Melting permafrost. Aerial view of lakes and rivers over permafrost in Lena Delta Wildlife Reserve, Siberia, Russia, in summer. Permafrost is soil that remains at or below freezing for two or more years. Lakes and rivers like these appear in summer when the upper layer of the tundra underground thaws. Climate change is causing the permafrost soils not only in the Lena Delta, but in the Arctic as a whole to thaw at increasing rates during the brief Arctic summers.