In many mountain regions there is evidence that temperatures are changing at different rates than the global average. Three questions arise: Are temperatures in mountain regions increasing faster than the global average? Within mountain regions are warming rates dependent on elevation? And if the answers to the above are yes, why do such differences occur? Several different feedbacks can contribute, including those related to snow-albedo, atmospheric water vapor, cloud cover, and cloud properties. These feedbacks are difficult to quantify because the relationship between two climate variables is invariably interconnected with other variables as well. Also, the sparsity of observations in high-altitude regions exacerbates this difficulty.
This project will combine surface-based and satellite observations with climate model simulations and a neural network analysis scheme to (1) quantify some of the principal relationships that contribute to feedbacks on temperature in high altitude regions, and (2) investigate how these relationships and feedbacks might change through the 21st century in response to increasing atmospheric greenhouse gases. The focus will be on the Tibetan Plateau and the Rocky Mountains in southwestern Colorado. The neural network analysis calculates partial derivatives between pairs of climate variables (e.g., downward longwave radiation and cloud cover) so that the strength of the various links in a feedback loop can be determined.
Broader impacts of this work include: (1) The neural network can be applied in other regions and can enable researchers to quantify important feedbacks in the climate system and analyze non-linear processes; (2) By combining surface-based and satellite observations, a new spatially and temporally expanded observational data base will be available to the research community; (3) A better understanding of climate change in mountain regions will benefit the public by improving management practices that affect the future of water resources, agriculture, tourism, and ecosystems in high altitude regions; (4) A high-school teacher will be supported to work with the investigators to help develop and implement podcasts on mountains and climate change; (5) There will be training for a postdoctoral fellow and undergraduates; and (6) Educational materials will be developed in collaboration with the Mountain Studies Institute in Colorado.
An important open scientific question is how climate will change regionally during the 21st century as a result of increasing anthropogenic greenhouse gas forcing. The focus of this project is on future climate change in mountain regions of the planet. Many studies in mountainous regions have found that the increase in temperature over time is greater as elevation increases. Because mountains hold a lot of the world water reserves in the form of snow and ice, there are concerns that an accelerating warming there can have dire consequences for people in the region and beyond. To understand why warming can occur faster in mountains than in their surrounding low-lying regions, one needs to understand what factors influence warming at high elevations. The major scientific goals of this project were to obtain a better understanding of these factors, otherwise known as feedbacks, to quantify links in several important feedback loops that contribute to high-elevation warming, and to investigate how these feedbacks might change in the future. By using a combination of both ground-based and satellite observations, model simulations, and statistical techniques, we identified at least two feedback loops that are of great importance in mountain regions. The snow/albedo feedback loop (Figure 1) works as follows: an initial increase in temperature causes snow to melt which reduces the reflectivity of the surface and leads to more absorption of solar radiation, thus causing more warming, and the initial temperature increase is enhanced. This is a positive feedback. The second major feedback is the water vapor feedback loop (Figure 2) which works as follows: an initial increase in surface air temperature allows the atmosphere to hold more water vapor, and since water vapor is a greenhouse gas, it absorbs more infrared radiation leaving the surface and warms the near-surface air. Again the initial temperature increase is enhanced, making this a positive feedback. Although there are observations of temperature at high elevations, there are not many stations that also include other variables, such as humidity, cloud cover, snow cover, and downward thermal (infrared) and solar radiation. We obtained and analyzed high-elevation observations from the Tibetan Plateau and southwestern Colorado (Figure 3) that included some of these additional climate variables. In addition, we supplemented them with satellite observations of the same quantities, and in doing so demonstrated their usefulness and reliability for these remote regions. A major focus has been on determining how much the downward infrared radiation at the surface will increase in response to increases in water vapor that are already occurring and will continue to occur during the present century. Using an advanced statistical technique, we showed that changes in water vapor have a relatively larger impact in dry regions and demonstrated that such changes can contribute to enhanced warming at higher elevations where it is much drier than in their lower elevation surroundings. Figure 4 shows this relationship and two Colorado sites used in the analysis. We investigated the impact of clouds on the water vapor feedback and found that the most important climate variable in this feedback loop in both clear and cloudy sky is specific humidity. Because clouds also have an effect on the surface insolation and can thus impact the snow-albedo feedback, we used surface-based and satellite observations over the Tibetan Plateau to quantify the role of clouds on incoming solar radiation. We demonstrated that traditional measurements of cloud extent are not enough to study changes in surface insolation, as changes in cloud opacity have a significant impact too. Finally our analysis of global climate model simulations for the Northern Hemisphere mid-latitudes also suggests that temperatures are likely to increase faster at higher elevations during the 21st century (Figure 5), in part owing to the water vapor and snow-albedo feedbacks we have analyzed. We used these model simulations to quantify the sensitivity of temperature and solar radiation to changes in snow cover and find a strong impact of snow melt on elevation dependent warming. However, most climate model projections we analyzed indicate the relatively stronger impact of the water vapor feedback on changes in temperature with elevation. Educational outreach has been an important part of the project, and four lesson plans on high-elevation climate change were developed, tested at workshops for teachers, and implemented in high-school classrooms in southwestern Colorado. Our project has provided support for three undergraduate students (at Columbia, Rutgers, and Fort Lewis College), about a dozen high school teachers, two-postdoctoral researchers, as well as for the primary investigators. The results of the research have been disseminated both nationally and internationally in twelve refereed journals and book chapters, as well as at two major international conferences, smaller topical conferences, and seminars. The investigators organized a special session on high-elevation climate change at the Fall 2013 American Geophysical Union meeting.
