As ice retreats, frozen mosses emerge to tell climate change tale

SEATTLE — Some mosses in the eastern Canadian Arctic, long entombed in ice, are now emerging into the sunlight. And the radiocarbon ages of those plants suggest that summertime temperatures in the region are the warmest they’ve been in tens of thousands of years.

As the planet warms and the ice retreats on Canada’s Baffin Island, the change is revealing plants long buried beneath the ice. And in some locations, the emerging plants last saw the sun at least 45,000 years ago — and possibly as much as 115,000 years ago. Paleoclimatologist Gifford Miller of the University of Colorado Boulder reported the finding October 22 at the Geological Society of America’s annual meeting. “We were stunned,” Miller said.

Miller’s team has collected an impressive number of samples and their findings are very compelling, said geomorphologist Lee Corbett of the University of Vermont in Burlington, who was not involved in the study. “It truly is an indication that humans are pushing the climate into a new regime, one that modern, agriculture-based civilizations have never witnessed.”

To track the growth and retreat of ice cover in the region, Miller and colleagues have been hunting for remnants of scraggly mosses along the edges of the island’s retreating ice sheets. Radiocarbon dates of the emerging plants correspond to when the mosses were last exposed to the atmosphere and were photosynthetically active.

So far, Miller’s team has determined 370 different ages for plant samples collected on Baffin Island. The ages tend to cluster into groups, each representing a time when the ice expanded across the island and entombed the plants. One large group dates to around 3,700 years ago; another to around 900 years ago; and a third to around A.D. 1450, corresponding to a cold period known as the Little Ice Age.

But in a few regions, the plants were so old that they had no radiocarbon left in them. Carbon has three isotopes — forms of the element with the same number of protons but different masses — but only carbon-14 is radioactive. It has a half-life of about 5,730 years, after which about half of the carbon-14 atoms in the original sample will have decayed away. So after about 45,000 to 50,000 years, almost all of an objects’ radiocarbon will have decayed.

Mosses with the “dead” radiocarbon were found at high elevations, on pedestals of rock with persistent ice caps that are now slowly melting away. Because the radiocarbon clock stops at about 50,000 years, it’s not possible to determine exactly when those spots last were ice-free. But an ice core collected in nearby Greenland suggests that the planet experienced continuous cold from 40,000 to about 115,000 years ago, when the last warm interglacial period ended, Miller said.

Originally, the researchers expected to find plants dating to medieval times, which would have suggested that the region is the warmest it’s been since the Middle Ages. But finding 3,700-year-old plants was a surprise, Miller said. And “we never anticipated we’d find plants 40,000 years old,” he added. “It’s a bit spooky because it provides quantitative evidence that the magnitude of summer warmth is already sufficient to melt all ice in the eastern Canadian Arctic. It’s just a matter of time now.”