Greenland was nearly ice-free for extended periods during the Pleistocene

The Greenland Ice Sheet (GIS) contains the equivalent of 7.4 metres
of global sea-level rise1. Its stability in our warming climate is
therefore a pressing concern. However, the sparse proxy evidence of
the palaeo-stability of the GIS means that its history is controversial
(compare refs 2 and 3 to ref. 4). Here we show that Greenland was
deglaciated for extended periods during the Pleistocene epoch
(from 2.6 million years ago to 11,700 years ago), based on new
measurements of cosmic-ray-produced beryllium and aluminium
isotopes (10Be and 26Al) in a bedrock core from beneath an ice core
near the GIS summit. Models indicate that when this bedrock site is
ice-free, any remaining ice is concentrated in the eastern Greenland
highlands and the GIS is reduced to less than ten per cent of its
current volume. Our results narrow the spectrum of possible GIS
histories: the longest period of stability of the present ice sheet that is
consistent with the measurements is 1.1 million years, assuming that
this was preceded by more than 280,000 years of ice-free conditions.
Other scenarios, in which Greenland was ice-free during any or all
Pleistocene interglacials, may be more realistic. Our observations
are incompatible with most existing model simulations that present
a continuously existing Pleistocene GIS. Future simulations of the
GIS should take into account that Greenland was nearly ice-free for
extended periods under Pleistocene climate forcing.
The possibility that future warming will cause destabilization