Might Catastrophic Cooling be Triggered by Greenhouse Warming
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This is the unedited version of an article published under the title
"Greenhouse and Icehouse" in the Whole Earth Review 73:106-111
(1991). Copyright 1991 by William H. Calvin. Personal copying
permitted; inquire otherwise ([email protected]).
Might Catastrophic Cooling be Triggered by Greenhouse Warming?
William H. Calvin
A recurrent nightmare for some scientists is to imagine Europe suddenly
deprived of its customary wintertime bonus of tropical heat, traditionally
delivered courtesy of the North Atlantic Current. As it happens, that shutoff
scenario doesn't require a catastrophe-prone imagination: it's already
happened many times in the past. What's new is how the global warming
might, paradoxically, trigger yet another abrupt episode of continental
cooling.
If you are flying from Paris to Seattle (or London to Los Angeles),
you look down on the North Atlantic Current sweeping up from the warm
tropics to the vicinity of Iceland (well, at least you see the clouds that it
encourages, drifting toward Europe). This current (Fig. 1), with only a
minor contribution from the Gulf Stream, is what keeps Europe wet and
warm. After all, judging from its northerly latitudes and the associated
sunshine, Europe really ought to be like Canada. Or perhaps Siberia. While
Canada is a very nice place, its agriculture only supports about 4 percent as
many people as Europe's climate sustains (France alone has twice Canada's
population). Such is the difference made by the North Atlantic Current.
The day before my most recent transatlantic overview, while
browsing the library at La Cit‚ des Sciences et de l'Industrie, I came upon
the article by Mikolajewicz et al. in the 14 June 1990 issue of Nature.
And it ruined my last night in Paris. Even though the authors do not
discuss abrupt climate shifts of the past or their mechanisms, the results of
their simulations of ocean circulation are disturbing to anyone who has been
following the news about ice age climates. The predicted changes in the
North Atlantic Ocean in the next few decades of greenhouse warming of the
atmosphere are like those that others, have suggested were responsible for
the last major episode of cold and arid climate in Europe (eastern North
America was affected to a lesser extent). If global warming can trigger
abrupt cooling, that's far more alarming than the now-familiar predictions
of slowly rising sea level and slowly thinning ozone.
Minor fluctuations in North Atlantic climate a thousand years ago
were responsible for why Iceland was not named Greenland and vice versa
(by the time that the coast of Greenland was settled by explorers from
Iceland, things had warmed somewhat). Then came the Little Ice Age
which wiped out the non-Inuit settlements on Greenland. Yet neither such
changes of the last millennium, nor the occasional multiyear drought, is
what is meant by abrupt climate change. The most recent abrupt episode
was the Younger Dryas: in the midst of the rising CO2 and the general
warming trend that melted the ice sheets of the last glaciation, there was a
"cold spike" that lasted about 800 years.
It caused European forests to die within a decade or two; they were
replaced with Arctic-adapted plants such as Dryas. It caused Scotland's
glaciers to form once again. Because this happened when northern
hemisphere summer sunshine was near its astronomical maximum, it served
to alert climatologists that there was more to ice advance than just the
familiar Milankovitch cycles (it has since been discovered that southern
hemisphere ice sheets also fail to follow the predictions; rather, they often
advance at the same times that northern hemisphere glaciers do). A good
time for rapidly melting all that ice in the northern hemisphere is, just as
Milankovitch predicted, when the earth's axial tilt is maximal and the
earth's closest approach to the sun occurs in June -- but there appears to be
something else going on that can occasionally override this general
pacemaker of the ice cycles.
The Younger Dryas cooling started 11,500 years ago; it lasted until
10,700 years ago, when it ended even more suddenly than it began. Thanks
to the year-by-year detail in the ice cores of Greenland studied by
Dansgaard et al., we know that rainfall returned over a 20 year period and,
as Europe's land surface warmed up, the formerly severe winter storms
diminished dramatically in that same two-decade-long period. As can be
seen in the top of Figure 2, cooling episodes are just as rapid (though often
with associated hot-and-cold "whiplash" chattering). Once triggered, mode-
switching climatic "leaps" evidently operate on a far faster time scale than
20,000-to-100,000 year Milankovitch cycles, faster even than the century-
long time scale of the predicted greenhouse warming. What triggers such
an abrupt change in climate?
Disturbing the Waterfall
What the sleep-depriving ocean current simulations2 implied is that,
in response to greenhouse warming, northern Atlantic surface salinity will
decrease, deep water production will drastically decrease in the ocean just
south of Iceland, and surface temperatures will drop. Unless you know
about ice age climates and salt economies, that combination might not seem
noteworthy. Those three factors are, alas, the conditions earlier proposed by
Broecker and coworkers3 as having encouraged the onset of the Younger
Dryas. Their theory is akin to traditional ones for how extra rainfall
reverses the salt circulation in estuaries (and in the Mediterranean during the
last Pluvial), just scaled up to an entire ocean. One says "encourage" rather
than "cause" because cause-and-effect reasoning can be tricky, given that
nonlinear systems often chase their tails.
That is a particularly apt description of the wintertime North Atlantic
Current (Fig. 1): it even does a vertical U-turn. Northbound, it rises to the
surface near Iceland and releases a bonus of heat to the Europe-bound winds
from Canada; this contribution is equal to 30 percent of what sunshine
provides to the northern Atlantic4. Then the current -- now so cold and
hypersaline that it is denser than any layer of underlying water -- plunges
from the surface to the abyss. Once the dense water has sunk under its own
weight to the sea floor, it flows south (just as Benjamin Thompson -- Count
Rumford -- predicted back in 1800) -- and so attracts even more warm
currents north to replace it. It is unfortunate that there isn't a giant northern
Atlantic whirlpool or waterfall for television crews to focus upon, as
commentators somberly warn of its key role in Europe's viability. Suffice
it to say that this "deep water production" is equal in magnitude to 20 times
the combined flow of all the rivers of the world4 -- and that three-quarters
of it might disappear in only 35 years, according to those new greenhouse
simulations2. That's why I had no appetite for dinner, that last night in
Paris.
In the boolean logic demonstration at La Cit‚ (as I remembered in
the middle of the night), hydraulic currents of colored water in glass
columns switch themselves on and off. So can the North Atlantic Current:
despite its enormous momentum, this salty stream stops flowing, ceasing to
transport tropical heat north for airborne transfer east to Europe. It did that
during the last major glaciation, and again during the Younger Dryas; one
supposes that another such shutdown will be associated with the beginning
of the next ice age (and our present interglacial period has already lasted as
long as the previous one, between 128,000 and 118,000 years ago). What
might derail the conveyor?
The obvious way to break the loop of warm water chasing
hypersaline water, and thus turn off the virtual waterfall, is to flood the
surface of the northern Atlantic with a deluge of freshwater. Say, from a
lake of glacial meltwater impounded behind a dam that breaks. Or, more
gradually, just from changes in rainfall patterns, presumably a major factor
in the freshening seen in those greenhouse simulations (which didn't take
such nonlinear ice dynamics into account). Shutdown can also be achieved
by smothering: floating ice blanketing the northern Atlantic might prevent
the wind-driven evaporation that makes the surface waters so hypersaline
and heavy every winter. Once interrupted by one means or another, the
loop that warms Europe might take some time to get started again, awaiting
climate fluctuations that carry ocean currents into initial conditions for a
diving loop. Tail-chasing loops can be harder to initiate than to maintain.
Of Floods and Firehoses
Broecker et al. have recently examined whether the Younger Dryas
was associated with the partial drainage of a giant midcontinental lake of
meltwater, flooding the North Atlantic via the St. Lawrence River's outlet.
Some of those worried about repeat performances of the Younger Dryas
have likely welcomed the Lake Agassiz theory with a sense of relief, since
massive amounts of meltwater are no longer available for release from the
Canadian coastline (or, for that matter, from the Scandinavian). But the
coral-reef record of late glacial sea level change (from which century-by-
century meltwater additions can be inferred) does not indicate that there was
massive flooding during the Younger Dryas,,. I would also note
that, especially during lowered sea level, the St. Lawrence River largely
emerges southwest of the Grand Banks of Newfoundland, mixing with
Atlantic waters at 45degN, hardly a major site of deep water production.
One would expect the northern Atlantic to be more sensitive to freshwater
flooding.
Above 60degN, the vertical U-turn is presumably shaped by the sea
floor rising 3000 meters from the abyss to the shallow continental shelf
south and west of Iceland; fresh flows from Greenland's fjords might shut
down a particularly active region of the waterfall. The enormous fjord
system on Greenland's east coast can presumably produce strong freshwater
flows channeled south between Iceland and Greenland (Fig. 1). This
"concentrated" freshwater flow out of Denmark Strait might act something
like a firehose, quenching the salty waterfall in a sensitive spot.
Whether by such focal freshwater or the more diffuse dilutions
(which also act by causing winter pack ice to extend further south, capping
evaporation), repeat performances of the Younger Dryas shutdown might
still be possible, as there are massive amounts of meltwater available from
a greenhouse Greenland. The last time that I flew from Seattle to
Copenhagen, the iceberg factory in Greenland's long east coast fjords at
70degN looked quite active. There were a number of meltwater lakes
somewhat inland, on the shoulders of the ice sheet. Often such a lake drains
because a crevasse opens up beneath it, but the sudden deluge into the
depths may serve to lubricate the ice's attachment to underlying rock. This
may, in turn, promote a glacial surge into the tidal waters, thereby
amplifying the melting.
Furthermore, each fjord (and Greenland has more than its share) is
capable of temporarily housing a meltwater lake. This happens when a
glacial surge comes in somewhere along one side of the narrow channel; a
few years ago, the entire southern arm of Yakutat Bay in Alaska was
dammed up in this manner, trapping many marine mammals in the
freshening waters. After enough backup, such ice dams eventually break,
releasing months-to-years of meltwater within days. So one expects a lot
of month-to-month variation in freshwater flows near a deglaciating
Greenland, something that would be smoothed out if the meltwater had to
first traverse such long overland paths as the Mississippi River or the St.
Lawrence River.
The Careful Handling of Instabilities
Whatever the fate of any such theories for the Younger Dryas per se,
"relief" is likely wishful thinking: it must be remembered that the Younger
Dryas was only the most recent of more than a dozen abrupt climate
changes in the northern Atlantic region. As Broecker noted, "The
records of the last 150,000 years... scream at us that the earth's climate
system is highly sensitive to nudges... By adding infrared-absorbing gases
to the atmosphere, we are effectively playing Russian roulette with our
climate." In addition to all the nudges from fjord floods, the northern
Atlantic may also have an underlying instability, a tendency to flip.
Given such predispositions, we must not let the complexities of triggering
one particular episode obscure the more general problem: Understanding
what attracts warm tropical waters to the northern Atlantic (which surely
involves the salt circulation), what could trigger another abrupt loss of this
warming (which surely includes the freshening of surface waters in the
northern Atlantic), and what serves to stabilize the loop.
The dynamics of such "latch-up," and the occasional "chattering"
between the resulting modes when conditions are marginal, will likely be
addressed by catastrophe theorists; models that utilize average melt rates,
and so smooth out the major floods, may be inadequate, failing to discover
shutdown and whiplash scenarios. We certainly need some assessment, and
soon, of just how close we currently are to the switchover conditions.
Meanwhile, those with practical experience in dealing with nonlinear
systems would undoubtedly offer this cautionary rule of thumb: Avoid
sudden changes. In the early days of airplane design, bumpy air (of the
kind one encounters over the Atlantic) or sudden maneuvers could put an
airplane into a tail spin. Sometimes the plane was shaken apart. The
obvious advice to minimize unpleasant surprises: Take it slowly, unless you
thoroughly understand the system (or, as with paper airplanes, can afford to
engage in destructive testing).
Taking it slowly is not what we have been doing, given the speed
with which CO2 and greenhouse gases are increasing. Or the rapidity with
which our tropical forests are being eliminated. Whether it qualifies as a
nudge or a kick remains to be estimated by the theorists. We simply cannot
now say exactly when the icehouse cometh, just that it will probably happen
much more abruptly than Milankovitch-based thinking has envisaged.
Certainly our scientific understanding of the mode-switching processes in
atmosphere and ocean is far short of what we will need to stabilize the
North Atlantic Current.
And from the current level of resources available to researchers,
you'd think that it was Antarctica that was threatened rather than Europe
and the east coast of North America: I didn't see a fleet of oceanographic
research ships down there studying the North Atlantic Current year-around,
nor have I heard of a half-dozen supercomputer-equipped theoretical groups
modeling the dynamics of the Current, nor is there a high-powered planning
group evaluating technological responses, should we discover that the winter
waterfall is weakening. There is no major effort in reproductive physiology
to find new ways of stopping the population explosion: the increases in
greenhouse gases are often secondary to more people, and it remains to be
seen if current plans to clean up emissions over the coming decades will
even compensate for what the worldwide population increase will add in the
meantime.
The 500 million people in Europe who depend on that bonus from
the North Atlantic Current (perhaps 700 million: the Younger Dryas climate
changes reached at least as far east as the Ukraine) have a considerable
interest in preventing such unpleasant surprises as were experienced by the
hunters and gatherers living in Europe 11,500 years ago. The thousand-fold
population increase since then causes Europe to be particularly vulnerable
to climatic shocks that arrive with little warning; the two-decade-long
excursion of the proxy climate indicators (Fig. 2) should be interpreted to
mean that significant changes could occur in several years. Essentially, a
drought would start, get worse -- and then it would be too late for
stockpiling.
Regional Cooling, Worldwide Challenge
But non-Europeans are vulnerable too, and not just those along the
eastern shores of North America (and elsewhere around the world where
repercussions of the Younger Dryas have been detected). Abrupt and
widespread agricultural shortfalls in densely-populated technological
societies tend to suggest lebensraum-style global conflict. Affected
populations will initially switch (as they have during brief droughts of the
past) to themselves eating the feed grains that now produce meat at 20
percent efficiency -- but remember how poorly an "economic response"
worked for Ireland in the 19th-century famine. Another cold spike need not
endure for 800 years to exhaust stockpiles and people's patience. Just
imagine any country affected by the North Atlantic Current contemplating
starvation -- while possessing the military technology needed to take over
another country (which will undoubtedly be described by the aggressors as
"irresponsibly squandering its agricultural potential while others starve").
From the Younger Dryas, one sees that regional cooling can occur
in the process of global warming, that the transition can be quite abrupt, and
that the duration can be far longer than the usual drought, plague, or war.
Preventing another shutdown of the North Atlantic Current seems the only
sensible strategy, as the climate's transition is likely to be too precipitous
for peaceful economic rearrangements and population relocations.
Overhauling our technology that contributes to greenhouse warming is an
obvious first step, and now an even more urgent one. But we shall need to
specifically address the icehouse as well, with a level of basic science that
will serve to quickly suggest a variety of possible technological responses.
To suppose that a climatic cooling cancels greenhouse warming, in
the familiar way we fix a scalding shower by adjusting the cold water tap,
is to indulge in a fool's paradise. The distribution is all important. Even
if the hot water heater has been readjusted to produce warmer water, you
can still get an inescapable blast of icy water -- if the hot water supply is
abruptly diverted to the washing machine. We now see how a gradual
greenhouse could paradoxically promote an abrupt icehouse. Whatever the
generalities applicable to the long term, we need the science and technology
to first survive the short term, to somehow maneuver around the whiplash
conditions that might shake our civilization apart.
###
The author is a neurophysiologist at the University of
Washington, NJ-15, Seattle, Washington 98195, USA.
Internet e-mail: [email protected]
3050 words, plus two illustrations without legends
References
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