Contrasting responses of Central Asian rock glaciers to global warming

While the responses of Tien Shan glaciers – and glaciers elsewhere – to climatic changes are becoming increasingly well understood, this is less the case for permafrost in general and for rock glaciers in particular. We use a novel approach to describe the climate sensitivity of rock glaciers and to reconstruct periods of high and low rock glacier activity in the Tien Shan since 1895. Using more than 1500 growth anomalies from 280 trees growing on rock glacier bodies, repeat aerial photography from Soviet archives and high-resolution satellite imagery, we present here the world's longest record of rock glacier movements. We also demonstrate that the rock glaciers exhibit synchronous periods of activity at decadal timescales. Despite the complex energy-balance processes on rock glaciers, periods of enhanced activity coincide with warm summers, and the annual mass balance of Tuyuksu glacier fluctuates asynchronously with rock glacier activity. At multi-decadal timescales, however, the investigated rock glaciers exhibit site-specific trends reflecting different stages of inactivation, seemingly in response to the strong increase in air temperature since the 1970s.


Study sites
Many rock glaciers in the Tien Shan have developed during the Holocene; their formation is likely to be associated with high rates of debris accumulation during phases of glacier retreat 1 .
All investigated rock glaciers are of (para-)glacial origin and emanate from cirques with small residual glaciers at altitudes between 3300 and 3800 m asl (Table S1), where they have been overwhelmed and depressed during Little Ice Age glacier advances 2 . The steep cirque walls and lateral slopes supply the debris for the rock glaciers 3 , which consist mainly of weathering-prone coarse-crystalline granite, schists and, in the case of Karakorum rock glacier, Cambrian gneisses 1,3,4 . The steep terminal fronts (>37°; Fig. S1) with loose boulders and the "drunken trees" (Fig. S2) growing on the rock glaciers indicate that the investigated rock glaciers are highly active 4-6 :

Dendrogeomorphology
More than 40 years ago, the principles of dendrochronology were further developed to include the dating of geomorphic events, thus establishing the principles of dendrogeomorphology 7 .
While the dendrogeomorphic approach has been widely applied in the field of natural hazards 8 , the lack of trees at high elevations, namely in permafrost regions, has so far limited its application in rock glacier research to a few studies [9][10][11][12] . The principal idea behind the dendrogeomorphic approach in a periglacial context is that trees growing on rock glaciers will react to unstable growing conditions from pronounced rock glacier activity with the formation of growth anomalies. Horizontal forces in the shallow root zone can result in tree tilting ("drunken forest" 9 ) and thus initiate the formation of compression wood, but also lead to the (partial) destruction of the root plate and thus induce a strong growth reduction. The burial of the tree stem base may lead to abrupt growth suppression as well. All these reactions can be dated with annual or even seasonal resolution. 3

Climate data
Time series of temperature, precipitation and snow depth were available in daily resolution from

Mass balance
Mass balance has been assessed since 1957 at Tuyuksu glacier, which makes this data set the longest mass balance series in Central Asia. The mass balance data were made available through the World Glacier Monitoring Service 13 , the original author is P. A. Cherkasov. For the period 1895-1957, we relied on reconstructed mass balance of Tuyuksu glacier [14][15][16] .

Earthquakes
We analysed all documented earthquakes with a magnitude higher than 5 and with an epicenter  Table S4.
Contemporary high-resolution satellite images with spatial resolutions between 0.5 to 1 m for all four sites were co-registered, if more than one was available for a site, and orthorectified using 4 the ASTER Global DEM (tests with Shuttle Radar Topography Mission DEM have also been performed) and UTM projection. Based on these master orthoimages, the Soviet era aerial photos and the 1970s Corona declassified satellite photos were oriented and rectified using rational function models based on 40-50 control points each. The resulting orthoimages have spatial resolutions of 0.5 to 2 m.