by Curt Stager
Venice is one case in point. It is already partly submerged, and it currently drops by twice the rate of sea-level rise as its buildings press into wet muds and supporting groundwater is pumped out. Residents have complained, structures have been lost, and corrective measures have been tried with variable success and at great cost, but it’s been more of an aggravation than a deadly disaster. And, of course, the submergence has also helped to turn Venice into a world-famous tourist destination.
New Orleans is also informative. It has long been sinking into the Mississippi Delta as sediments compress and de-water under their own weight and that of overlying buildings, as levees prevent the deposition of regenerating river muds, and as bedrock responds to lingering ice age distortions of the continental crust. In fact, most of the Gulf coast is also subsiding as a result of petroleum and groundwater extraction; some sites have dropped 10 feet (3 m) during the last century. Much of New Orleans itself is sinking by a quarter of an inch (6 mm) per year, with some sections dropping four times faster than that. Clearly, any city that crouches ever farther below sea level faces real danger from powerful storms. But most residents didn’t try to move out of New Orleans until hurricanes Katrina and Rita actually rolled in on them in 2005, and many are now rebuilding their homes despite the inevitability of future hurricane strikes.
Much of lowland Tokyo sank 6 to 13 feet (2 to 4 m) during the last century as a result of groundwater extraction; subsidence rates near the harbor can exceed 4 inches (10 cm) per year. Similar processes drive the soft ground beneath Bangkok, Thailand, downward as fast as 4.5 inches (12 cm) per year; that’s over twice the speed of Hansen’s extreme inundation estimate and forty times the recent rate of sea-level rise. And every year China’s largest city, Shanghai, sinks a third of an inch (10 mm) deeper into the Yangtze Delta. During the last century it dropped nearly 9 feet (3 m) and suffered billions of dollars in structural and flood damage.
These examples help to show what the future advance of the sea will really be like in most cases; slow, unrelenting, costly, exasperating, but rarely deadly to humans. It won’t be a frothing shoreward rush of waves, but it will still be well worth slowing down as much as possible, as residents of Shanghai and the other already-sinking cities would surely agree. In human terms, global sea-level rise will turn what has until now been a problem faced in isolation by several dozen urban centers into a worldwide phenomenon.
I suspect that our descendants will deal with those coming changes as we have usually dealt with such environmental disturbances thus far—that is, piecemeal. Some who can afford to dike a strip of shoreline or build urban flood-control gates will do so, and some will pull up and move well ahead of the waves. Others will simply accept the risk of being increasingly vulnerable to storm surges, trust in fate, and hang on to the bitter end, hoping that the Big Storm doesn’t strike while they’re still there. Then as now, mistakes made by those charged with the public welfare will occasionally make climate-driven threats more devastating than they would otherwise be. And when global cooling eventually turns the advance of oceanic waters into a retreat, successive generations will move back seaward and resettle newly revealed lands that were once submerged.
What remains less certain is how these changes will affect coastal habitats and species in the Anthropocene future. They survived even greater shifts in ocean levels during the transitions between ice ages and interglacials by moving inland or seaward as the oceans rose and fell. But now, in a world dominated by humans, we can only hope that there will be room enough on the newly crowded edges of the sea for them to do so again as the shorelines shift beneath them.
8
An Ice-Free Arctic
The fate of almost everything in the winter
world is ultimately determined by the crystallization
of water.
—Bernd Heinrich, Winter World, 2003
“The polar ice caps are melting!” It is said so often nowadays that it risks becoming a cliché, albeit one that happens to be true. Arctic ice retreat is one of the most obvious signs of global warming and it will drive enormous environmental and societal changes for a long time to come. But there are also greater depths to the subject than we usually hear about. What, exactly, do we mean by “polar ice caps”? Why are they melting? Are the polar bears doomed? And what might the world be like without ice at the North Pole? Would it be bad, good, or some mix of the two?
For now, we have an ice cap atop each pole, but only the northern one is likely to vanish within our lifetimes. That’s because it is a relatively thin lid of floating sea ice, fundamentally different from the one in the south, which rests on solid ground rather than on 2 or 3 vertical miles (4 to 5 km) of salty water. The seasonally grown stuff that makes up at least half of it only thickens to 6 feet (2 m) or so over the course of a winter, and the multiyear ice averages about 10 feet (3 m). Even before the shrinkage of recent decades, the shell of Arctic ice was already so thin in places that Russian and American submarines used to push right up through it in order to sneak a look around from time to time.
In contrast, Antarctica is a solid continent trapped under a gigantic frozen slab up to 3 miles (4.8 km) thick. The world’s largest single ice mass, the East Antarctic ice sheet, is so incredibly cold, thick, and huge that winters on its high austral pole can average 75°F or so below zero (-60°C), far too cold to permit much melting for now. In addition, summer warming (closer to-13°F, or-25°C, at the South Pole) tends to make it snow more in the interior, which thus far cancels much of the loss around the lower, warmer margins.
When we hear warnings about big ice sliding off into the sea, it’s the tenfold smaller West Antarctic ice sheet that is most likely to do so. Much of it lies on a peninsula exposed to the additional warming effects of ocean currents, and some parts of it that rest at or below sea level are not firmly anchored. Greenland is also at risk, as we’ll discuss in Chapter 9, but it doesn’t count as a polar ice cap in the strictest sense of lying on or very close to the pole; the southernmost tip of it doesn’t even lie within the bounds of the Arctic Circle.
It’s the floating northern cap that we’re losing most rapidly, both in terms of area and of thickness. September ice extent shrank by about half between the 1970s and 2006, and in 2007 an even sharper decline reduced it to its smallest extent since satellite-based measurements began in 1979. We don’t yet know exactly when this trend will lead to a complete thaw, but most experts expect it to happen well before the end of this century, perhaps even before 2020 AD.
Arctic warming rates far exceed the global average, and most of the circumpolar regions warmed by 4 to 6°F (2 to 3°C) during the last half of the twentieth century. This isn’t because greenhouse gases are somehow scrum-piling atop the far north; rather, they’re teaming up with other local factors that can also raise temperature themselves, most notably a replacement of reflective white snow and ice by darker, heat-absorbing water, soil, and vegetation. The very presence of so much snow and ice has helped to keep the Arctic unusually cool because much of the energy in the weak northern sunlight is used up in converting water from solid to liquid form, and a great deal of it also reflects off the bright white surfaces without warming them. In a sense, the Arctic is now playing thermal catch-up with the rest of the world because it started several notches lower on the temperature scale to begin with.
But greenhouse warming and reflectivity are only part of the story. Another factor behind the retreat is a natural climate disturbance called the Arctic Oscillation, which makes far northern climates jump suddenly and unpredictably between regionally warmer and cooler conditions. Some scientists blame much of the recent melting on this phenomenon because a switch into mostly positive warm mode occurred in 1989, about the same time that the ice began to retreat. In that mode, stronger westerly winds help to push warmer currents into frozen zones, where they melt the ice from below. The winds also widen the watery gaps between broken floes, thereby allowing more young, thin ice to form and then
melt more easily in summer, and they flush multiyear sea ice out into the North Atlantic, leaving more of the thinner seasonal stuff behind.
On the other hand, negative modes of the Arctic Oscillation are supposed to stop those ice-eating patterns, but they’ve returned briefly several times in recent years without halting the trend, and such reversals have been flopping back and forth for at least a century without devouring the northern sea ice this much. There seems to be something new afoot here and, although it may reflect a joint effort among several causal factors, the artificial greenhouse effect appears to be doing most of the heavy lifting. Even if today’s shrinkage slows down again in coming decades, it will probably only delay the inevitable melt-off.
The historical record doesn’t provide much comfort in this regard. When the last ice age ended 11,700 years ago, the Northern Hemisphere entered a period in which orbital cycles produced summer temperatures higher than today throughout the Arctic. During that early Holocene warm phase, which lasted roughly 2,000 to 3,000 years at most sites, summer air temperatures were generally 4 to 6°F (2 to 3°C) warmer than now and northern sea surface temperatures were two or three times higher than that. When I asked Johannes Oerlemans, professor of meteorology at Utrecht University, if that was enough to uncover much of the polar ocean, he replied, “It’s hard to imagine that summer temperatures several degrees higher than today would not have had a significant effect on the sea ice cover. Many of my colleagues have expressed that opinion, so it’s really nothing new.”
According to a recent study headed by Darrell Kaufman of North Arizona University, the remains of blue mussels, Macoma clams, and bowhead whales in marine deposits show that these animals invaded much of the Canadian Arctic Archipelago and Beaufort Sea coast during the warm early Holocene in response to newly opened waters. Similar evidence from farther east also reveals similar conditions in and around Svalbard, but those conditions were not perfectly representative of what is happening now. For one thing, that earlier insolation-driven warming was primarily limited to summers at high northern latitudes rather than the more universal effects of greenhouse gas buildups that we face today. In addition, winds and ocean currents and temperatures in the eastern Canadian Arctic were still strongly influenced by what was left of the gigantic Laurentide ice sheet. In light of those differences, we need not be surprised to see even more dramatic sea ice retreats despite somewhat lower temperatures today.
And there’s yet another point that is often overlooked when the subject of polar melting arises in the media. When most experts say that the North Pole will soon be ice-free, they’re not talking about total, year-round loss. Both poles endure long months of darkness every winter and temperatures fall far below zero then. Even now, the Arctic ice cap doubles in size as the December-January air chills down, and all but the most extreme versions of future warming might not keep all of it from refreezing during the dark months of winter. This is mainly about melt-backs in late spring and in summer, when the circling Arctic sun warms the pole twenty-four hours a day.
Most of what we hear about the northern ice retreat is that it threatens polar bears. Sometimes we also hear that a new Northwest Passage has opened along the Arctic coast of Canada and that shipping, mining, and fishing interests are moving in to exploit the new geography. Such broad statements leave a lot unsaid, but together they raise an important point: some will lose and some will gain as the Arctic opens up. And time will also change that list of losers and winners when the dramatic transitional events that hold our attention now are over and an ice-free polar ocean begins to seem normal, then slowly begins to glaze over again. By digging deeper than usual into such details of life in the far north, I hope to provide some helpful insights into what is going on up there as well as some of what’s to come.
Polar bears are an obvious subject to start with, but even the simplest online search yields a confusing mess of opinions about what is really happening to these iconic symbols of global warming. Some articles tell of bears drowning in a desperate search for ice to stand on, echoing the computer-generated scene in Al Gore’s film, An Inconvenient Truth. Others excoriate such reports for being misleading; the Danish statistician and author Bj0rn Lomborg begins his book Cool It with one of the stronger critiques, calling most versions of the story “vastly exaggerated and emotional claims that are simply not supported by data.”
Most of these reports apparently stem from an incident in 2004, when scientists who were flying off the coast of Alaska spotted several dead bears floating many miles from the nearest floes. A powerful storm had just struck the area, and the animals were probably overwhelmed by violent waves while swimming in open water. The ice margin lay farther offshore than usual that year, and bears had already been seen swimming more than 50 miles (80 km) from land. Normally, a long-distance swim isn’t particularly troublesome for a polar bear because the adults are comfortable enough in deep, cold water to qualify as honorary amphibians. However, the recent ice retreat makes such journeys more common, so the risk of being caught out in a dangerous storm is probably increasing, too. Furthermore, the smaller body sizes of young cubs make them more vulnerable to hypothermia in cold water, so longer swims may be a more serious problem for them than for their parents. For now, the significance of this issue remains inconclusive; yes, bears have drowned, but how many are actually dying because of climate change?
Keeping in mind the highly politicized nature of the subject, I have used extra caution in retrieving what I believe to be reliable background facts and updates from the field. My principal source is Andrew Derocher, a renowned polar bear specialist who is based at the University of Alberta and who kindly agreed to talk with me by phone.
That was no small favor. As one of the few real experts in a field that draws a lot of attention these days, Derocher is besieged by journalists and climate naysayers in search of a catchy quote or a punching bag. When I called him, he had just finished reporting a particularly threatening letter to the campus security office.
For many of us, the north polar cap can seem about as appealing as an empty parking lot in winter; nothing but lifeless, featureless white emptiness. But Derocher sees it differently. “Under normal conditions,” he explained, “you get huge pressure ridges of ice that stretch for miles and act like snow fences that trap powdery drifts on their leeward slopes. Ringed seals come up through cracks in those ridges and dig birthing lairs in the soft snow pockets.”
It’s the seals that draw bears out onto the ice in the first place. Unlike their southern cousins, polar bears generally stay awake and hungry even in the long darkness of winter, although pregnant females do spend that season in snow-covered maternity dens where their cubs are born. Meanwhile, nonmaternal adults prowl the drifts and ice ridges in hopes of snagging the odd unlucky seal, but they usually have little success until the light returns and their favored prey’s whelping season begins.
Seals sustain the bears, and ice sustains the seals. As the sea ice shrinks in the warming Arctic, the effects cascade through the food web. “Ringed seals establish their winter territories in late autumn, when the ice sets up for the winter,” Derocher continued. “As the freeze-up comes later and later, seals are less and less likely to find their territories in time, and that makes them less likely to den up and give birth in spring.” Ringed seal behavior, it seems, is closely tied to day-length cues, which remain pinned in place on the calendar, while the duration of winter ice cover shrinks at both ends of the season. When breeding time arrives on schedule, the seals still heed the call, but Derocher fears that they may be forced to stake their territorial claims on more dynamic, shifting offshore ice that makes the birthing and raising of pups more difficult.
Polar bear on Hudson Bay sea ice. Andrew Derocher
When waxing spring sunlight heralds the arrival of pups, the bears have two or three months in which to do most of their feeding for the year. They sniff out buried dens and pound, stiff-legged, on the snowy roofs with their forepaw
s, hoping to grab the youngsters before they scoot down their escape chutes into the water. Perhaps one in twenty attacks suceeds, but that’s enough; ringed seal pups are incredibly energy-rich snacks, with fatty blubber making up half of their body weight. But it’s not just the pups that they’re after. “Females with pups make bigger meal packages,” Derocher explained. “A bear will sometimes discard a pup and then wait at the birth lair for the mother to come back.”
Weeks later, the southern lip of the ice peels slowly back toward the pole and most of the bears move ashore. In western Hudson Bay, where Derocher does much of his research, summer is a lean time even though some terrestrial foods are theoretically available. From May through August, the Hudson Bay bears usually fast and wait for the coastal waters to refreeze. Pregnant females there may go without eating for as long as eight months before seeking shelter for the winter. In spring they emerge from their maternity dens and move back out onto the returning rim of ice with their new cubs.
But not all individuals are alike, and different habits develop among different subgroups. Polar bears in some regions spend more time on land than polar bears in other regions. Some may take a break from sealing to hunt walrus and narwhals offshore and then stalk reindeer and geese onshore. In the Svalbard archipelago, north of the Norwegian mainland, bears have been seen chewing on beached sperm whale carcasses. In western Hudson Bay, some catch ptarmigans or munch berries. And, on rare occasions, human flesh goes down easily when the belly is growling.
It can be tempting to hope that innate resourcefulness will help at least some polar bears to survive future warming. But Derocher cautioned, “they certainly are adaptable, but there are limits.” The main problem is their need for high-calorie meals. Foods other than blubber are too energy-poor to keep a bear in good physical condition through four-to eight-month fasts. It was probably this narrow ecological niche, the hunting of seals on pack ice, that originally led these predators to evolve into white, water-loving versions of terrestrial brown bears.