Greenland is melting at an unprecedented rate, causing vast quantities of ice to disappear and global sea levels to rise. The fate of the ice sheet is not sealed, but unless CO2 emissions are sharply cut, the long-term existence of Greenland’s ice is in doubt.

The heat wave arrived early this spring — a shroud of temperate air, sweeping in during early June, which enveloped the Northern Hemisphere’s biggest ice sheet in a stifling hug. At its peak, nearly 45 percent of Greenland’s frozen surface turned to meltwater, coloring the huge white expanse with sapphire lakes and lapis streams. During the warmest stretch, runoff from the ice sheet amounted to about 2 billion tons, which meant that at the same time Greenland was losing water, the North Atlantic was gaining it. Some areas on the island were 40 degrees Fahrenheit above normal for this time of year. “We didn’t see anything like this prior to the late 1990s,” Thomas Mote, a University of Georgia scientist who monitors summer melting on the ice sheet, explained to CNN. “The melting is big and early,” Jason Box, a climatologist with the geological survey of Denmark and Greenland, informed the Washington Post. Greenland’s ice sheet covers about 80 percent of the island, and measures about 660,000 square miles; in its center, it runs to a depth of about two miles. According to the most recent NASA studies, the ice sheet holds enough water to raise sea levels by about 24 feet, should it ever disappear completely.

What seems clear now is that Greenland is no longer changing in geological time. It is changing in human time.

It’s worth noting that the ice has undergone summertime melting events ever since visitors to the island have made field observations. Hinrich Rink, the first Danish researcher to seriously investigate the contours of Greenland’s ice, correctly surmised that it was the last remaining link to ice sheets that had covered Northern Europe and northern North America during previous ice ages — a “Rosetta Stone” that could explain the mysteries of a lost, frozen world. During one summer trip in the mid-1800s, Rink surveyed the western edge of Greenland’s ice and described rushing streams that cut through the ice and dropped into depthless moulins. Where did Greenland’s meltwater go, Rink wondered? Yet we now know that the Greenland of today is different from the Greenland that Rink experienced. The ice sheet is melting more, and melting earlier in summer, and melting in ways that computer models suggest will ultimately threaten its long-term existence. A recent paper in Nature presented compelling evidence, gathered from cores extracted from the ice sheet, that demonstrated Greenland’s recent melt is “exceptional” over the past 350 years and that the ice sheet’s response to higher temperatures is now “nonlinear.” In the last two decades, melting rates of the ice are 33 percent higher than 20th century averages; the melting, moreover, is not only increasing but accelerating. Many scientists who have spent their careers on the ice sheet have witnessed these changes firsthand. Konrad Steffen, who has built up a record of meteorological readings around Greenland over the course of the past 30 years, has calculated that between 1990 and 2018 average temperatures on the ice sheet have increased by about 2.8 degrees Celsius, or 5 degrees Fahrenheit. While the highest points on the ice sheet are still mostly resistant to melting, over the same 30-year time period the total area of the ice sheet that has become vulnerable to surface melting has increased by around 65 percent. And what seems clear now is that Greenland is no longer changing in geological time. It is changing in human time.

A team from the Danish Meteorological Institute travels by dogsled across a pond of meltwater in northwest Greenland to retrieve equipment on June 13. Steffen M Olsen/Twitter

One evening, in the picturesque Greenlandic village of Ilulissat, Steffen, the research director at the Swiss Federal Institute for Forest, Snow, and Landscape Research, pointed out to me that by the measure of its current losses — equaling about one millimeter of sea level rise per year — Greenland’s ice sheet could last 7,000 years. But neither he nor any glaciologist working there seems to think that will be its fate. As Steffen put it, the warming curve for Greenland in the coming years, and especially into the next century, “gets steeper, and steeper, and steeper.” That same evening in Greenland, Steffen, speaking of rising sea levels, said darkly: “There will be a change coming, and obviously a change that we have not seen in thousands of years.” Greenland’s ice is not only beset by warming air. It is beset by warming water, too.

In recent years, about half of the island’s ice has been lost from surface melt. But another half has been drained by massive glaciers — such as Jakobshavn on the west coast and Helheim on the east — that branch off from the ice sheet and end at the water’s edge. These so-called “marine terminating” glaciers seem especially sensitive to warming ocean temperatures that can accelerate iceberg losses and increase melting where the glaciers’ calving fronts meet the water. Jakobshavn, for instance — for decades one of the fastest and most active glaciers in the world — accounts for about 4 percent of sea level rises during the 20th century. And yet, these enormous rivers of ice can slow down sometimes, too. Jakobshavn recently paused when temperatures plummeted in the waters in the fjord where it ends; a recent study by scientists at NASA’s Jet Propulsion Lab concluded the change in water temperatures was prompted by a cooling ocean current brought on by something known as the North Atlantic Oscillation. So it was likely just a temporary break for Jakobshavn, which will probably accelerate again when the climate oscillation flips back. Put simply, the overall trend appears bleak; and steadily warming oceans, as NASA’s Josh Willis warned, “is bad news for Greenland’s ice sheet.”

In the late 1990s, sensing experiments revealed dramatic changes in the most remote polar regions.

Willis’ point about the long game here is crucial: What ultimately matters about Greenland isn’t necessarily the month-to-month or year-to-year variations in summer melt and icebergs. It’s the math that tells us how the ice sheet’s losses stack up against its gains over long periods of time. In other words, during any given year, the ice sheet is replenished by snowfalls that blanket the island during the cooler months, and this process — summer loss, winter gain — has defined the ice sheet’s bulk during its recent history. For the past hundred years, glaciologists who studied the island’s ice strained to come up with a precise calculation for how its mass balance was trending. Was the ice sheet growing from big snowfalls and advancing glaciers? Or was it shrinking from warmer ocean and air temperatures, retreating glaciers, and surface melting? Sometimes, the consensus was that the ice sheet was in equilibrium, and summertime subtractions were roughly balanced by wintertime additions. The first definitive field study of Greenland’s annual snow accumulations, done by a University of Alaska glaciologist named Carl Benson in the 1950s, led to the hypothesis that the ice sheet appeared to be in a state of balance. And yet, it has always been exceedingly difficult to measure the mass of an ice sheet by means of traditional fieldwork. Even if a science team can assess snowfalls by taking a heroic road trip over the ice sheet, as Benson did, gauging losses from calving glaciers and meltwater streams at the coasts is far more difficult. In the mid-1980s, a glaciologist named Robert Thomas wrote a report for NASA arguing that it was finally time to figure out a definitive way to measure the ice sheets.

The total surface area experiencing melting in Greenland in June (blue), compared to the 1981-2010 average (gray). NSIDC

“Despite 25 years of intensive field work in Greenland and Antarctica,” he noted, “and the expenditure of billions of dollars, we are still unable to answer the most fundamental glaciological question: Are the polar ice sheets growing or shrinking?” For Thomas and a number of other NASA scientists, the only answer was to measure Greenland from above — “sensing” the ice from planes and satellites. In the modern era, we take for granted satellite data that can tell us instantly — and almost precisely — what percentage of Greenland’s surface is melting. The information is accessible daily on websites like Greenland Ice Sheet Today, for instance, which is run by the National Snow and Ice Data Center. But it’s worth understanding just how revolutionary these early “remote sensing” projects were, and how they helped scientists grasp something that had never been measured before. For the first time, beginning in the late 1990s, sensing experiments revealed dramatic changes occurring in the most remote regions of the polar world. What’s more, the data coincided almost precisely with significant climate changes in Greenland, as the Arctic began to warm at a rate almost double the average of the rest of the earth. The first airborne mission to effectively measure an ice sheet was spearheaded by NASA’s Thomas and was known as PARCA, or the Program for Arctic Regional Climate Assessment; it was conducted over the Greenland ice sheet during 1993 and 1994, and again in 1998 and 1999. By flying precise routes over the ice sheet and using a tool known as a laser altimeter, the researchers determined that over a six-year period Greenland had lost 51 cubic kilometers of ice per year, which was akin to an ice cube 2.3 miles long on each side falling into the ocean annually.

We are not yet in the midst of a Greenland meltdown. We are only poised, precariously, at its worrisome beginning.

The era of remote sensing of ice sheets thus began. In subsequent years, NASA and other space agencies sent up a variety of satellites to monitor changes at the poles. And in terms of assessing Greenland, the most important was NASA’s GRACE satellite, which launched in 2002 and used changes in gravity to gauge variations in the ice. GRACE’s measurements were almost immediately surprising. In the early years of operation, the satellite indicated that Greenland was suffering net losses of just over 100 billion tons of ice annually. But by 2010 it appeared that the ice sheet’s deterioration was accelerating. It was losing, on average, about 260 billion tons annually. And the chart of lost ice began to look like a steep, descending staircase. In 2012, GRACE measurements indicated that more than 400 billion tons of ice came off Greenland and into the ocean. That summer also coincided with a profound warm spell that, on a single day, melted 97 percent of the ice sheet surface. One of the concerns among glaciologists is that 2019, following June’s early melting, will resemble 2012 — meaning Greenland’s ice sheet could set new records in terms of summertime melt. It is too soon to know for sure. It seems clear, in any event, that if this summer marks a new extreme for ice losses, such a data point will prove less meaningful than the bigger picture. With 3 quadrillion tons of ice sitting atop Greenland — that’s 3,000,000,000,000,000 tons — there is a lot more to melt, chip, and shatter into the oceans. And contrary to the news headlines, we are not yet in the midst of a Greenland meltdown. We are only poised, precariously, at its worrisome beginning.

Meltwater running off the Russell Glacier, Greenland on April 28. Credit: NASA/Jefferson Beck

How long does Greenland’s ice have left? It is not a simple question. Arguably, it does not have a definitive answer, either. One reason is that Greenland’s future depends on the complicated physics of glaciers and how the ice sheet is buffeted by the immensely complex effects of a warming atmosphere, varying cloud cover and snowfalls, and shifting ocean currents. Understanding the interplay of all these forces remains a work in progress. In addition, Greenland’s ice will likely be affected by a number of feedback loops that may accelerate or, possibly, slow its demise. To take just one example, as Greenland melts in a warmer climate, the ice sheet will likely get lower in elevation, where warmer temperatures can affect it even more. That means the more Greenland melts now, the more Greenland may melt in the future. Even so, the ice sheet’s collapse is not a given. The most defining aspect of its future may depend on how we act in the coming decades — whether we curtail CO2 emissions rapidly to keep the Arctic cool enough to preserve this vestige of the ice ages, or whether we continue on our current trajectory. Just as Greenland was enduring its June meltdown, the journal Science Advances published an exhaustive modeling study led by Andy Aschwanden at the University of Alaska. The paper considered different scenarios for the future of Greenland’s ice sheet. And the takeaway was that it seemed very likely that Greenland could lose all of its ice in a thousand years, based on our current emissions pathway. On the other hand, a dramatic curtailment of CO2 emissions — one in line with the framework hammered out in the 2015 Paris agreement — might preserve most of the ice. The longer we waited to act, however, the harder it looked to preserve the ice sheet.