Conference Agenda

Unlike most glaciers experience mass lost in recent decades, the Karakoram and its surroundings seem rather stable. The phenomenon is claimed as ‘Karakoram anomaly’ and/or ‘Karakoram-Pamir anomaly’. Given its remote, the field observations for glacier mass balance and flow rates are hard and archive data are rare. Remote sensing technique plays an essential role in alpine glaciers observations. Previous studies yielded a positive glacier mass balance for the Karakoram and later extended to the Pamir Plateau between 2000 and ~2012, however the laser altimetry observation at almost the same period (2003 - 2009) found slight glacier mass loss but claimed that the anomaly centred at the West Kunlun.

To make a more accurate observation and to understand the ‘Karakoram anomaly’, we applied Differential SAR Interferometry (D-InSAR) technique to a set of X-band bistatic TerraSAR-SAR-X/TanDEM-X (TSX/TDX) images observed at ~2013 by respecting to SRTM DEM observed in 2000 to derive glacier mass balance. The topographic residual phase of D-InSAR is unwrapped and then transferred to height changes. By presuming density of 850 Kg/m³, the volume changes of glaciers are converted to glacier mass balance. We compare quasi-simultaneously observed C-band and X-band SRTM (both in February of 2000) to evaluate and to remove the penetration depth differences at different elevation bins. The possible seasonal variation in terms of glacier mass balance was evaluated at an adjacent site by using TSX/TDX images observed in different months in one year. The standard deviation of differential processing between SRTM and TSX/TDX is about 6.27m, which is more accurate than the previous study using SPOT DEM and SRTM. Besides, it noticed that TSX/TDX make more efficient observations at accumulating area than optical observations. The result found that both east and west part of the Karakoram presents almost zero glacier mass balances, which were −0.020 ± 0.064 m w.e. yr⁻¹ and −0.101 ± 0.058 m w.e. yr⁻¹, respectively. Most negative glacier mass balance was contributed by the southern slope of the Karakoram while the northern slope was rather stable. At the most northeastern part of the Karakoram, where are very close to the edge of Tarim basin, the glaciers presented thickening in also most every elevation levels. The glacier mass balance at the Karakoram presented a decreasing gradient from the edge of the Tarim basin to the southwest of Karakoram. The mass balance for 2000 to ~2013 is almost identical comparing to 1974 to 2000, which implies that the stable environment at the Karakoram despite the global warming trend.

The re-analysis monthly GHCN_CAMS Gridded 2m Temperature data found an increasing start from 1995 to 2000 for about 1 degree and kept stable after then. Monthly precipitation Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) suggest rather stable annual and seasonal precipitation from 1980 to 2000, followed by an increasing trend. The precipitation increased from ~260mm/yr before 2000 to ~350mm/yr by 2010. The meteorological data suggested that a warming and wetting trend after 2000 for the Karakoram, which possibly explains the ‘Karakoram anomaly’ was induced by increasing precipitation rather than a cooling trend of temperature.

Thermokarst, a process that characterizes landforms caused by thawing of ice-rich permafrost, is a key indicator of permafrost degradation. Surface dynamics of thermokarst processes on Qinghai-Tibet Plateau (QTP) of China, is still poorly quantified or understood. It is also challenging to detect and measure surface subsidence due to loss of subsurface ice over a large area. The Persistent Scatterer Interferometric Synthetic Aperture Radar (PSInSAR) technique has the potential to detect local or regional thermokarst-induced surface subsidence with the advantage of full resolution and millimeter to centimeter accuracy by less affected related to the temporal or geometric decorrelations. Previous studies based on multi-baseline time series analysis have separated the seasonal and thermokarst-induced surface subsidence only using SAR images acquired during thaw seasons. To fully usage of the SAR images, we introduce frost heave processes during early freeze season and subsequent stable stage when the layer is completely frozen.

Applying our improved PSInSAR method to 17 L-band ALOS-1 PALSAR images over Eboling Mountain where 22 thermal erosion gullies are well developed, we found a mean gradual subsidence trend of 1.3 cm/year, with a maximum of 5 cm/year near the thermal erosion gullies. It is equivalent to an ice volume loss of 1.4810⁴ m³/year over the entire thermokarst landform in the study area. We also found that the ground surface nearby the thermal erosion gullies is more likely to undergo subsidence. It indicates that the thermal erosion gullies could affect the permafrost processes at its surroundings. This study promises a potential of using PSInSAR to identify thermokarst landforms, map and quantify permafrost thaw subsidence, and assess its impacts over large areas such as the QTP.