The Greenland ice sheet has been there for a long time. As near as we can tell, it could have been extensive as early as 45 million years ago. Evidence, as well as our understanding, is patchy, but scientists have been pretty confident on one thing: It seems to have been in place for at least 1 million years.
New evidence has come to light that contradicts that, however. At the bottom of a 1.4-kilometer (0.87-mile) ice core drilled from northwestern Greenland, scientists have found remnants of ancient plant material.
This suggests that, at least once within the last million-year period, and multiple times in the few million years prior, Greenland's ice sheet melted long enough during warm periods for significant vegetation - perhaps even a forest - to take root and thrive.
Warm periods like those we are currently experiencing due to climate change, according to an international team of scientists led by geologist Andrew Christ of the University of Vermont.
"Our study shows that Greenland is much more sensitive to natural climate warming than we used to think - and we already know that humanity's out-of-control warming of the planet hugely exceeds the natural rate," Christ said.
The evidence comes from a 1,368-meter core sample excavated in 1966 - over 50 years ago - in Cold War military base Camp Century. The secret plan for the base was to dig a system of tunnels to hide hundreds of nuclear weapons (it failed because the ice was too unstable). The official purpose of the base was, among other things, to conduct scientific research, and that's why the ice core was extracted.
Included in the core was 14 meters (46 feet) of silty ice, and 3.44 meters of frozen sediment from under the ice sheet. This sediment was packed up, in cookie jars of all things, and moved around from freezer to freezer, finally ending up in cold storage in Denmark, where it languished, forgotten.
It was only rediscovered in 2018 when scientists at the University of Copenhagen were sorting through the stored samples in preparation for relocation to a new freezer. When they realized what was in the jars, they mobilized a team to study the ancient dirt.
To their immense surprise, it wasn't dirt - at least, not entirely. Fossilized leaves and twigs were also found mixed in with the sediment.
"Ice sheets typically pulverize and destroy everything in their path," Christ said.
"What we discovered was delicate plant structures - perfectly preserved. They're fossils, but they look like they died yesterday. It's a time capsule of what used to live on Greenland that we wouldn't be able to find anywhere else."
The climate has fluctuated a lot over Earth's history, with multiple ice ages or glacial periods interspersed with warmer interglacial periods. During these times, scientists believe that at least some ice cover persisted on Greenland in the last few million years, even during the interglacial times.
The Camp Century core, taken from a high latitude of 77.1667 N, shakes that perception to its roots. According to the team's analysis of the sediment, rare isotopes of aluminum and beryllium that only form when cosmic rays hit the bare ground were found in the plants.
This means the region melted entirely, and stayed melted long enough for cosmic rays to form those isotopes, and for vegetation to grow and take them up, with moss and maybe even trees thriving.
Radiocarbon dating and luminescence dating - estimating the time since the sediment was last exposed to light - placed that melting to within the last million years.
Taken all together, the evidence suggests that Greenland's ice sheet - currently covering around 80 percent of the land mass - may not be as permanent as we thought. Scientists have predicted that if the Greenland ice sheet completely melts, the sea could rise by as much as 7 meters, so the problem is not a trivial one.
"Greenland may seem far away, but it can quickly melt, pouring enough into the oceans that New York, Miami, Dhaka - pick your city - will go underwater," said geoscientist Paul Bierman of the University of Vermont.
"This is not a twenty-generation problem. This is an urgent problem for the next 50 years."
The research has been published in the Proceedings of the National Academy of Sciences.