GC11B-01 INVITED
Quaternary glaciation of the Himalaya and Tibet
Glacial geological evidence from throughout the Himalaya-Tibet shows the existence of expanded ice caps and extensive valley glacier systems during the late Quaternary. Whether the timing of the extent of maximum glaciation was synchronous throughout the entire region or whether the response was more varied is a topic of much contention. This is mainly because the lack of organic material needed for radiocarbon dating that has hindered past progress in glacial reconstruction. However, the application of optically stimulated luminescence and terrestrial cosmogenic nuclide (TCN) methods has recently expanded the number of chronologies throughout the region helping to test glacier synchoneity. Yet, limits to the precision and accuracy available with these methods and, more importantly, geological uncertainty imposed by processes of moraine formation and alteration both conspire to limit the time resolution on which correlations can be made to Milankovitch timescales (several ka). All the published TCN ages for moraine boulders and glacially eroded surfaces in the Himalayan-Tibetan orogen have been recalculated to assess synchroneity of glaciations, and well-dated sites have been re-evaluated. Locally detailed studies indicate that there are considerable variations in the extent of glaciation from one region to the next during a glaciation. Glaciers throughout monsoon-influenced Tibet, the Himalaya and the Transhimalaya are likely synchronous both with climate change resulting from oscillations in the South Asian monsoon and with Northern Hemisphere cooling cycles. In contrast, glaciers in Pamir in the far western regions of the Himalayan-Tibet orogen advanced asynchronously relative to the other regions that are monsoon-influenced regions and appear to be mainly in phase with the Northern Hemisphere cooling cycles. Broad patterns of local and regional variability based on equilibrium-line altitudes have yet to be fully assessed, but have the potential to help define changes in climatic gradients over time.
GC11B-02 INVITED
Climate, Permafrost, and Landscape Interactions on the Tibetan Plateau
Observational evidence indicates that climate warming has been well underway on the Qinghai-Tibetan Plateau (QTP) for the past few decades, manifest through increases in station air temperature across the Plateau. Given the high elevation and extremely complex terrain of the QTP, an important consideration in evaluating in situ station observations for climate change is the fact that observing sites are almost always located in convenient, easily accessible places. On the QTP, this often translates to non-representative low- lying valley areas near population centers. ERA-40 reanalysis air temperatures, from an atmospheric model, provide an alternative measure of surface air temperatures, free of such biases. ERA-40 is in very good agreement in terms of interannual variability with in situ measurements. However, there are significant differences in the absolute magnitude of temperatures, as well as the long-term trends. An unresolved question is the degree to which the observed warming on the QTP is caused by anthropogenic effects over the past several decades, and the contribution of data quality issues. Our preliminary findings suggest that the observed climate warming on the Plateau may in part be due to local land cover and land use change. Using available ground-based measurements, ERA-40 reanalysis data, and satellite remote sensing data, we provide an assessment of the relative roles of air temperature, precipitation, and vegetation change on QTP and their roles in climate warming. An important additional factor in this semi-arid environment is the presence of permafrost. Plateau-wide permafrost degradation will intensify melting of ground ice, development of thermokarst, thaw lake drainage, and slope instability. These surface geomorphologic processes also have a significant impact on vegetation and ecosystem dynamics and, ultimately, on the climate system. We thus also assess the response of permafrost to climate warming and increased human activity, and feedbacks to landscapes and ecosystems on the QTP.
GC11B-03
General Humid Little Ice Age In Westerly Dominated Western China
Eleven major humidity proxy records and other supplementary evidence, including natural archives such as ice-cores, sedimentary records, river terraces, lake-level fluctuations, as well as historical documents, have been used to reconstruct the variability of humidity in the Westerly dominated Western China (WDWC) including Arid China and north part of the Tibet Plateau, over the last 1000 years. The results show that as a whole, the LIA from ca AD1300 to AD 1850 was a relatively humid period in this region, in contrast to the recent 100 years during the period of global warming and Medieval Warm Period (MWP). For instance, proxy records from western Kunlun Shan, Tien Shan and Qilian Shan suggest precipitation was higher in these mountains. Accordingly, the discharge of large inland rivers such as the Tarim and Keliya rivers increased during LIA; the areas of terminal lakes in the desert expanded, e.g., at Juyan Lake and Aibi Lake and the water levels rose at lakes in intermontane basins, such as Bosten Lake and Balikun Lake. The groundwater recharge rates in the Badain Jaran desert were increased and the water salinity of Sugan Lake significantly decreased. The general humid LIA in the WDWC contrasted to monsoon dominated Eastern China. The humid LIA in WDWC possibly resulted from the increasing precipitation due to the strengthening and equatorward shift of westerly and the decreasing evaporation due to the lower temperature during that time.
GC11B-04
Spatio-temporal dynamics of snow cover in the Western Tibetan Plateau using a MODIS derived fractional snow cover index.
In situ analyses of snow dynamics on the Tibetan Plateau (TP) are scarce and constrained by complex topography. Available long-term coupled in situ-remote sensing analyses from 1951-1997 in the TP show a large interannual variation superimposed on a small increasing trend. Microwave derived estimates of snow cover have been shown to overestimate snow in the TP without additional corrections for terrain and changing atmospheric conditions. In addition, few studies exist characterizing snow cover over the last decade for the TP incorporating these corrections. Thus there is a need for a more automated method for quantifying snow cover dynamics in the TP and for providing input for hydrologic and climate models. We apply a new optically-derived multiple endmember spectral mixture analysis technique, which had comparable accuracy to Landsat derived validation scenes, to assessing the spatio-temporal dynamics of snow cover in the Western TP from 2000-2007 using MODIS data. Preliminary results reveal an increasing trend in fractional snow covered area (fSCA) in the Himalayas and Bayan Har ranges, a strong negative correlation between winter and summer fSCA, and large spatial and temporal variability in the onset and duration of fSCA super imposed on an increasing trend in winter (DJF).
GC11B-05
Understanding subsurface hydrology in the Tibet Plateau, Western China
Groundwater flow in the Tibet Plateau in western China, replenished by subglacial recharge at high elevations, may play an important role in buffering the climate impact on water resources and ecosystems. Current knowledge on groundwater is limited. This study intends to provide a preliminary understanding of the groundwater flow system in the Tibet Plateau. Three conceptual groundwater regimes are recognized: near-surface shallow groundwater above the permafrost base; deep groundwater below the permafrost; and the regime under low-lying valleys and fault zones where upwelling groundwater discharges to rivers and springs. Near-surface permafrost hydrology is of great importance to assessing the short-term impact of climatic change on permafrost degradation and ecosystems. Groundwater processes below the permafrost and in valleys and fault zones, however, likely exert more influence on the water cycle at larger scales and long-term water resource sustainability. A two-dimensional steady-state groundwater-flow model is developed in the approximately north-south direction across the Tibet Plateau. Constrained by baseflow and spring discharge data, model results suggest that the regional-scale groundwater-flow system in the Plateau is driven and sustained by the topographic gradient. With evaporation greater than precipitation over the Plateau, groundwater is recharged at high elevations. This recharge is an important source for the groundwater system and critical for sustaining the water cycle in the Plateau. The most plausible rate of recharge is at 100 to 200 mm/year. Hydrologic connections exist between groundwater and surface water in valleys and fault zones where the groundwater discharge rate is on the order of 10-9 to 10-7 m/s. This discharge could be vital to sustaining river baseflow and spring discharge. Groundwater circulation can reach 1 to 2 km in depth and supply geothermal water to springs, which may disrupt permafrost continuity and create thaw areas on the Plateau. Potential variation in recharge due to climate change could result in reduced groundwater discharge to rivers and springs and adversely impact the ecosystems on the Plateau.
GC11B-06
A Systematic Regional-scale Assessment of Lake Dynamics across the Tibetan Plateau
The Tibetan Plateau is home to the world's largest high-altitude lake group and has been experiencing significant climate change. Tibetan lakes have been impacted greatly, and in return they serve as a sensitive indicator of regional climate and water cycle variability. The paper provides a regional-scale systematic assessment of both paleo and recent lake changes across the plateau using geo-spatial information technologies. Knowledge of past lake dynamics is essential for us to better understand the current and inferred future lake changes. Since paleo lake shores have been extensively preserved on this remote plateau, paleo lake change since the late Pleistocene (~25 ka BP) can be inferred from paleo shorelines visible in high-resolution satellite imagery with the assistance of digital elevation models. We have recovered the paleo lake extent for more than 650 major contemporary lakes, and found that they have evolved from 173 larger paleo lakes that have shrunk so significantly that more than two-thirds of the original lake area has disappeared. The total water loss due to this lake shrinkage is estimated to be more than 2,900 km3. The recovered paleo lake shrinkage exhibits a strong spatial pattern across the plateau, and corresponds with Quaternary glacial changes in a drainage-basin-based analysis. More recent lake dynamics over the past 30 years can be observed by employing long-archived Landsat satellite data, and only minor changes have been found in most areas. It has been found that glacial dynamics play an important role in detected paleo as well as recent lake changes, and will continue to play a critical role in Tibetan lake dynamics in the near future.
GC11B-07 INVITED
Tibetan Glaciers as Integrators and Sentinels of Climate Change
Information from ice cores collected over the last two decades across the Tibetan Plateau demonstrates that this is a climatically diverse and complex region. Records spanning more than 500,000 years have been recovered from the Guliya ice cap in the far northwestern Kunlun Mountains, where the climate is dominated by the westerly flow over the Eurasian land mass. Shorter records (less than 10,000 years) have been recovered from ice fields in the central Himalaya to the south, where a monsoonal climate regime dominates and the annual accumulation is high. On decadal and longer timescales IPCC climate models predict that continued anthropogenic greenhouse gas emissions will force air temperature to increase faster at higher elevations. This vertical amplification will be greatest in low latitudes due to upper tropospheric humidity and water vapor feedback. Meteorological records across the Tibetan Plateau indicate that temperatures have risen since the mid-1950s and the rate of warming is greater (0.3°C per decade) at the higher elevation stations. Likewise, the stable isotopic compositions of ice cores across the Plateau show an overall the 20th Century enrichment that is greatest at the highest elevation sites. Glaciers in the central Himalayas, including many around the Tibetan Plateau, are experiencing an accelerating rate of ice loss, due in part to current temperature trends and associated feedbacks. Ice loss in the central Himalayas is evident from ice cores recovered in 2006 from the Naimona'nyi ice field. Unlike previous cores from glaciers around the world, including those drilled across the Tibetan Plateau, the Naimona'nyi cores lack the elevated levels of beta radioactivity from the decay of 36Cl and 3H associated with atmospheric thermonuclear bomb testing in the 1950s and 1960s. This suggests that net mass (ice) loss has exceeded accumulation on this glacier since at least 1950. If the climate conditions that govern the mass balance on Naimona'nyi extend to other glaciers in the region, the implications for future water resources in South Asia could be dire as these glaciers feed the headwaters of the Indus, Ganges and Brahmaputra Rivers which sustain the world's most populous region.
GC11B-08
Climatic Changes on Tibetan Plateau Based on Ice Core Records
Climatic changes have been reconstructed for the Tibetan Plateau based on ice core records. The Guliya ice core on the Tibetan Plateau presents climatic changes in the past 100,000 years, thus is comparative with that from Vostok ice core in Antarctica and GISP2 record in Arctic. These three records share an important common feature, i.e., our climate is not stable. It is also evident that the major patterns of climatic changes are similar on the earth. Why does climatic change over the earth follow a same pattern? It might be attributed to solar radiation. We found that the cold periods correspond to low insolation periods, and warm periods to high insolation periods. We found abrupt climatic change in the ice core climatic records, which presented dramatic temperature variation of as much as 10 °C in 50 or 60 years. Our major challenge in the study of both climate and environment is that greenhouse gases such as CO2, CH4 are possibly amplifying global warming, though at what degree remains unclear. One of the ways to understand the role of greenhouse gases is to reconstruct the past greenhouse gases recorded in ice. In 1997, we drilled an ice core from 7100 m a.s.l. in the Himalayas to reconstruct methane record. Based on the record, we found seasonal cycles in methane variation. In particular, the methane concentration is high in summer, suggestiing active methane emission from wet land in summer. Based on the seasonal cycle, we can reconstruct the methane fluctuation history in the past 500 years. The most prominent feature of the methane record in the Himalayan ice core is the abrupt increase since 1850 A.D.. This is closely related to the industrial revolution worldwide. We can also observe sudden decrease in methane concentration during the World War I and World War II. It implies that the industrial revolution has dominated the atmospheric greenhouse gas emission for about 100 years. Besides, the average methane concentration in the Himalayan ice core is higher than that in polar regions, indicating that the low latitude wet land is a major natural source of atmospheric methane.