October 1 (National Geographic) – Greenland is on track to lose more ice this century than it has at any other point in the Holocene, the 12,000-year period in which human civilization has flourished, an alarming new study has found.
The study, published today in the journal Nature, offers the latest evidence that Earth’s northernmost ice sheet, which contains enough frozen water to raise global sea levels by 24 feet, has entered a period of rapid decline and may melt away entirely if humanity continues burning fossil fuels at current levels. The research also puts to rest the notion that Greenland’s recent deterioration might be part of a natural cycle, by showing just how fast the current meltdown is compared with the ups and downs of the geologic past.
“We have confidence now that this century is going to be unique in the context of natural variability of the last 12,000 years,” says lead study author Jason Briner, a glaciologist at the University at Buffalo.
Stitching together past and future
Over the last 40 years, rapid Arctic warming has caused Greenland to lose ice at an accelerating clip. But in order to place this trend in a long-term context, scientists require records of the ice sheet’s growth and decline over thousands of years.
Previously, researchers had attempted to reconstruct changes in the size of Greenland’s ice sheet throughout the Holocene using oxygen-18 isotopes inside ice cores, which give an indication of past temperatures. But most of those analyses extrapolated climate conditions across all of Greenland from a single ice core, adding significant uncertainty to the reconstruction. And no prior studies had stitched together model reconstructions of Greenland’s history with projections of how much ice will melt this century.
“People have modeled the past of the Greenland ice sheet and people have modelled the future,” Briner says. “There hasn’t really been one study that used the same model, the same methods, and did the whole shebang from past to future.”
Briner and his colleagues have now filled this gap in time, and in the process, they have reconstructed Greenland’s melt history in a more sophisticated way. The researchers combined an ice sheet model with both temperature and snowfall data from a series of ice cores collected across Greenland. They then extended this information across the ice sheet using a climate model. The researchers ran their models backward and forward in time, from 12,000 years ago to A.D. 2100, using low- and high-carbon emissions scenarios to game out possible futures for the ice sheet.
Their results show that Greenland’s present-day meltdown is as extreme as anything that has ever occurred during the Holocene.
Between 10,000 and 7,000 years ago, a warming event known as the Holocene thermal maximum caused Greenland’s ice sheet to shrink dramatically. During one particularly extreme century, approximately 6,000 billion tons of ice melted—comparable to the 6,100 billion tons of ice Greenland could lose this century if the average melt rate from 2000 and 2018 is extrapolated forward in time.
But 6,100 billion tons is a conservative estimate of how much ice Greenland is on track to lose: As carbon continues to accumulate in the atmosphere, the planet will continue warming and average melt rates should continue to speed up. It’s also likely that Greenland will experience more extreme melt years like 2012 and 2019, when heat waves on top of climate change conspired to trigger enormous summertime ice losses.
Under an optimistic scenario where humanity quickly reduces global carbon emissions, Briner’s models show that Greenland will lose about 9,700 billion tons of ice this century. But if we continue to burn fossil fuels with abandon, Greenland could lose closer to 21,000 billion tons of ice this century, a melt rate approximately four times higher than the highest model estimates for the past 12,000 years.
That latter scenario, called RPC 8.5, is considered pessimistic from an emissions standpoint, but it is the pathway Greenland is following most closely based on recent ice losses. The RPC 8.5 findings are also consistent with another recent study that concluded Greenland could be ice free in as few as 1,000 years.
Ted Scambos, a glaciologist at the National Snow and Ice Data Center who wasn’t involved with the study, called it “an excellent merging of paleo records, present-day measurements, and modeling that extends the work to future projections.”
“The paper is also an answer to those who dismiss the ongoing effects of climate change with ‘the earth has always changed’—and the answer is, ‘not at this pace’,” Scambos says.
Limitations and next steps
The study’s results come with an important caveat: The authors restricted their models to southwest Greenland because the region has relatively simple physics, with most of the ice melt driven by air temperatures as opposed to ocean warming and the breakup of glaciers that spill into the sea. From there, they extrapolated across Greenland.
The ice losses the researchers concluded from their models closely matched observational data from the last 40 years, lending support to the findings. Still, as a next step, the team would like to apply their models across all of Greenland and incorporate additional processes that melt and break up ice.
Southwest Greenland is “the region that has showed one of the highest increases in melting in recent years, so it’s a good indicator for how the rest of the ice sheet will likely change overall,” says glaciologist Ruth Mottram of the Danish Meteorological Institute, who wasn’t involved in the paper.
Mottram points out that the authors also used glacial moraines—fields of rocky debris left behind after a glacier retreats—to see how well their models of ice growth and decline matched real-world evidence. “The combination of field and model results helps us to have more confidence in the model results for the past climate and hopefully therefore in the future projections,” she says.
But Ellyn Enderlin, a glaciologist at Boise State University, feels that extrapolating from the southwest to the entire ice sheet is “a bit of a stretch.”
“Although the authors point out that trends in modern mass loss are similar for this region and the ice sheet in its entirety, it is possible that this correlation would not hold over much longer time periods when the ice sheet’s geometry was different from present,” Enderlin says.
On a larger ice sheet with more glaciers that run into the sea, Enderlin says, melting “would have been strongly controlled by the instabilities inherent to these systems, which may not follow the same patterns of mass loss modeled here for the southwestern portion of the ice sheet.”
While there’s more work to be done to suss out the finer details of Greenland’s past and its future fate, Scambos says at this point the scientific community has amassed enough evidence to say with confidence that Greenland—like Earth’s climate at large—will be dramatically altered unless humanity changes course.
“We have found a gas pedal and we have put a brick on it in terms of climate,” Scambos says. “And it’s not going to stop until we change.”