Influence of increasing surface humidity on winter warming at high altitudes through the 21st century

This dissertation examines the influence of low-level atmospheric humidity in mediating the rate of surface warming, particularly at high altitude regions, during the late 20th century and the 21st century. The focus is on observations and global climate model projections (IPCC SRES A1B scenario) for China, the Tibetan Plateau (TP) and the San Juan Mountain (SJM) in southwest Colorado. For China, the analysis suggests large surface warming despite significant decreases in insolation until the middle of the 21st century. Both the past and future warming in China occurs primarily as a result of the lower and upper atmospheric water vapor feedbacks, triggered by the increase in anthropogenic greenhouse gases, which in turn causes an increase in downward longwave radiation (DLR). For the TP and the SJM region, I find that increases in surface specific humidity (q) leads to relatively large increases in DLR. This effect is enhanced in colder months and at higher altitudes, and the winter warming in the TP is about twice the warming during other seasons. For the TP, the model shows that for the highest elevations the largest warming between 1950-2100 occurs during winter and spring. The increases in DLR influenced by increases in q during winter, and increases in absorbed solar radiation influenced by decreases in snow cover extent during spring are, in part, the reason for a large warming trend over the plateau. These two effects appear to produce the model's elevation dependent warming trend. For the SJM region, the observations show that q has been increasing at more than 10% per decade from October through January between 1990-2005, when the region experienced the largest increases in surface temperatures. Moreover, only during these months do diurnal changes in humidity explain the large variability in the corresponding changes in temperature. The largest changes in DLR also appear to occur during these months. Large increases in DLR during January and December coincide with large increases in temperatures and, in part, indicate the causes for a large warming trend during these months.