Deep Future

by Curt Stager

  But how do satellites calibrate their measurements as their orbital paths slowly decay and move closer to the ground? Do they drop sounding lines to Earth to check the accuracy of their measurements? From my work on deep lakes, I know that supposedly well-calibrated electronic gadgets can give readings quite different from those that you get by lowering a heavy lead weight on a graduated rope.

  And what, exactly, does a high-flying spacecraft measure sea level relative to? Does it record how long it takes a radar signal to bounce off the ocean and return, and then calculate how many miles above the sea surface it is at any given moment? If the satellite measures a slightly shorter distance the next time it flies over that spot, does it mean that sea level rose or that slight losses of energy lowered the long flight path a bit, or was it some combination of both?

  Today, multiple satellites and global positioning systems give us a diverse selection of methods for determining surface heights, using three-dimensional, space-based networks of interlocking angles and reference points for drawing maps, calculating locations, and measuring sea levels. But even the most sophisticated satellites still don’t help us with truly long-term trends because their records cover only the last few decades. Instead, we must turn to geohistorians if we want to know what major sea-level changes have been like in earlier times.

  Composite charts of sea-level positions since the peak of the last ice age combine readings from fossil deposits in places such as Tahiti, Barbados, western Australia, and the Red Sea. Those charts show that sea-level change itself is nothing new. About 20,000 years ago, when continental ice sheets held the most water away from the oceans in frozen form, you could have walked from what is now Cape Fear on the sandy coast of North Carolina all the way out to the edge of the continental shelf, roughly 50 miles (80 km) to the east. The ancestors of Native Americans walked from Siberia to Alaska on a land bridge that now lies beneath the storm-whipped Bering Strait, and Paleo-Europeans could have wandered across what is now the English Channel without even seeing a beach or sizable body of water. The last ice age dropped ocean surfaces by about 400 feet (120 m) for tens of thousands of years, and similarly severe ice ages struck repeatedly during the last 2 to 3 million years. If satellite cameras could have filmed, say, the Carolina coast on a continuous basis over that time period, a fast-forward run of that footage would resemble a home video of your favorite beach, with wave after wave alternately swallowing and disgorging the border between land and water.

  The last time we came out of a glacial deep freeze, it took several millennia of melting for global sea level to stabilize near today’s elevation after eating hefty portions of beachfront. The scale of that encroachment outstrips anything in the works for us during the Anthropocene; we have only enough terrestrial ice left now to raise future sea levels by a little over half the postglacial amount. But it didn’t bother our nomadic Stone Age ancestors nearly as much as smaller future rises could bother our more numerous descendants, whose dwellings and possessions won’t be moved as easily as mammoth-skin tents once were.

  How did that last great rise compare to today’s? From 1993 to 2003, ocean levels crept upward by the thickness of three fingernails per year (a tenth of an inch, or 3.1 mm), a pace that was nearly twice as fast as the average for the full twentieth century. A French team led by geophysicist Anny Cazenave reports that the pace slowed somewhat from 2003 to 2008 (2.5 mm per year), perhaps because the global warming trend also slowed briefly on its irregular path to higher temperatures. By comparison to, say, the recent three-fingernail rate, the average postglacial recovery was about four times as fast, and several short but intense episodes sped things up even more from time to time as ice sheets surged or collapsed. For example, about 14,500 years ago a major meltwater pulse lifted sea levels by 50 to 65 feet (15 to 20 m) at more than ten times our modern rate—still much too slow to watch from shore, but those incremental changes reshaped coastlines dramatically over the course of centuries.

  As in the past, today’s rise reflects both the melting of land-based ice and the expansive effects of heating that swell seawater just as rising temperatures fatten the liquid in a thermometer tube so that it rises higher and higher the warmer it gets. As the world warms further, both expansion and polar melt will continue, but the magnitude of the rise at any given time will probably be most strongly influenced by what happens to our remaining ice.

  Only three frozen masses are still large enough to change sea level dramatically, and they’re sitting in Greenland, West Antarctica, and East Antarctica. Satellite-based measurements show that the first two have been losing weight since 2002, while the East Antarctic sheet may actually have gained a bit due to snowfall and exceptionally cold temperatures around the South Pole.

  Seasonal melting in Greenland, which holds roughly 10 percent of the world’s ice, now drives about a tenth of the current global rise. If the whole thing eventually liquifies, it could raise sea level by 23 feet (7 m).

  The loosely anchored West Antarctic ice sheet is the least secure of the bunch. Covering nearly 800,000 square miles (2 million km2), it contains enough meltable water to hoist sea levels by 16 feet (5 m) or so. Winter temperatures on its narrow western peninsula are warming faster than in any other place on Earth, up by 10 to 11°F (6°C) since 1950, and just how unstable it is remains unclear. But for now it contributes only slightly more than Greenland does to sea-level rise.

  About 80 percent of the world’s remaining ice—the gigantic East Antarctic ice sheet—seems to be holding steady for now because its surface is so high and cold that relatively little melting occurs there at present. Some simulations of a wetter, warmer world show more snow falling there in coming centuries, which might actually thicken the vast interior as the lower, warmer coastal margins diminish, and it might therefore slow global sea-level rise somewhat, at least at first.

  On the other hand, it’s hard to imagine much ice surviving a long-term greenhouse future. Some experts foresee Greenland’s ice cap destabilizing if temperatures rise more than 2 to 7°F (1 to 4°C), close to the range of temperatures expected for a moderate-emissions scenario. And most who have baked the more persistent south polar cap to death in computer simulations expect it to destabilize if temperatures rise by 9°F (5°C), which would be likely in an extreme-emissions scenario. But even more of a threat to polar ice than the magnitude of future heating will be its immense duration, at least 50,000 to 100,000 years, with temperatures at or above those of today.

  How might the world look without any ice at all? For a glimpse of that possible future, we can turn to computer programs that overprint 230 feet (70 m) of virtual seawater onto topographic charts of the modern world. On such maps, the southeastern United States looks like it’s been chewed by sharks, with the entire thumb of Florida bitten off. The sharks also gnaw the Yucatan Peninsula to a stump, and eastern China is mauled most of the way in to the Tibetan Plateau. Such striking images help to feed a rising tide of anxiety as people struggle to understand what human-driven sea-level change means. But how high the ocean can eventually rise is only one of the things we should keep in mind here. We also need to know how fast it will rise. And that’s where we can be misled by short-term thinking.

  Remember the last time you considered the magic of compounding interest or some amazing, and possibly true, statistic like “the average human eats a pound of dirt every year.” Most of the shock value of such revelations arises from misperceptions of the time scales involved. We tend to focus on becoming millionaires while overlooking the need to put money aside consistently for many years, or we imagine digging into a bucketful of soil rather than gulping a few grains of grit along with our daily salad. Drinking several gallons of water all at once can kill us by diluting our body fluids, but drinking the same amount over a couple of months can kill us through dehydration. The key factor here is time.

  When, for example, scientists warn that sea levels could surge in response to a “collapse” of the West Antarctic ice sheet, m
ost of us envision the ice crashing down as rapidly as a building might disintegrate in an earthquake. Fevered media coverage of calving glaciers and ice shelves breaking loose from shore also magnify that impression. But geoscientists and lay folk mean different things when they use that word “collapse”; what seems fast to a glaciologist might sometimes be better described as “less sluggish than usual” to most of us. So how long would such a collapse really take?

  When a noted glaciologist whom I met at a recent conference mentioned that a “catastrophic” slide-off may be imminent in western Antarctica, I pressed him for details. After some hesitation, he guessed that such a collapse would take decades at least, possibly a century or more. That time frame fits well with an article in Science that was published recently by geophysicist Charles Bentley, who reckoned that the right combination of warming, rising sea level and loss of supporting ice shelves could make the western slab “collapse in as little as a century by catastrophic grounding-line retreat.”

  The probable decades-to centuries-long duration of such an event doesn’t mean that the change would be insignificant. It simply reflects the enormous quantities of relatively slow-moving ice involved; the sheet is so large that it would take years for all of it to cross the many miles from land to ocean. Recent evidence suggests that some sections of West Antarctica’s interior ice are actually thickening rather than thinning, and that any major slide-off would probably be partial, leaving sizable remnants stranded on the jagged spine of the peninsula. Even so, the total marine displacement could still be on the order of 10 feet (3 m).

  And what might such a sea-level rise look like from shore? Not much, if you were to sit there and try to watch it happen. Changes of similar amplitude happen all the time in Maine’s picturesque Muscongus Bay, though they’re compressed into brief 10-foot tidal cycles. I’ve happily sat there long enough to watch the slowly rising tide creep in among the cobbles and rockweed and heard the barnacles hiss as they prepared for the return of the shifting waterline. But I wouldn’t want to have to wait there long enough, even amid the crying gulls and beautiful fir-clad islands, to watch a global sea-level surge swallow the shore. Not even the fastest rises of past deglaciations would have been visible to the average eye. Like the tides, such changes require time-lapse imagery or a well-trained imagination to be seen for what they really are.

  Low and high tides at Muscongus Bay, Maine. Rick Ylagan

  The last IPCC assessment report estimated that sea level could climb 1 or 2 feet (0.3 to 0.6 m) by 2100 AD, depending on how much carbon we emit. That would average out to nearly twice today’s rate, though more recent estimates double or triple it. But even those faster paces wouldn’t represent a frothing shoreward dash in the sense that most nonscientists imagine it. To help put this issue of rates into perspective, keep in mind that mean sea level has already risen by about 7 inches (18 cm) during the last century. Most of us didn’t notice the change because it was so slow. Unfortunately, some of us may therefore doubt that it’s even happening at all.

  This is an important point. Sea-level rise is a serious and regrettable consequence of our carbon pollution, but not necessarily in the ways that many sources portray it. On the scale of a human lifetime, real sea-level rise won’t much resemble a biblical deluge except in certain situations, most often when a local storm surge pushes floodwaters farther inland than usual and catches formerly safe inshore communities by surprise. In theory, most coast-dwelling people will be able to monitor the slow encroachment and issue fair warning to those living in the newly expanded danger zones. Whether they actually do so, of course, is another matter.

  There is one kind of situation, however, in which an initially slow sea-level rise really could cause sudden and massively destructive changes on a regional scale. It could happen where a low geographic barrier holds the ocean back from a local depression, a setting that actually developed at least once in Asia Minor. What is now the Black Sea used to be a freshwater lake lying below sea level, isolated from the much larger Mediterranean by a narrow neck of land where Istanbul now stands on Turkey’s Bosporus waterway. About 8,000 years ago, ocean levels were nearing the end of their steep postglacial climb when the water discovered a low point and slid a probing finger over the barrier.

  Then erosion took over. Within a matter of months, a torrent hundreds of times more powerful than Niagara Falls was pouring into the Black Sea basin, raising its surface by as much as 6 inches (15 cm) per day until, two or three years later, it fused with the Mediterranean 500 feet (152 m) above the original lake level. At least that’s what preliminary historical reconstructions showed; follow-up studies have suggested that the rise was more like 115 feet (35 m), quite a bit less than the original estimate but still enough to submerge a tall building.

  Whatever the exact magnitude of the surge was, thousands of square miles of land disappeared within that first year. Countless houses, hearths, and bits of crockery were submerged and preserved beneath what are now a hundred or more feet of saltwater. The resulting dispersal of farming communities may have contributed to the spread of agriculture through Europe and Asia, and some historians have noted possible echoes of the disaster in a flood myth that appeared in the Babylonian Epic of Gilgamesh some 3,000 years later, as well as in the younger tale of Noah and his ark.

  Fortunately, current coastal geography doesn’t presage any new megafloods of the Black Sea type, though coastal cities that already lie below sea level are indeed vulnerable to the overtopping of artificial dikes and levees, and one or more notable ice-driven surges may still be in the cards for us or our descendants. But even gradual sea-level rise will turn the world’s coasts into malleable fluid lines rather than solid boundaries, and it can radically reshape the edges of the continents over the long term.

  Such changes are easily envisioned with the help of computer generated maps, several of which are freely available online; they generally focus on the initial single-meter shifts that are likely to occur within the next few centuries. The first one I encountered was created by Jeremy Weiss and John Overpeck at the University of Arizona’s Department of Geosciences. The color scheme for the maps was wisely chosen; the oceans are a lovely deep blue, and the land a rich green. But the submerged zones are bright, sanguine red. Your eyes are drawn straight to them as though they were wounds on a beloved pet.

  Most of those wounds don’t run very deep even for a multimeter rise, and when you look at entire continents most of the problem areas make only a thin red rind along the edges. But zoom in on spots where the land slopes gently, and things become more interesting. The biggest bleeders on North America are located on the low coastal plains that stretch from the Tex-Mex border to eastern Virginia. After only a 3-foot (1-m) rise, the Florida Keys and the Everglades sink under a crimson tide and the Mississippi Delta seems to drip blood, with New Orleans swimming in the center of one great, hanging drop.

  Scanning the rest of the planet, other hot spots glow like fiery beacons; San Francisco Bay, much of eastern China, the southern tip of Vietnam, Cameroon’s port city of Douala, the Dutch interior, the southwestern rim of Denmark, and the broad deltas of the Nile, Niger, Orinoco, and Amazon rivers. And this is only after the first puny step of 3 feet. By geographic happenstance, additional rises of up to 20 feet (6 m) seem to add less dramatically to the redness, so many of the most notable inundations will happen early rather than later.

  Inspecting the future in this manner is somewhat akin to snorkeling on a coral reef. You see something interesting, hold your breath, and dive down to take a closer look. But it’s important to come up for air, too. These maps can be terrifying to behold, especially when they foretell the fate of a place that you happen to know well. But is terror the most suitable response?

  I’m not exaggerating when I use the word “terror” here; sea-level rise really does scare people, and I suspect that some who use it to raise public awareness about climate change don’t realize how much unhelpful panic they can cause. One
set of online maps, posted by British computer program designer Alex Tingle and titled Firetree, displays comments from viewers around the world. Here’s a sample of what they were saying recently:

  “This page works well if you want to know if your home will be under water. Tell me if it’s possible to add some time control, like year 2015, etc.”

  “When I built my dream home I tried to make sure that it would be above future sea levels so I won’t drown in fifty years…. I used your work to check, and yes, I’ll be fine.”

  “All people that live near coastal areas should move within the next twenty years … by 2050 sea levels will have risen 10 feet.”

  The first of these writers apparently expects extreme flooding to happen within less than a decade. The next person seems to believe that it will be fast enough to drown her. And the third one’s impression of the rate of rise is probably off by an order of magnitude. Such comments help to illustrate how poorly most of us grasp the time scales involved. Artificially accelerated video depictions can also make things worse by printing indelible images on our memories that speak louder than words and logic.

  Firetree’s so-called flood maps and the bloodred colors in other, more aptly named “inundation” or “sea-level” maps aim for maximum attention, but in doing so they can overshoot the mark and strike real fear into the hearts of many who might be better served by more sober descriptions and depictions. The term “flood” itself, when used in this context, can evoke an unrealistic sense of speed and deadly destruction. Nonetheless it is used routinely, even in the scientific literature.

  In typical fashion, a recent paper in Science tells us that the melting of Greenland could raise sea level significantly, thereby “flooding much of South Florida.” However, a rise of 23 feet (7m) over several centuries, which averages out to something close to today’s sluggish rate, isn’t what a normal person would call a flood. Such a choice of words can confuse people and intensify conflicts between concerned citizens and climate naysayers who focus on different aspects of the message. No, sea-level rise isn’t going to flood southern Florida; only storms, tsunamis, and swollen rivers work that quickly. But the scientists aren’t lying, either; the ocean really is going to submerge or inundate southern Florida, albeit over the course of many years.