The Flickering Switch
One of my personal heroes in science is Richard B. Alley, an outstandingly accomplished glaciologist and professor of geosciences at Penn State University. Not only has he cranked out one landmark idea after another, published nearly forty times in Science and Nature, been elected to the National Academy of Sciences, and written a wonderful popular book explaining it all for the rest of us,487 he is also about the nicest and most enthusiastic guy one could ever hope to meet.
In 1994, Alley came to deliver a guest lecture at Cornell University, where I was a lowly second-year graduate student. Everyone was abuzz that Richard Alley was coming, because he had just published a pair of back-to-back articles in Nature that had stunned the climate-science community.488 Even my thesis advisor—who was pretty famous himself, having written the paper putting together the theory of plate tectonics489—was talking about them. But a great thing about academia is that it is on open, democratic affair even when it comes to its pop icons. Visiting celebrities will hang out for a day or two happily chatting with whomever, even lowly second-year graduate students. Landing a meeting with one is largely a matter of getting to the sign-up sheet first, which of course I did.
When my time slot arrived I went to meet Alley, armed with a list of questions about his Nature papers so I could hear more from the great man himself. That lasted about forty-five seconds, before he insisted on hearing all about my work. I couldn’t believe it. It was a dumb little side project of my research, but Alley’s enthusiasm was totally contagious. We relocated to my lab hole, where he huddled alongside me, giving all manner of helpful advice and inspiration. By the time he ran off late to his next appointment, I was so excited about my project I barely remembered I’d forgotten to ask more about his. That’s just the kind of guy he is.490
What had everyone gabbling was what Alley and his colleagues had dug out of the Greenland Ice Sheet. The U.S. National Science Foundation had funded construction of a drilling and laboratory camp on top of it to extract a two-mile-long ice core called GISP2, an enormous task taking about four years.491 Preserved in the upper sections of ice cores are annual layers, like the rings of a tree. Each one contains the compressed equivalent of a full year’s worth of snow accumulation falling on the ice sheet surface (cores are drilled from deep ice sheet interiors where it never melts). By counting the layers down-core and measuring their thickness and chemistry, a very long reconstruction of past climate variations is obtained. We even get tiny samples of the ancient atmosphere, by cracking into air bubbles trapped in the ice. From these high-resolution annual measurements in Greenland, Alley and his colleagues had discovered that around twelve thousand years ago, just when we were pulling out of the last ice age, the climate began shuddering wildly.
The shudders happened faster than anyone had dreamed possible. Our climatic emergence from the last ice age, it seems, was neither gradual nor smooth. Instead it underwent rapid flip-flops, seesawing back and forth between glacial and interglacial (warm) temperatures several times before finally settling down into a warmer state. These large temperature swings happened in less than a decade and as quickly as three years. Precipitation doubled in as little as a single year. Around Greenland, at least, there was no gradual, smooth transition from a cold ice age to the balmy interglacial period of today. Alley’s team had shown that climate could sometimes teeter as well, like a “flickering switch,” between two very different states. Furthermore, it had happened other times in earlier millennia, so this was not a totally isolated event. The extreme rapidity of these changes, concluded Alley, implied “some kind of threshold or trigger in the North Atlantic climate system.”492
Thus was born a brand-new subfield of climate science known today as “abrupt climate change.” Twenty years ago anyone who hypothesized a sudden, showstopping event—a century-long drought, a rapid temperature climb, or the fast die-off of forests—would have been laughed off. But today a growing body of evidence from ice cores, tree rings, ocean sediments, and other natural archives tells that such things have happened in the past. We’ve long known the Earth’s climate has experienced big changes before but assumed they only occurred slowly over geological time, like the gradual turning of a dial. Now we know they can sometimes happen abruptly as well, like flipping a switch. The implications of this are global, as we shall see next.
The Pentagon Report
From a societal perspective, an abrupt unexpected climate change is more destabilizing than one that is gradual and anticipated. Military analysts concede that the expected gradual climate changes pose national security threats, and by late 2009 the U.S. Central Intelligence Agency had opened a new center specifically dedicated to assessing them.493 A recent study, for example, projects a more than 50% increase in armed conflict and nearly four hundred thousand more battle deaths in Africa by 2030.494 But one of the few attempts to assess the societal impact of an abrupt climate change was commissioned by the U.S. Department of Defense in 2003.
This document, titled “An Abrupt Climate Change Scenario and Its Implications for United States National Security,” is not based on climate model projections, but instead on a known prehistoric event seen in ice cores, sediments, and fossils. About 8,200 years ago, several thousand years after the really big swings that Alley had studied, temperatures near Greenland suddenly tumbled by about 6°-7°C. Cold, dry, windy conditions spread across northern Europe and into Asia; certain African and Asian monsoon rains faltered, and temperatures probably rose slightly around the southern hemisphere. These conditions persisted for about 160 years before reversing again.
This event was not unique but simply the last and smallest of several climate shudders seen in Greenland ice cores as the last ice age wound down. It was less severe, shorter-lived, and less geographically extensive than its predecessors (especially the Younger Dryas event, the monster cold snap studied by Alley that abruptly kicked in about 12,700 years ago, then persisted for nearly 1,300 years).495 That said, let’s hope that it never happens again. The Pentagon’s report, which outlines possible social scenarios if what occurred 8,200 years ago were to happen again today, is quite scary.
It describes wars, starvation, disease, refugee flows, a human population crash, civil war in China, and the defensive fortification of the United States and Australia. “While the U.S. itself will be relatively better off and with more adaptive capacity,” the authors conclude, “it will find itself in a world where Europe will be struggling internally, large numbers of refugees washing up on its shores, and Asia in serious crisis over food and water. Disruption and conflict will be endemic features of life.”496 The report’s authors insist that their assessment, while extreme, is plausible.
Could this really happen? Nobody knows for certain, but the good news is that the physical mechanism underlying these North Atlantic cold shudders is now fairly well understood, and its behavior successfully replicated by climate models, so we can at least test the probability. The culprit appears to be a slowdown of the global thermohaline circulation—the long, ribbon-like “heat conveyor belt” of ocean currents, one arm of which carries warm tropical water from the Indian Ocean all the way to the Nordic seas, bathing western Europe and Scandinavia in all that heat so undeserved for its latitude as described in Chapter 7. The North Atlantic region is a critical pivot for this global circulation pattern. It is where the warm, salty north-flowing surface current finally cools sufficiently so that it becomes heavier than the surrounding colder (but less saline) water, sinks down to the ocean floor, and begins its millennia-long return south, crawling along the dark bottom of the abyss.
All of this is driven by density contrasts. If sufficiently large, a local freshening of the North Atlantic can slow or even halt the sinking, thus killing this entire overturning arm of the global heat conveyor belt. This has immediate implications for the Earth’s climate. Heat becomes less mixed around the planet. Cold temperatures (especially winters) and drought descend upon Europe. The southe
rn latitudes warm; the Asian and African monsoons weaken or drift. It’s rather like adding hot water to a cold bath, in which stirring the water around helps to even out the temperature contrasts. But with no water circulation, one’s back grows cold but feet are scalded.
The most likely source of water for the sudden freshening of the North Atlantic was one or more massive floods released from the North American continent at the end of the last ice age, as its giant glacial ice sheet melted away. As the sheet retreated north into Canada, huge freshwater lakes, some even larger than the Great Lakes today, pooled against its shrinking edge. Then, when a pathway to the sea emerged from beneath the rotting ice, out the water went. The deluge that tore out through Hudson Bay must have been biblically awesome in scale.497 I wonder if any aboriginal version of Noah witnessed and survived it, creating a legend for generations of the Great Flood that drained the Earth’s water to the sea, bringing seemingly endless winter upon the land.
Figuring out hidden genies takes time and a lot of work. The above hydrologic explanation for the North Atlantic climate shudders was first proposed by Columbia University’s Wallace Broecker back in 1985.498 Its finer details are still being tinkered with today. But now that we understand this genie rather well, and its physics are reproducible in climate models, we can assess the likelihood of another such shudder happening again in the future.
So far, most simulations agree that a complete collapse of the thermohaline circulation is unlikely anytime soon, for the simple reason that it’s hard to find a big enough freshwater source with which to sufficiently hose down the North Atlantic. The Laurentide ice sheet that once covered Canada and much of the American Midwest is long gone. The projected increases in high-latitude precipitation and river runoff appear sufficient to weaken the circulation, but not enough to kill it outright.499 This weakening shows up in most future climate model projections as a little bull’s-eye of below-average warming centered over the North Atlantic. It’s not enough to create outright cooling, but it does reduce the magnitude of warming locally over this area. Let’s hope these simulations are correct—because if they’re wrong, losing even part of the Asian monsoon would be really, really bad.
There is, of course, another big source of potential freshwater—one that happens to be plunked right in the middle of the North Atlantic. No serious scientist thinks the Greenland Ice Sheet will melt away anytime soon, and if it ever does we’ll be dealing with even bigger worldwide problems than a cold, dry Europe and faltering monsoonal rains. But this genie, we’re nowhere near to understanding well enough to model yet.
Genie in the Ice
Two smelly straight guys sharing a tent sized for one is bad enough. But waking up covered in yellow dust, with no hot water for days, is the pits. It was impossible to keep the stuff out, even barricaded inside the lone wind-rated tent we had thought to bring with us.
The Greenland Ice Sheet was in charge, not me and not Ohio State geography professor Jason Box. We were camped next to its southwestern edge, where one of its many outlet glaciers finally succumbs to a grinding wet death, killed by the sun among the tundra grasses, caribou, and musk oxen. Every night, we squeezed head-to-toe in the little tent and buttoned up tight. Every night a fierce katabatic wind would pour off the ice sheet, lift tons of grit from its gravelly outwash plain, and fling it against our shuddering tent. The silt pushed through closed zippers and tiny mesh slits. It entered our nostrils and encrusted our hands as they gripped the tent’s violently shaking poles.
But by morning the winds would die down and we went to work. Jason installed time-lapse cameras to track the speed of the glacier’s sliding snout; I submerged electronic sensors in its outgoing torrent of meltwater to monitor how much was flowing off to the sea. We were studying these things to help answer a burning scientific question that should worry us all. Chapter 4 showed that we are facing decimeters of sea-level rise by century’s end. Many scientists wonder if even these estimates might be too low. Could climate warming cause the Greenland and West Antarctic ice sheets to accelerate their dumpage of ice and water into the sea, thus cranking up its rise even faster than is happening already? Could the world’s oceans go even higher, say a couple of meters by the end of this century?
The short answer is maybe. The geological record tells us sea levels are certainly capable of responding quickly to shrinking glaciers. And over the long haul—meaning several thousands of years—it looks like the Greenland Ice Sheet is in trouble and could well disappear completely.500 Glaciers and ice sheets are nourished on their tops by snow. They are removed at their margins by melting and—if they float out into an ocean or lake—by calving off icebergs into the water. When nourishment exceeds removal, glaciers grow, storing water up on land, so sea level falls. When removal exceeds nourishment, glaciers retreat and their stored water returns to the ocean. In this way sea levels have danced in a tight waltz with glaciers, falling and rising anywhere from about 130 meters lower to 4-6 meters higher than today over the past few ice ages. Other things—especially thermal expansion of ocean water as it warms—also drive sea level, but the waxing and waning of land ice is a huge driver.
As the last ice age unraveled, sea levels commonly rose 1 meter per century, and sometimes as fast as 4 meters per century during intervals of very rapid glacier melting.501 Looking forward, if average air temperatures over Greenland rise by another +3°C or so, its huge ice sheet, too, must eventually disappear. Depending on how hot we allow the greenhouse effect to become, this will take anywhere from one thousand to several thousand years, raising global average sea level by another 7 meters or so.
Based on the emissions scenarios currently being bandied about by policy makers, the temperature threshold to begin this process will indeed be crossed in this century, and the long, slow decline of Greenland’s ice sheet will begin.502 It is already something of a stubborn relic of the last ice age; if it magically disappeared off the island tomorrow, it’s doubtful this ice sheet could grow back.503 One thousand years from now, eighteen of the twenty-seven megacities of 2025 listed in Chapter 2 will lie partially or wholly beneath ocean water that might once have been blue ice in Greenland.504
But over the shorter term, meaning between now and the next century or two, the scary genie of Greenland and Antarctica isn’t from their ice sheets melting per se (indeed, it will never become warm enough at the South Pole for widespread melting to occur there) but from their giant frozen rumbling ribbons of ice that slide over hundreds of miles of land to dump icebergs into the sea. Already, there are many such ice streams in Antarctica and Greenland moving tens of meters to more than ten thousand meters per year. They empty out the deep frozen hearts of these ice sheets, where temperatures are so cold the surface never melts at all.
Of grave concern is collapse of the West Antarctic Ice Sheet. This vast area is like a miniature continent of ice towering out of the ocean, much of it frozen to bedrock lying below sea level. If it became unstuck, a great many Antarctic glaciers would start lumbering toward the water, eventually raising average global sea level by around five meters. There is geological evidence that this has happened before,505 and if it happens again it would hit the United States especially hard. For various reasons a rise in global average sea level does not translate to the same increase everywhere—water will rise by more than the average amount in some places and less than the average in others.506 Such a collapse would produce above-average inundation of the Gulf Coast and eastern seaboard, putting Miami, Washington, D.C., New Orleans, and much of the Gulf Coast underwater. When it comes to climate genies, the West Antarctic Ice Sheet is an ugly-looking lamp.
Frankly, we don’t understand the physics of sliding glaciers and ice sheet collapses well enough yet to model the futures of Greenland and Antarctica with confidence. Many things affect the speed and dynamics of that long slide that are hard to measure or see. They include the interplay between the sliding ice and its bed, the heat and lubrication added by meltwater percolatin
g to the bed from the surface, the importance of buttressing ice shelves (which help dam ice up on the land), the ocean water temperature at the ice edge, and others.507 Computer models and field studies—like the one Jason and I were conducting in Greenland—are in their infancy. Scientists are still discovering new things and debating what may or may not be important. This is why the likelihood of accelerated sea-level rise was kept out of the last IPCC assessment, and may be kept out of the next one as well. Might the ice sheets start slipping faster, with higher sea levels right behind? Perhaps—but without well-constrained models, we don’t yet know how likely that is.
Genie in the Ground
Digging into a permafrost landscape usually goes something like this: After cutting through a thick living mat of vegetation, the spade turns over a dark, organic-rich soil, almost like the mulch that one buys to spread in a garden. Usually there are bits and pieces of old dead plants poking out of it. Then, anywhere from several to tens of inches down, the blade goes chunk and will bite no farther. But it’s not a stone. At the bottom of the hole, there is just more of the same organic-rich goop but it is frozen hard as cement, often with a little black ice peeking through. Going any deeper is a major job, requiring a big drill and lots of manpower.
Why on Earth would anybody go all the way to the Arctic to drill holes into frozen black muck? The reason is organic carbon, and we now know that frozen northern soils hold more of it than any other landscape on Earth. In fact, the more we study these soils the more carbon we find. As of 2010 the latest estimate is 1,672 billion tons (gigatons) of pure organic carbon frozen in the ground.508 That’s roughly half of the world’s total soil carbon crammed into just 12% of its land area.
The World in 2050: Four Forces Shaping Civilization's Northern Future Page 26