Changes of Petrov glacier and its proglacial lake in the Akshiirak massif, central Tien Shan, since 1977

A combination of remotely sensed data, field mapping, bathymetric survey and geophysical soundings is used to describe the recent changes in the terminal area of Petrov glacier, the largest glacier in the Akshiirak massif, central Tien Shan. According to our results, three periods of accelerated glacier retreat (1977-80, 1990-95 and 2008-09) can be distinguished since 1977. The largest recession occurred in 1990-95 when the glacier retreated by 54 +/- 8 m a(-1). Accelerated glacier retreat affected the enlargement rate of Petrov lake, which increased by 0.04-0.1 km(2) a(-1) and by 1.3-2.2 x 10(6) m(3) a(-1) in the last three decades. Since 1995, the mean annual retreat rate of the lake-calving northern glacier section has been up to three times higher than the retreat rate of the land-terminating southern section. The calving flux ranged from 2.5 to 4.6 x 10(6) m(3) a(-1) in 2003-09, resulting in a total glacier mass loss of (17.7 +/- 0.4) x 10(6) m(3). The calving terminus of Petrov glacier was >65 m thick in 2009 according to ground-penetrating radar measurements.