The stability of the world's three great ice sheets is among climate science's biggest questions. Any of them melting would displace hundreds of millions of people living in low-lying areas. So scientists are as disturbed as they are astonished to discover fossil plants under the Greenland ice sheet, suggesting the island lost its ice cap much more recently than previously suspected.
The question of when most of Greenland's ice last melted is hotly contested. Five years ago Nature published two papers in the same edition that reached almost opposite conclusions using differing methods. Even if Greenland has held onto most of its ice for the last few million years, we can't assume it will do the same through the Anthropocene, but it would be an encouraging start.
However, a core drilled 55 years ago as part of a Cold War ruse suggests we don't have even that comfort. The core was one of those collected at Camp Century, a military base pretending to be a research station in northwest Greenland, one of the closest land locations to the North Pole with ice 1.4 kilometers (0.9 miles) thick. Lacking modern analysis techniques scientists missed the core's major treasure but fortunately, the cores were kept, and their potential significance was recognized during a relocation from one freezer to another.
When Dr Andrew Christ of the University of Vermont examined the core he expected to see nothing but sand and rock, although to a geologist those are interesting enough. Instead, he found leaves, twigs, and mosses that could only have come from a time when the area was ice-free for long enough for vegetation to flourish.
The important questions then became when this ice-free period occurred, what caused it, and how quickly it happened.
"Ice sheets typically pulverize and destroy everything in their path," Dr Christ said in a statement. "But 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."
In Proceedings of the National Academy of Sciences, Christ and co-authors conclude Camp Century and most of Greenland were ice-free at least twice during the Pleistocene era. One of these times, possibly the last interglacial 129,000-116,000 years ago, was less than 1.1 million years ago.
The paper is accompanied by another from a team who independently reached similar conclusions using different methods. Dr Sarah Crump of the University of Colorado found dwarf birch pollen in lake sediments from nearby Baffin Island, Canada. The shape of the basins here has prevented the sediments deposited during warm periods being scoured clean by glaciers. Dwarf birch are moving north in response to current warming, but Crump's work shows they reached considerably further north during the interglacial, consistent with an ice-free Greenland.
Birch leaves are much darker than the tundra they are replacing, so their expansion warms the planet by absorbing light that would otherwise be reflected, overwhelming any carbon capture.
In the last million years global temperatures have not exceeded the extra few degrees we are on track to experience this century without drastic cuts to greenhouse gas emissions. If Greenland could melt in such circumstances, it can do so again, possibly very fast. Even without an accompanying loss of ice from Antarctica, that would raise global sea levels by 7 meters (22 feet), enough to put the most densely populated parts of the planet under water.