Every month, the National Snow and Ice Data Center in Boulder, Colo., puts out a news release about how much ice is floating on the cold seas at the top of the world. Those who follow this obscure bit of news will know that last month marked the lowest extent of Arctic sea ice on record for June, going back to the beginning of satellite observations in the late 1970s. And summer still has a few months to go. Arctic sea ice typically shrinks until mid-September, when darker nights and colder temperatures come, and the ice cover begins to expand again. We don’t yet know if the approaching yearly minimum this September will be the historical low, but it seems on track to possibly match, or even exceed, the previous record minimum, which occurred in 2012. Nathan Kurtz, a NASA researcher who did an aerial survey of the ice cover in early July, told me recently, “It’s been a very warm year in the Arctic.”  The scientists who study ice in the Arctic tend to fall into one of two professional camps. The first consists of researchers who study land ice, which is the ice that flows in the mountain glaciers of northern Canada and Europe or the ice that’s locked within the vast frozen sheet that covers about 80 percent of Greenland. There’s an obvious, real-world imperative to this research: As temperatures climb and as arctic glaciers fracture and melt, water pours off the land and into the oceans and sea levels rise. If even a third of the ice covering Greenland were to melt, for instance, it would raise oceans by about seven feet. (We’re not near that point yet.) The data these scientists collect can help computer models become more accurate. That, in turn, may help us predict how quickly sea levels could rise in the next 100 years.  Studying sea ice is an altogether different pursuit. Technically, the term refers to ice that “grows” on top of salty water exposed to extreme cold temperatures for a long time. Oceanographers use a compelling and arcane vocabulary to describe this process. Sea ice begins as tiny, needle-shaped crystals, about a tenth of an inch long, known as frazil. Depending on the marine conditions, frazil congeals into “pancake ice” or “grease ice” (terms that basically capture what they look like) and eventually fuses into thick, floating frozen sheets. In recent years, as the earth has endured the warmest annual temperatures on record, the extent of sea ice around the southern continent of Antarctica has increased slightly, for reasons probably relating to natural climate variability. But the ice that typically covers the North Pole and surrounding areas of the Arctic has declined drastically. In April, I accompanied a group of researchers on their way to northeast Greenland. They were glued to the aircraft windows on the approach to the island’s coast, watching the dark sea stretch below for mile after mile when only a few decades ago it was blanketed by a white, frozen sheet. The trapezoidal plates of floating sea ice finally became visible only close to land, as a thin cold crust holding fast to the Greenland shore. “Where is all the ice?” a NASA project manager named John Woods asked.  The Arctic is now warming two to three times faster than the rest of the world, a phenomenon scientists have described as “Arctic amplification.” One anecdotal measure of amplification is made possible by the accounts of ship captains from centuries ago, who kept meticulous journals during an era in which the abundant northern ice could trap a boat for months and the pressure of colliding floes might crack a reinforced wooden keel like a thin-shelled nut. We now know, for instance, the Arctic sea ice near Siberia that fatally trapped the U.S.S. Jeannette in 1879, the subject of Hampton Sides’s recent best-selling book, is no longer a threat; those are now ice-free waters. We also know that the Northwest Passage, the perilous, ice-choked route from the Atlantic to the Pacific through Northern Canada, is navigable today. In August, a large luxury cruise ship, the 820-foot Crystal Serenity, will become the first such vessel to sail through the passage, carrying 1,000 tourists. Each paid $22,000 for the privilege. The cruise sold out in three weeks.  Satellite and airborne observations, of course, offer the most precise way to determine the decline of sea ice. Jaqueline Richter-Menge, of the U.S. Army Corps of Engineers’ Cold Regions Research and Engineering Laboratory in New Hampshire, told me that “the overall rate of decline is about 13 percent per decade” since 1979. There’s another way to visualize that, she said: Over the past 30 years or so, the Arctic has lost roughly 50 percent of its cover of sea ice, an area equivalent to more than half the area of the continental United States. And what ice remains has grown thinner. That means the volume of ice is decreasing too. “If you take into account the thickness,” Jennifer Francis, a Rutgers University scientist, told me, “it’s more like two-thirds of the ice has been lost.” She added, “This is pretty much a bad-news story.”  In the vast and chaotic climate systems that govern our atmosphere and oceans, making sense of how one change — diminished sea ice — affects places or people thousands of miles away is a task of such extraordinary complexity that it strains even the most sophisticated supercomputers. Nevertheless, what it means to be entering an era of new sea-ice minimums is one of the big scientific questions of the moment. Unlike the ice on land, sea ice, which derives from the ocean itself, has no direct impact on sea levels, so its melting poses no threat of coastal flooding. On the other hand, a recent group of scientific papers suggests that the steady retreat of sea ice may have a residual effect on all sorts of other things, like the ice covering Greenland or storms in New England.  As the ice recedes, exposing the ocean waters beneath it, the region’s surface appearance changes. Reduced ice cover means more open ocean water, which lowers the reflectivity — the so-called albedo — of the Arctic, which means that the darker surface absorbs more solar energy and less of it bounces back into space. A feedback loop ensues. More sea ice is lost, more energy is absorbed by the oceans, and this in turn can lead to lessened or thinner ice the following year as well as warmer temperatures. The process can continue, until the amplifying effects ultimately leave the Arctic ice-free in summer.  In the past year or so, scientists have begun to consider whether the changes in sea ice are influencing moisture and atmospheric conditions in the region, too. Francis says there now appears to be a connection between a reduction of sea ice and the more frequent creation of what are known as atmospheric “blocks” — weather patterns that can draw warm air to unusually high northern latitudes for long periods of time. “If the block is in the right place, it can bring a lot of that heat and moisture into one side or another of Greenland and contribute to a lot of melt,” she told me. “And that’s exactly what happened this past April. There was a huge strong block there, which brought a lot of very warm air up over the Greenland ice sheet.” Large parts of the Greenland ice sheet, which a recent study noted has lost one trillion tons of ice between 2011 and 2014, experienced a remarkably early thaw. Some of the extreme warmth reappeared in June, too, when I visited Greenland for a second time this year. Nuuk, the country’s capital, reached 75 degrees on June 9. That was 30 degrees above average. It was warmer there than in New York City.  Another recent paper, by Marco Tedesco of Columbia University’s Lamont-Doherty Earth Observatory, largely accords with Francis’s hypothesis that changes in the high Arctic are having serious weather impacts farther south: Tedesco pointed to an atmospheric block that resulted in unusually warm temperatures in 2015 over northern Greenland, which in turn led to an extensive melt on the ice sheet. Over the past three years, I’ve interviewed Tedesco a number of times: in his university classroom; over coffee on the Upper West Side of Manhattan; even on a hotel veranda in Greenland that overlooked a bay cluttered with icebergs. He views the Arctic as a systems engineer would. He recently told me he has been trying to “close the loop” and connect the exceedingly complex interactions that drive the northern climate system, which includes its sea ice, atmosphere and ocean circulations, and land ice. He thinks he may be getting closer. Tedesco told me we can’t yet quantify the link between sea-ice loss and land-ice loss. He is convinced, however, that these unusual warming events in Greenland create ripple effects that may extend for years. During warm spells, for instance, the top layer of Greenland’s surface melts; and because meltwater is less reflective than ice or fresh snow and absorbs more solar energy, that in turn leads to more melting. But even when the meltwater refreezes, Tedesco said, the new grains of ice seem to be permanently altered in a structural way that makes them more vulnerable to future melting. “You might have a warm-weather event that impacts things for a few weeks,” Tedesco said, “but that creates a ‘memory’ in the ice that might create impacts for a few years.” He called this “preconditioning.” Another way to think of it is that the melt that is now happening more frequently in Greenland (and in late July, the island endured yet another period of extreme warm weather) may be preconditioning the ice sheet for even more melting in the future.  The science of charting all these connections — if x occurs and y melts, what happens to z? — is still young; it is so new, in fact, that none of it has yet been incorporated into models that try to predict sea-level rises for later in the century. Yet it seems increasingly likely that the effects of a warming Arctic may create significant and unforeseen problems. Francis, for one, believes that some recent extreme weather events, like the pattern of massive snowfalls in Boston in 2015, were brought on by alterations in the atmosphere — what she calls “an extreme waviness” of the jet stream — akin to what recently affected Greenland. (The waviness can bring warm spells to cold regions or cold spells to temperate regions.) Francis isn’t sure, however, what will happen in the bigger picture when Arctic ice declines even further. The last time the Arctic was ice-free was probably 125,000 years ago. Marika Holland, a senior scientist at the National Center for Atmospheric Research in Boulder, told me that her computer models don’t give us a precise year for when we should expect an open summer ocean in the Arctic. “The range is somewhere in the mid-21st century,” she said, “and there’s uncertainty because there’s some natural variability in the system.” But she remarked that there’s no indication, with global CO2 levels and temperatures steadily rising, that we will somehow buck the direction of the trend. The outcome is the same, and the outcome is bleak, in all of her computer models. “They all go ice-free in the future.”
The melting away of Arctic ice because of climate change won’t raise sea levels, but what it means for the world is one of the big, complicated questions in science right now.