Antoni Gerard Lewkowicz 

The spatial distribution and links to climate of retrogressive thaw slump (RTS) activity over 32 years were examined using Google Earth Engine Timelapse videos for five areas in the western Canadian Arctic totalling 150 000 km2, each previously identified as having a high spatial concentration of these thermokarst landforms. Four spatial datasets run from 1984 to 2016 (Banks Island, northwest Victoria Island, Bluenose moraine, Paulatuk region), while the fifth starts in 2001 (Richardson Mountains / Peel Plateau). The total number of RTS active in the first four areas increased more than 50-fold, from 115 in 1984 to nearly 6000 in 2013. A further 573 RTS were active in this peak year in the Richardson Mountains / Peel Plateau. RTS developed most frequently adjacent to rivers (45%), with fewer on slopes (27%) or next to lakes (23%), and the smallest group at the coast (5%). However, there was considerable variation among the areas, and more than half in the Bluenose moraine and the Paulatuk region were initiated on lakeshores. High RTS initiations were linked to particularly warm summers, but once initiated, more than half of those RTS with long records remained active for more than 25 years. The impacts of this geomorphic activity included changes of colour in more than 500 lakes due to direct or indirect sediment inputs from RTS. The results show that the non-linear orders of magnitude increase from the 1980s to the 2010s previously reported for Banks Island extended across other ice-rich parts of the western Canadian Arctic.