by Curt Stager
Map of Florida’s shorelines during the last ice age, today, and in the future after a long-term sea-level rise that could result from a moderate 1,000 Gton emissions scenario. After maps posted online by the University of Arizona, Department of Geosciences
To put it another way, panic is not an appropriate response to this problem, but neither is complacency or denial. Sea-level rise will be almost imperceptibly slow for the most part, but this certainly doesn’t mean that it’s unimportant. We’d be better off without it, and the longer we continue to release the greenhouse gases that are causing it, the more land it will eventually consume. Over the long term, it will force maritime nations to invest again and again in the costly uprooting and rebuilding of coastal towns and port facilities, and some places—especially low-lying islands and coastal plains—will be lost for thousands of years. That’s serious business, but I believe it needs no overstatement to merit our attention.
Not everybody agrees with me, of course. James Hansen, NASA’s most widely quoted advocate for taking climate change seriously, is known to occasionally focus public attention on the extreme end of the list of possible futures to avoid. He has challenged the IPCC’s recent estimate of a foot or two of twenty-first-century sea-level rise by proposing that “explosively rapid” ice sheet collapse could force polar melt rates to double every decade and thereby hoist the ocean’s surface another 16 feet (5 m) by 2100 AD. That would involve an average rise of 2 inches (5 cm) per year, almost a twentyfold acceleration of the current rate. He urges the research community to sound a more vigorous alarm about it and suggests that any “scientific reticence” about this issue reflects political pressure to minimize concerns about climate change.
Some scientists share Hansen’s views, but others do not, partly because they consider caution in the face of uncertainty to be the hallmark of good science rather than inappropriate reticence and partly because they doubt that enough unstable ice exists to launch a large deglacial-style ocean surge in modern times. A more recent review of the subject headed by glaciologist Tad Pfeffer from the Institute of Arctic and Alpine Research in Boulder, Colorado, concluded that a twenty-first-century rise of more than 6 feet (2 m) is “physically untenable,” and that the most likely outcome will be a rise of roughly 2.5 feet (80 cm). But at this early stage in our understanding of ice sheet dynamics, we simply can’t be sure whether, when, or how much of Greenland or Antarctica might de-ice in response to Anthropocene warming.
This is the kind of situation in which a long-term historical perspective is most helpful. Can a shrunken remnant of polar ice really launch a major sea-level pulse today? “Definitely,” says Paul Blanchon, an expert on ancient reefs and sea levels based at the National University of Mexico. “Drowned coral reefs are the smoking gun that too many scientists ignore,” he told me recently. Paul and his colleagues have used fossil reef deposits to reconstruct century-scale jumps in ocean levels at the end of the Eemian interglacial and shortly after the close of the last ice age. “The history of partial ice sheet collapses seems to give a tight range of rise rates of several centimeters per year,” he explained. One abrupt rise during the late stages of the Eemian averaged 2 inches (5 cm) per year, an order of magnitude faster than today’s rate, and lasted for about a century. “And another surge about 8,000 years ago coincided with the drowning of many Caribbean reefs. Corals eventually regrew farther upslope several centuries later.”
The candidate most likely to repeat those reef-drowning events of the past is the West Antarctic ice sheet. For the sake of argument, and to illustrate the need for more information about ice and oceans, let’s imagine a scenario in which a little more than half of it begins to slide off tomorrow. An associated sea-level rise of up to 10 feet (3m) would presumably occur between now and 2100 AD because the streams of ice would displace surrounding waters as soon as they hit them. But the sea-level responses to the dumping of so much ice could be more complex than one might imagine.
The West Antarctic ice sheet is so large and thick that the gravitational attraction generated by its enormous bulk makes seawater within 1,200 miles (2,000 km) of the Antarctic coast bulge slightly upward. Were all that ice to disappear, the local sea surface would slosh away toward the Northern Hemisphere, there to raise sea levels higher than average in some places while lowering them elsewhere. The slow rebounding of down-warped bedrock on the de-iced peninsula might also destabilize sea levels even further.
The upshot of all this is that the risk of relatively abrupt sea-level rise due to global warming is real, but the details of how probable it is and exactly how it might proceed remain unclear. We can be sure, however, that some sort of rise is going to happen for a very long time to come, and it will involve significant human and ecological costs.
A team of scientists at the University of Kansas have recently combined submergence maps with current population data in order to estimate some of those costs. According to their study, the first 3 feet (1 m) of sea-level rise will cover nearly 400,000 square miles (1 million km2) of coastal land and could displace more than 100 million people, nearly half of them in Southeast Asia. Northwestern Europe will lose about 13,000 square miles (34,700 km2) and might eventually see 12 million citizens uprooted. And in the southeastern United States, an expected loss of 24,000 square miles (62,000 km2) could displace more than 2.6 million people.
The study’s basic premise is sound, but some of the details remain open to question. We know people will be displaced as the ocean swells upward, but we can’t know their numbers exactly because we don’t know how demographics will change in coming decades and centuries. Regardless of how many people are affected, how might that kind of sea-level rise affect their daily lives?
The speed of the change will be at least as important as its magnitude. If we postulate a rise of 3 feet (1 m) at twice today’s rate, then it would occur over the course of nearly two centuries. Even if we shorten the time period to one century, the displacements would be spread out over several generations and thousands of miles of shoreline. If an abrupt surge of the Eemian sort should occur instead, speed those changes up by yet another factor of 5. But even that would not be like having your kids swept away from the beach while you’re at the snack bar. Living with sea-level rise will be more like bay water leaking into the Manhattan subway system year after year until the pumps are finally turned off and the rail tunnels are abandoned to become submarine caves.
For most of our descendants, the coming affliction will be more of a chronic ache than acute agony. This was already recognized as early as 1975 when a select gathering of prominent geoscientists met in North Carolina’s Research Triangle Park to discuss the future of greenhouse gas pollution, one of the first major meetings of its kind dedicated to the subject. In the published proceedings of the conference, the moderator concluded: “It was generally agreed that sea-level rises would be more of an expensive annoyance than a catastrophe.” The relatively mild concern expressed back then was not due to some misguided belief that the coming changes would be small. If anything, the rates were overestimated; the long-term sea-level rise under discussion was expected to average several centimeters per year, much like Hansen’s short-term worst-case scenario. In today’s more highly charged media atmosphere, some of those same scientists now speak in apocalyptic terms, but the earlier conclusion, which was made in a more sober professional setting, still makes sense to many experts. As one of my own colleagues put it with a wry smile, “You’re not going to die from sea-level rise. But if places that you care about succumb to it, then you might well want to die.”
In the case of Europe, a climb of 3 feet (1 m) per century could force 120,000 people to migrate inland within any given year. That’s a lot of uprooted people looking for lodging and employment, but housing and employers can move, too, and many Europeans already change homes or jobs for more mundane reasons without triggering widespread panic or social unrest. In the United States, 10 to 20 percent of the population already pulls
up stakes every year; applying this statistic to western Europe’s 400 million souls yields a ballpark figure of 40 to 80 million routine annual migrants and suggests that relatively rapid sea-level rise might add 3 to 7 percent to that total.
A focus on population numbers alone, of course, doesn’t capture the full spectrum of impacts on sea-level refugees. Those who will be displaced are likely to be more closely tied economically, culturally, and emotionally to the sea than the average mainlander, and being forced to move from a sinking shoreline into an already crowded interior could mean the loss of livelihoods as well as homes.
The slow pace of the changes will allow societies to respond in more complex fashion than a sudden, uniformly unpleasant flight of refugees to higher ground. In the wealthier countries, home prices and insurance rates are linked to “location, location, location,” and in oceanside nations they will be influenced by their positions on inundation maps. For some, this will be an incentive to move preemptively or to avoid oceanside properties altogether, but others might actually be lured shoreward by perennial crops of real estate bargains and short-term business opportunities there.
Consider the case of Amsterdam. In early medieval times it was an isolated, nondescript village that lay several miles inland from the ocean. Where the heavily diked Ijsselmeer harbor now stands, a broad lowland once blocked access to the North Sea. Naturally rising sea levels slowly chewed away at the Dutch shoreline until they opened a profitable sailing route to the maritime trade lanes and the rest of the world. With or without climate change, life on low coastlines always involves risks, and North Sea storms wrought terrible havoc in that region from time to time, including the Saint Lucia’s flood of 1287 and the Grote Mandrenke (“Great Drowning of Men”) of 1362 that killed tens of thousands of people. But without long-term sea-level rise and the new connection to the Atlantic, Amsterdam might never have become the glittering cultural and economic center that it became in later centuries.
Situations such as this will complicate the story of sea level in coming centuries, as well. As the creeping zone of inundation slowly consumes the latest array of oceanfront settlements, a migrating band of social and economic change—a “zone of anticipation,” if you will—may also move inland ahead of it. As the next settlements in line realize what’s coming, they will rush to prepare for the conversion of their landlocked setting into coastal status. In the honeymoon years between the ocean’s arrival at a city’s doorstep and the inevitable submergence, targeted towns may blossom from a resultant boost to trade and tourism. Sites with steeper slopes will have longer honeymoons than those on flats, where rising waters will consume the land more rapidly. If hardware and expertise that can be mobilized from the zone of inundation sells cheap, then outfitting the next string of port facilities could be undertaken at fire-sale prices.
But it won’t be all positive, either. Rising sea levels will require constant and costly upgrading of water barriers before they are finally abandoned along with the properties they were built to protect. And accidents will happen, too. Occasional breaks in dikes have repeatedly devastated Dutch towns that lie below sea level, and in 1421 gaps in several dikes flooded seventy communities during a high storm tide and killed as many as 10,000 people. At best, the boom times in the zones of anticipation will merely represent short-term parties before a final demise.
The least prosperous nations, of course, are likely to suffer the most. Low-lying Bangladesh is already sinking under its own weight because of sediment compaction on the broad floodplains of the Ganges and Brahmaputra rivers, and its residents have long been caught between the hammer and anvil of seasonal floods and coastal storm surges. But rising sea level threatens to obliterate much of that country altogether, and the displaced people will have no easy escape route because international borders restrict their migration. Fortunately, it will take a long time, probably several centuries, for the sea to nibble its way very far inland because much of the terrain beyond the coastal floodplains slopes up toward the Himalayas. According to the University of Arizona maps, a vertical 6-foot (2-m) rise would submerge approximately one-fifth of Bangladesh, and a 20-foot (6-m) rise would consume about half of it.
Sea level will continue to climb long after the thermal maximum and climate whiplash phases pass, and where it finally levels off will depend on the height and timing of the temperature peak and how much land-based ice eventually melts into the oceans. A moderate-emissions scenario probably stops well short of a total polar meltdown, but an extreme 5,000-Gton scenario could leave the planet more or less ice-free— and sea levels hundreds of feet higher—for thousands of years. If the ocean surface climbs 230 vertical feet (70 m) at Pfeffer’s predicted rate of 2.5 feet (80 cm) per century, then we might expect sea level to continue rising for the next 9,000 years or so before leveling off.
But that’s just the beginning. When the phase of net melting finally ends in the deep future, coastal peoples will have to deal with another, even longer-lived aspect of maritime change. As the air and oceans cool and snow begins to accumulate once more in the polar regions and high mountain ranges, the new challenge will be long-term sea-level retreat.
Those many millennia of future cooling will bring much slower changes than warming does because meltwater won’t surge abruptly back out of the oceans. At first, most of the retreat will be due to slight contraction as the seas cool down, but continental reserves of ice will eventually be rebuilt by the gradual accumulation and compression of winter snow. Two miles (3 km) of Greenlandic ice can represent more than 100,000 years of growth, and in parts of East Antarctica, it can represent almost a million years. In order to regenerate those huge stacks, we’re looking at tens to hundreds of millennia of ice reconstruction. But despite the sluggish pace of that change, the societal and ecological effects of sea-level retreat will still be profound over the long term. Millennium after millennium, formerly deepwater rocks and reefs will become shipping hazards, maritime towns and port facilities will be repeatedly stranded inland, and near-shore islands will be hitched, one by one, to the mainland by land bridges as new islands arise from the exposed tops of reefs and seamounts.
Although sea-level change will be more of an economic problem for most people than a life-threatening one, it will present much more serious challenges to species that are less mobile or adaptable than we are.
Salt marshes require daily immersions and dryings in order to survive and continue their important work of harboring infant fishes, crabs, shrimp, and shellfish. During previous deglacial periods, salt marshes avoided extinction by moving inland ahead of the tide line, but that’s not always an option for them now. We’re already holding much of that higher ground. Instead of simply giving them a landward nudge, rising seas are crushing more and more of the world’s intertidal habitats against an impenetrable wall of humanity.
Farsighted citizens in south Australia’s beachfront city of Adelaide are already struggling to save local salt marshes from a deadly combination of ocean encroachment and land subsidence caused by compaction and groundwater removal. Plans are afoot to remove or reposition barriers such as seawalls and roads that now prevent the marshes from retreating inland, but the specter of long-term sea-level rise casts a deepening shadow over such efforts. Australian coastal scientist Peter Cowell recently expressed his frustration about it to the Adelaide Sunday Mail, asking at what point do “we give up and decide we’d be better off relocating rather than trying to defend this stuff?”
Tropical mangrove forests also face similar threats, as do the marine creatures that depend on them and the terrestrial wildlife that has been driven into them by human activity in the surrounding countryside. On the low-lying Bengal coast, for example, dense mangrove tangles make a shrinking haven for endangered tigers, as shrimp farms chew away at their edges even more rapidly than sea level does. Also threatened are intertidal mudflats where commercially and ecologically valuable shellfish grow like root crops, all dependent on narrow ranges of water dep
th and therefore vulnerable to sea-level change whether it be upward or downward.
Global cooling will flip today’s ecological challenges into reverse gear for whatever remains of marshes, mangroves, and mudflat communities in those future centuries. Fortunately, the slow sinking trends will be easier on most coastal ecosystems than the faster warming-driven rises will be.
In the tropics, the crests of shallowwater reefs may gradually die off from increasingly common exposures to the open air as the ocean surface sinks, but new coral colonies might thrive in deeper waters along the reef edges—unless ocean acidification stops them. Reef expert Paul Blanchon, however, finds little evidence of this among the fossil corals of Barbados and New Guinea. “Reefs didn’t necessarily die off altogether when sea level fell in the past, but new reef structures didn’t grow like they usually do, either,” he told me. “I imagine this is because the corals were forced to move downslope and smeared themselves like a veneer over large areas rather than building upward like they did during periods of sea-level rise.”
The fastest environmental changes will occur during the next several centuries, while the warming and melting are most intense. And how will people react to those changes? To help answer that question, one might simply look to coastal cities that have already been sinking as a result of human-induced earth movements. You might be surprised to learn how common they are; most of us hear little about their struggles with land subsidence or overlook their similarity to those caused by sea-level rise. But whether you’re sinking below the waterline or having the water climb up and over you, the results are much the same.