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Deciphering Glacial Change on the Tibetan Plateau during the Holocene | Righting America

by Shuang-Ye Wu

And now, some actual science on global warming.

Dr. Shuang-Ye Wu is a climatologist working in the Department of Geology at University of Dayton.  Her research focuses on how climate change alters the hydrologic cycle and the consequent precipitation patterns. In particular, she is interested in changes in extreme events such as extreme storms, floods and droughts.  Dr. Wu has published 36 papers in peer-reviewed scientific journals, and obtained grants from NSF, EPA and other funding agencies. Dr. Wu obtained her Master and PhD degrees from Cambridge University in UK, majoring in environmental geography. She is currently teaching courses in the Earth system science, climate change, and geographic information systems at UD.

This post summarizes work she and colleagues at Nanjing University (where she is affiliated as a visiting professor) will be doing over the next four years thanks to a $552,620 grant from the National Natural Science Foundation of China. 

The Tibetan Plateau (Figure 1) is often referred to as the “water tower of Asia,” because of the large number of high mountain glaciers that form the headwaters of major river systems (e.g., Yangtze, the Yellow River, Mekong, Brahmaputra, Ganga, Indus, and Tarim) which supply water for irrigation, power and drinking water for over 1.4 billion people. These glaciers act as an important reservoir and buffer against drought in the world’s most populated region. During the past few decades, most of the Tibetan glaciers have experienced reduction in length, surface area and volume due to increasing temperature in this region. Recession of the Tibetan glaciers varies spatially, with the most significant retreat in the Himalayas, while there is a slight mass gain in the Karakoramglaciers in the northwest.

World Topographic Map, 2001. Wikimedia Commons.

The state of a glacier is controlled by its mass balance, i.e., the difference in ice input from snowfall and ice output from melt. If the input is consistently greater than the output over a period of time, a glacier will get bigger (i.e. advance); otherwise, it will get smaller (retreat). Mass balance of a glacier is largely controlled by climate factors, in particular temperature (which affects the output) and precipitation (which affects the input).  Although climatic change could affect the glacial mass balance instantly, the glacier extent (i.e. size) responds to changes in mass balance with a delay of decades to centuries depending on such factors as glacier size, surface slope, direction and debris cover. This delayed response makes it difficult to attribute an observed glacier change to any specific change in the climate, particularly when long-term glacial change data are lacking. Most of the observation data for glacial change are based on satellite images and in-situ measurements obtained during the past several decades, and little is known for glacial change on the Tibetan Plateau on the long time scales (e.g., millennial or longer).

Past glacial advances can often be reconstructed from mapping and dating sediments deposited by past glaciers (moraines) and from proglacial lakes. However, past glacial retreats are more difficult to detect because traces of minimum extents are now buried underneath modern glaciers. Recently, a new approach was developed to assess minimal glacier extent by determining the glacier basal ice age, which is interpreted as indicative of ice-free conditions at the time. In a recent project funded by the Natural Science Foundation of China, we propose to apply this approach to establish past glacial retreats on the Tibetan Plateau. Combined with previous data of glacial advances, this new information will allow us to examine how glaciers respond to climate change during the Holocene (~ 11000 calendar years ago to the present).

In this study, we propose to drill ice cores to bedrock and collect sediment samples at various locations on four glaciers over the Tibetan Plateau, in order to explore changes of glacial extent during the Holocene. They include: Cho Oyu in the Himalayas, Zangser Kangri in the central Tibetan Plateau, Shule Nanshan in the Qilian Mountains, and Chongce in the western Kunlun Mountains (Figure 2). After field samples are collected, we will determine accurately the ages of the sub-glacial sediment samples, terminal and periglacial sediment samples, as well as the bottom age of the ice cores drilled from the glaciers. These bottom ages suggest previous smaller than present glacial extents at various times during the Holocene because of the absence of older ice at the studied sites. Based on this assumption, we will estimate the glacier reduction by applying GIS and spatial statistics methods during the bottom ages (which are estimated to be around 6000-9000 years ago based on previous studies). Together with the time series of the quaternary glacial advance events, we will re-examine the previously suggested asynchronous glaciation on Milankovitch timescales over the Tibetan Plateau. We will also update the Holocene climate reconstruction over the study region, and decipher the glacial responses to the past climatic conditions on the long time scale that extends far beyond the instrumental period. Our results will have important implications for the prediction of the glacier fluctuations over the Tibetan Plateau in the near future with anthropogenic global warming.

Tibetan Plateau: Modern topographical map of the Tibetan Plateau and the surrounding region showing areas of low (green) to high (red and white) elevation. Credit: Darekk2, GLOBE, and ETOPO1, CC BY-SA 4.0