The earth's climate has always been changing. Only the rate of the change differed from time to time, showing variations of very different temporal scales in the climate system. How did these changes occur? What were the driving forces and mechanisms involved? Were the driving forces from interactions of internal processes of the climate systems, or from external sources? How were the changes manifested in differences of precipitation, temperature, the occurrence of extreme events, and other factors specifying our living environment? These are some of the major questions that climate change research is trying to address so as to understand the current climate and provide insight into future climate and environment. Our climate change studies focus on the last question in the previous list and strive to reveal how changes of climate in the 20th century are measured in terms of precipitation and temperature variations in various regions in the central and western United States as well as some other parts of the world, and what physical processes have led to these changes. What will be the effects of these changes on society and the management of natural resources?