The vagaries of global climate set in motion by our species’ frankly brainless maltreatment of the only atmosphere we’ve got, the subject of last week’s post here, have another dimension that bears close watching. History, as I suggested last week, can be seen as human ecology in its transformations over time, and every ecosystem depends in the final analysis on the available habitat. For human beings, the habitat that matters is dry land with adequate rainfall and moderate temperatures; we’ve talked about the way that anthropogenic climate change is interfering with the latter two, but it promises to have significant impacts on the first of those requirements as well.
It’s helpful to put all this in the context of deep time. For most of the last billion years or so, the Earth has been a swampy jungle planet where ice and snow were theoretical possibilities only. Four times in that vast span, though, something—scientists are still arguing about what—turned the planet’s thermostat down sharply, resulting in ice ages millions of years in length. The most recent of these downturns began cooling the planet maybe ten million years ago, in the Miocene epoch; a little less than two million years ago, at the beginning of the Pleistocene epoch, the first of the great continental ice sheets began to spread across the Northern Hemisphere, and the ice age was on.
We’re still in it. During an ice age, a complex interplay of the Earth’s rotational and orbital wobbles drives the Milankovich cycle, a cyclical warming and cooling of the planet that takes around 100,000 years to complete, with long glaciations broken by much shorter interglacials. We’re approaching the end of the current interglacial, and it’s estimated that the current ice age has maybe another ten million years to go; one consequence is that at some point a few millennia in the future, we can pretty much count on the arrival of a new glaciation. In the meantime, we’ve still got continental ice sheets covering Antarctica and Greenland, and a significant amount of year-round ice in mountains in various corners of the world. That’s normal for an interglacial, though not for most of the planet’s history.
The back-and-forth flipflop between glaciations and interglacials has a galaxy of impacts on the climate and ecology of the planet, but one of the most obvious comes from the simple fact that all the frozen water needed to form a continental ice sheet have to come from somewhere, and the only available “somewhere” on this planet is the oceans. As glaciers spread, sea level drops accordingly; 18,000 years ago, when the most recent glaciation hit its final peak, sea level was more than 400 feet lower than today, and roaming tribal hunters could walk all the way from Holland to Ireland and keep going, following reindeer herds a good distance into what’s now the northeast Atlantic.
What followed has plenty of lessons on offer for our future. It used to be part of the received wisdom that ice ages began and ended with, ahem, glacial slowness, and there still seems to be good reason to think that the beginnings are fairly gradual, but the ending of the most recent ice age involved periods of very sudden change. 18,000 years ago, as already mentioned, the ice sheets were at their peak; about 16,000 years ago, the planetary climate began to warm, pushing the ice into a slow retreat. Around 14,700 years ago, the warm Bölling phase arrived, and the ice sheets retreated hundreds of miles; according to several studies, the West Antarctic ice sheet collapsed completely at this time.
The Bölling gave way after around 600 years to the Older Dryas cold period, putting the retreat of the ice on hold. After another six centuries or so, the Older Dryas gave way to a new warm period, the Alleröd, which sent the ice sheets reeling back and raised sea levels hundreds of feet worldwide. Then came a new cold phase, the frigid Younger Dryas, which brought temperatures back for a few centuries to their ice age lows, cold enough to allow the West Antarctic ice sheet to reestablish itself and to restore tundra conditions over large sections of the Northern Hemisphere. Ice core measurements suggest that the temperature drop hit fast, in a few decades or less—a useful reminder that rapid climate change can come from natural sources as well as from our smokestacks and tailpipes.
Just over a millennium later, right around 9600 BC, the Boreal phase arrived, and brought even more spectacular change. According to oxygen isotope measurements from Greenland ice cores—I get challenged on this point fairly often, so I’ll mention that the figure I’m citing is from Steven Mithen’s After The Ice, a widely respected 2003 survey of human prehistory—global temperatures spiked 7° C in less than a decade, pushing the remaining ice sheets into rapid collapse and sending sea levels soaring. Over the next few thousand years, the planet’s ice cover shrank to a little less than its current level, and sea level rose a bit above what it is today; a gradual cooling trend beginning around 6000 BCE brought both to the status they had at the beginning of the industrial era.
Scientists still aren’t sure what caused the stunning temperature spike at the beginning of the Boreal phase, but one widely held theory is that it was driven by large-scale methane releases from the warming oceans and thawing permafrost. The ocean floor contains huge amounts of methane trapped in unstable methane hydrates; permafrost contains equally huge amounts of dead vegetation that’s kept from rotting by subfreezing temperatures, and when the permafrost thaws, that vegetation rots and releases more methane. Methane is a far more powerful greenhouse gas than carbon dioxide, but it’s also much more transient—once released into the atmosphere, methane breaks down into carbon dioxide and water relatively quickly, with an estimated average lifespan of ten years or so—and so it’s quite a plausible driver for the sort of sudden shock that can be traced in the Greenland ice cores.
If that’s what did it, of course, we’re arguably well on our way there. I discussed in a previous post here credible reports that large sections of the Arctic ocean are fizzing with methane, and I suspect many of my readers have heard of the recently discovered craters in Siberia that appear to have been caused by methane blowouts from thawing permafrost. On top of the current carbon dioxide spike, a methane spike would do a fine job of producing the kind of climate chaos I discussed in last week’s post. That doesn’t equal the kind of runaway feedback loop beloved of a certain sect of contemporary apocalypse-mongers, because there are massive sources of negative feedback that such claims always ignore, but it seems quite likely that the decades ahead of us will be enlivened by a period of extreme climate turbulence driven by significant methane releases.
Meanwhile, two of the world’s three remaining ice sheets—the West Antarctic and Greenland sheets—have already been destabilized by rising temperatures. Between them, these two ice sheets contain enough water to raise sea level around 50 feet globally, and the estimate I’m using for anthropogenic carbon dioxide emissions over the next century provides enough warming to cause the collapse and total melting of both of them. All that water isn’t going to hit the world’s oceans overnight, of course, and a great deal depends on just how fast the melting happens.
The predictions for sea level rise included in the last few IPCC reports assume a slow, linear process of glacial melting. That’s appropriate as a baseline, but the evidence from paleoclimatology shows that ice sheets collapse in relatively sudden bursts of melting, producing what are termed “global meltwater pulses” that can be tracked worldwide by a variety of proxy measurements. Mind you, “relatively sudden” in geological terms is slow by the standards of a human lifetime; the complete collapse of a midsized ice sheet like Greenland’s or West Antarctica’s can take five or six centuries, and that in turn involves periods of relatively fast melting and sea level rise, interspersed with slack periods when sea level creeps up much more slowly.
So far, at least, the vast East Antarctic ice sheet has shown only very modest changes, and most current estimates suggest that it would take something far more drastic than the carbon output of our remaining economically accessible fossil fuel reserves to tip it over into instability; this is a good thing, as East Antarctica’s ice fields contain enough water to drive sea level up 250 feet or so. Thus a reasonable estimate for sea level change over the next five hundred years involves the collapse of the Greenland and West Antarctic sheets and some modest melting on the edges of the East Antarctic sheet, raising sea level by something over 50 feet, delivered in a series of unpredictable bursts divided by long periods of relative stability or slow change.
The result will be what paleogeographers call “marine transgression”—the invasion of dry land and fresh water by the sea. Fifty feet of sea level change adds up to quite a bit of marine transgression in some areas, much less in others, depending always on local topography. Where the ground is low and flat, the rising seas can penetrate a very long way; in California, for example, the state capital at Sacramento is many miles from the ocean, but since it’s only 30 feet above sea level and connected to the sea by a river, its skyscrapers will be rising out of a brackish estuary long before Greenland and West Antarctica are bare of ice. The port cities of the Gulf coast are also on the front lines—New Orleans is actually below sea level, and will likely be an early casualty, but every other Gulf port from Brownsville, Texas (elevation 43 feet) to Tampa, Florida (elevation 15 feet) faces the same fate, and most East and West Coast ports face substantial flooding of economically important districts.
The flooding of Sacramento isn’t the end of the world, and there may even be some among my readers who would consider it to be a good thing. What I’d like to point out, though, is the economic impact of the rising waters. Faced with an unpredictable but continuing rise in sea level, communities and societies face one of two extremely expensive choices. They can abandon billions of dollars of infrastructure to the sea and rebuild further inland, or they can invest billions of dollars in flood control. Because the rate of sea level change can’t be anticipated, furthermore, there’s no way to know in advance how far to relocate or how high to build the barriers at any given time, and there are often hard limits to how much change can be done in advance: port cities, for example, can’t just move away from the sea and still maintain a functioning economy.
This is a pattern we’ll be seeing over and over again in this series of posts. Societies descending into dark ages reliably get caught on the horns of a brutal dilemma. For any of a galaxy of reasons, crucial elements of infrastructure no longer do the job they once did, but reworking or replacing them runs up against two critical difficulties that are hardwired into the process of decline itself. The first is that, as time passes, the resources needed to do the necessary work become increasingly scarce; the second is that, as time passes, the uncertainties about what needs to be done become increasingly large.
The result can be tracked in the decline of every civilization. At first, failing systems are replaced with some success, but the economic impact of the replacement process becomes an ever-increasing burden, and the new systems never do quite manage to work as well as the older ones did in their heyday. As the process continues, the costs keep mounting and the benefits become less reliable; more and more often, scarce resources end up being wasted or put to counterproductive uses because the situation is too uncertain to allow for their optimum allocation. With each passing year, decision makers have to figure out how much of the dwindling stock of resources can be put to productive uses and how much has to be set aside for crisis management, and the raw uncertainty of the times guarantees that these decisions will very often turn out wrong. Eventually, the declining curve in available resources and the rising curve of uncertainty intersect to produce a crisis that spins out of control, and what’s left of a community, an economic sector, or a whole civilization goes to pieces under the impact.
It’s not too hard to anticipate how that will play out in the century or so immediately ahead of us. If, as I’ve suggested, we can expect the onset of a global meltwater pulse from the breakup of the Greenland and West Antarctic ice sheets at some point in the years ahead, the first upward jolt in sea level will doubtless be met with grand plans for flood-control measures in some areas, and relocation of housing and economic activities in others. Some of those plans may even be carried out, though the raw economic impact of worldwide coastal flooding on a global economy already under severe strain from a chaotic climate and a variety of other factors won’t make that easy. Some coastal cities will hunker down behind hurriedly built or enlarged levees, others will abandon low-lying districts and try to rebuild further upslope, still others will simply founder and be partly or wholly abandoned—and all these choices impose costs on society as a whole.
Thereafter, in years and decades when sea level rises only slowly, the costs of maintaining flood control measures and replacing vulnerable infrastructure with new facilities on higher ground will become an unpopular burden, and the same logic that drives climate change denialism today will doubtless find plenty of hearers then as well. In years and decades when sea level surges upwards, the flood control measures and relocation projects will face increasingly severe tests, which some of them will inevitably fail. The twin spirals of rising costs and rising uncertainty will have their usual effect, shredding the ability of a failing society to cope with the challenges that beset it.
It’s even possible in one specific case to make an educated guess as to the nature of the pressures that will finally push the situation over the edge into collapse and abandonment. It so happens that three different processes that follow in the wake of rapid glacial melting all have the same disastrous consequence for the eastern shores of North America.
The first of these is isostatic rebound. When you pile billions of tons of ice on a piece of land, the land sinks, pressing down hundreds or thousands of feet into the Earth’s mantle; melt the ice, and the land rises again. If the melting happens over a brief time, geologically speaking, the rebound is generally fast enough to place severe stress on geological faults all through the region, and thus sharply increases the occurrence of earthquakes. The Greenland ice sheet is by no means exempt from this process, and many of the earthquakes in the area around a rising Greenland will inevitably happen offshore. The likely result? Tsunamis.
The second process is the destabilization of undersea sediments that build up around an ice sheet that ends in the ocean. As the ice goes away, torrents of meltwater pour into the surrounding seas, and isostatic rebound changes the slope of the underlying rock, masses of sediment break free and plunge down the continental slope into the deep ocean. Some of the sediment slides that followed the end of the last ice age were of impressive scale—the Storegga Slide off the coast of Norway around 6220 BCE, which was caused by exactly this process, sent 840 cubic miles of sediment careening down the continental slope. The likely result? More tsunamis.
The third process, which is somewhat more speculative than the first two, is the sudden blowout of large volumes of undersea methane hydrates. Several oceanographers and paleoclimatologists have argued that the traces of very large underwater slides in the Atlantic, dating from the waning days of the last ice age, may well be the traces of such blowouts. As the climate warmed, they suggest, methane hydrates on the continental shelves were destabilized by rising temperatures, and a sudden shock—perhaps delivered by an earthquake, perhaps by something else—triggered the explosive release of thousands or millions of tons of methane all at once. The likely result? Still more tsunamis.
It’s crucial to realize the role that uncertainty plays here, as in so many dimensions of our predicament. No one knows whether tsunamis driven by glacial melting will hammer the shores of the northern Atlantic basin some time in the next week, or some time in the next millennium. Even if tsunamis driven by the collapse of the Greenland ice sheet become statistically inevitable, there’s no way for anyone to know in advance the timing, scale, and direction of any such event. Efficient allocation of resources to East Coast ports becomes a nighmarish challenge when you literally have no way of knowing how soon any given investment might suddenly end up on the bottom of the Atlantic.
If human beings behave as they usually do, what will most likely happen is that the port cities of the US East Coast will keep on trying to maintain business as usual until well after that stops making any kind of economic sense. The faster the seas rise and the sooner the first tsunamis show up, the sooner that response will tip over into its opposite, and people will begin to flee in large numbers from the coasts in search of safety for themselves and their families. My working guess is that the eastern seaboard of dark age America will be sparsely populated, with communities concentrated in those areas where land well above tsunami range lies close to the sea. The Pacific and Gulf coasts will be at much less risk from tsunamis, and so may be more thickly settled; that said, during periods of rapid marine transgression, the mostly flat and vulnerable Gulf Coast may lose a great deal of land, and those who live there will need to be ready to move inland in a hurry.
All these factors make for a shift in the economic and political geography of the continent that will be of quite some importance at a later point in this series of posts. In times of rapid sea level change, maintaining the infrastructure for maritime trade in seacoast ports is a losing struggle; maritime trade is still possible without port infrastructure, but it’s rarely economically viable; and that means that inland waterways with good navigable connections to the sea will take on an even greater importance than they have today. In North America, the most crucial of those are the St. Lawrence Seaway, the Hudson River-Erie Canal linkage to the Great Lakes, and whatever port further inland replaces New Orleans—Baton Rouge is a likely candidate, due to its location and elevation above sea level—once the current Mississippi delta drowns beneath the rising seas.
