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.
