For fifty years, the Tibetan Plateau has been recognized as the largest topographic feature that perturbs atmospheric circulation. It serves as an ideal field laboratory for understanding the geodynamic processes that build high terrain. Accordingly, the growth of the plateau should have altered atmospheric circulation and therefore written an evolving paleoclimatic signature not only on eastern Asian regional climates, but on global climate as well. Despite many recent studies, we still do not know precisely when the Tibetan Plateau reached its current dimensions and how it perturbs atmospheric circulation. This project brings together geodynamicists, atmospheric scientists, and paleoclimatologists in a multidisciplinary study of the when and the how.
One of the major goals of the project is to quantify the extent to which Tibet has grown by crustal thickening, by thrust faulting and folding, by flow within the crust that redistributes material there, or by replacement of cold mantle lithosphere with hotter material (all in a state of isostatic equilibrium). Such quantification will take big steps toward the understanding of how high plateaus are built and how continental lithosphere deforms, topics at the forefront of geodynamics.
Determining how Tibet has grown will require determining when crustal shortening and thickening occurred, using basic field methods and modern laboratory techniques, and quantifying paleoaltitudes with new isotopic tools. Applying such paleoaltimetric techniques, however, requires an understanding not only of how the atmosphere transports isotopes, but how the evolving high terrain affected surface temperatures at times in the past. Even if the project?s focus were solely on how Tibet has grown, a meteorological component of the study, focused particularly on eastern Asia?s hydrological cycle, would be necessary.
Most continental paleoclimatic indicators are thought to be more sensitive to precipitation than to temperature, and among the unknowns of future climate, the hydrological cycle stands out. Accordingly, a major focus will be on understanding how high terrain like Tibet affects the hydrological cycle of eastern Asia, and China in particular. These studies will focus on: (1) how the plateau, as both a topographic obstacle and a sink for solar radiation, affects atmospheric circulation; (2) how the atmosphere transports stable isotopes (ä18O and äD); (3) how it affects mid-latitude climate variability, including how, via lee cyclogenesis, it lofts and transports dust, and (4) how vegetation feeds back on atmospheric circulation and the hydrological cycle. As links from geologic processes occurring at multi-Myr time scales to those on human time scales, the Principal Investigators plan studies that specifically examine paleoprecipitation over the past few hundred thousand years, using both loess deposition and speleothems that quantify paleoclimate.
The Tibet is called the "Roof of the World" with good reason. The average elevation of the Tibetan Plateau is more than 5000 m. One of the two main goals of this project was to further our understanding of the processes that led to the formation of the Tibetan Plateau. The second main goal was to further our understanding of how the Tibetan Plateau affects climate over a broad swath of Asia and how the climate of the region may have affected the formation of the Plateau itself. These were the overarching goals of the multi-institutional project. Within these broad goals, the specific aim of the University of Minnesota group was to reconstruct climate history over portions of the last 500,000 years for localities on and around the Tibetan Plateau. This time window is many thousands of times longer than the instrumental record, but from a geologic perspective only covers very recent earth history. Indeed, by 500,000 years ago, the Plateau had essentially reached its present elevation. Thus, our aim was to characterize the climate of the region, and its variability, subsequent to the formation of the plateau. We worked closely with Chinese colleagues from the Chinese Academy of Sciences based in Xi'an in carrying out this research. We chose to reconstruct climate history by analyzing layers in deposits collected from caves. The Minnesota group has previously shown that analysis of cave deposits can result in remarkable records of climate history. Along with our Chinese colleagues, we reconnoitered several caves from sites on the Plateau, as well as to the north and south. We then selected several samples from each of these caves for laboratory analysis. Much of our work involved careful laboratory analysis of the layers within the cave deposits. Once this work was completed, we discussed the interpretation of the data, presented the results at conferences, then published the results in scientific journals. For perspective on our results, today the region to the north of the Tibetan Plateau (northwestern China) is quite arid. The summer monsoon rainfall that typifies southeastern China does not reach this region. Much of the Tibetan Plateau is also arid. However, the southern portion of the Tibetan Plateau as well as localities further south have wetter climate characterized by summer monsoon rainfall. The two main successes of this project are reconstruction of climate history on the Tibetan Plateau for portions of the last 130,000 years and reconstruction of climate history north of the Tibetan Plateau for much of the last 500,000 years. We have reached two main conclusions from these records. First, at times in the past, summer monsoon rainfall reached much, if not all, of the Tibetan Plateau and reached well north of the Tibetan Plateau, well into the interior of the (now arid) interior of Asia. Second, the main cause of this extension of summer monsoon rainfall, well beyond its current limits, is the changing of the spring and early summer heating of Asia resulting from the changing geometry of the earth's orbit and axis of rotation. In and of themselves, these observations are noteworthy and not necessarily expected. In the future, they may well prove useful in testing the ability of climate models to hindcast these records.
