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 Tibetan Plateau is the largest region of elevated topography on Earth, averaging about 4 to 5 km high, and covering an area equivalent to about 5/6 of that of India. In the modern world, this vast feature in central Asia alters much of the atmospheric circulation in the northern hemisphere. In concert with the tallest mountain range in the world – the Himalaya just to the south – the plateau exerts an enormous influence on the monsoon systems of the Indian subcontinent and China, weather regimes that dictate the climate in a region that is home to nearly half the planet’s population. The plateau also represents a barrier to the northern-hemisphere wintertime jet stream in the atmosphere. As a result, the plateau controls the shape of the atmospheric storm track over the Pacific Ocean, which in turn sets the patterns of wintertime climate over North America. The plateau’s control of the atmospheric circulation also sets the backdrop on which important sources of natural climate variability, such as the El Nino climate oscillation, propagate around and influence the rest of the planet. The Tibetan Plateau is also a relatively young topographic feature on the Earth’s surface, having resulted from the collision of the Asian and Indian continents around 50 million years ago. This means that the evidence of the impact of its development on the environment remains on the landscape. The University of Washington’s contributions to this multi-institution research grant have primarily been two-fold. Firstly, using computer models of the climate system, we have studied the pathways by which moisture and rainfall reaches different regions in Asia. These are recorded in the different isotopes (heavier and lighter forms of the same chemical elements) in water; records of these are retained in sediments, soils, and cave stalactites. We have shown: a) that water vapor transported across the Tibet Plateau from India and southeast Asia is a dominant control on the water isotopes that make up rainfall in China; and b) that long-term variations in the Earth’s orbit, known as precessional cycles (akin to the Earth’s spin axis rotating over tens of thousands of years like a wobbly top), alter the patterns of solar radiation falling on the Earth in a way that dramatically changes the monsoon circulation. With these results we can explain the record of isotopes over time found in Chinese and Middle-eastern caves. The second main contribution has been to use the same computer climate models to understand which geologic changes over the past 50 millions year have been most important for affecting Asia’s climate. We compared the above-mentioned orbital changes with carbon dioxide variations, the location and terrain of the continents, and lastly the presence of vast inland seas, which occupied much of central Asia during this interval. Our results establish that each of these factors caused significant and unique patterns of climate change, but that it is the inland seas that were most important. Their presence radically altered the seasonal cycle of climate and provided a proximal source of moisture to regions that, in the modern climate, form some of the largest and most formidable deserts on Earth.
