Earth’s climate system will respond to the stronger greenhouse effect we’ve produced in various ways. But one key question about its response is how much ice will be lost from the great ice sheets of Greenland and Antarctica. Push them too far outside their climatic comfort zone, we know, and a large retreat of glacial ice can follow, raising global sea levels.

Given that the innards of these ice sheets are complicated and inaccessible, researchers rely heavily on what we can discover about their pasts. One key point in the past has been the warm interglacial period around 400,000 years ago. The cycles in Earth’s orbit that govern the timing of the ice ages conspired to produce an exceptionally long respite from the cold at this time—twice as long or more than the most recent interglacial period about 120,000 years ago. It may have been warmer, as well, and some estimates put sea level in the vicinity of six to 13 meters higher than it is today.

Did that sea level rise come mostly from a smaller Greenland ice sheet, loss of ice from Antarctica, or some mixture of the two? We don’t know, and it’s difficult to answer since glaciers destroy the evidence of their past retreat when they expand again. A new study led by Alberto Reyes (now at the University of Alberta) and Oregon State’s Anders Carlson sifted sediments off Greenland’s coast to find clues about what the ice sheet was up to at that time.

Greenland’s bedrock varies from place to place, though it’s mostly exceptionally old. As the ice sheet flows over any portion of it, it grinds it down, producing sediment that is eventually carried out by the glacial meltwater. That sediment still bears the chemical characteristics of the rock from which it came. The researchers examined a core of seafloor sediment from the Eirik Drift off the southern tip of Greenland, looking for silt that originated from glacial activity. The isotopes of strontium, neodymium, and lead in that silt can fingerprint which bedrock was being eroded by the ice sheet.

Those isotopes showed a shift over the 400,000-year-old interglacial period, a shift consistent with the southern portion of the ice sheet retreating inward and stopping its erosion of the southern bedrock regions. The researchers calculated the changing contribution from each region’s fingerprint that would produce that shift, finding that silt from the southern third of Greenland fell to as little as a couple percent of the total. In the last two interglacial periods, it never fell below 10 to 15 percent. That points to the Greenland ice sheet shrinking drastically during that time period. Pollen previously recovered from ocean sediment cores has testified to a large growth of pines in southern Greenland at this time, which is what you might expect after such a retreat.

The researchers used a model of the Greenland ice sheet to simulate a configuration of the ice sheet consistent with their isotopic evidence and previous research. A small amount of ice was left at the southern tip of Greenland, and a diminished ice sheet remained in the central and eastern part. All total, that would result in a global sea level 4.5 to six meters higher than it is today.

If the sea level was really six meters higher then, that would mean that Greenland might have been responsible for most of it, with Antarctica playing it cool. If, on the other hand, it was as much as 13 meters higher, Antarctica would have to kick in a hefty contribution to tidy up the math.

So what could have driven the Greenland ice sheet to “collapse” in this fashion? We know that this warm interglacial period was exceptionally long, but we’re not sure just how warm it was. Some evidence near Greenland records temperatures not much more than one degree Celsius warmer than the present day, while a record from a Siberian lake indicates summer temperatures as much as five degrees Celsius warmer than that.

Ice-sheet model simulations show a threshold for slow-motion collapse around one degree Celsius warmer than today, whereas three-to-four-degree Celsius warmer temperatures can get the job done more quickly—in a handful of millennia rather than tens of thousands of years. Without knowing how warm it got 400,000 years ago, it’s difficult to guess which of those scenarios is more likely.

What does all this mean for Greenland’s outlook? Reyes told Ars, “Ice-sheet and climate models suggest that the time frame for severe retreat of the Greenland ice sheet is millennial-scale, rather than centennial-scale. This is probably good news, in terms of the foreseeable future for the ice sheet, in that we aren't soon likely to see the kind of retreat and associated sea-level rise we reconstruct for [this period of the past]. On the flip side, those same computer models also predict that the tipping point for slow but irreversible loss of the ice sheet may be only a few degrees above present temperatures. So, taken together with the recent news of the start of West Antarctic Ice Sheet collapse, the story that seems to be emerging is that recent and projected warming really will push the cryosphere pretty hard.”

[Disclosure: The author's wife, Kelsey Winsor, was involved in this study.]

Nature, 2014. DOI: 10.1038/nature13456 (About DOIs).