climate, especially snow accumulation. The results show that global warming has a large effect on precipitation, snow water, surface temperature, low level winds and soil moisture They also show in summer that irrigation has a strong effect on the differences between recent and past conditions in maximum temperature, surface latent and sensible heat fluxes, surface moisture, and surface humidity.