The World in 2050: Four Forces Shaping Civilization's Northern Future (31 page)

The Evolution of Climate Models

The motivation for running climate models is nothing like the motivation for making weather forecasts on the nightly news. Those seek to identify specific events, like a storm front, and are meaningful only a few days into the future. But climate models forecast
average
climate variables, like mean January temperature, and are meaningful many decades into the future. They do this by taking account of certain things—like deep ocean circulation and increasing greenhouse gas concentrations—that simply don’t matter for short-term weather. It’s not possible to know what the exact temperature will be in Chicago next August 14 or January 2 at three o’clock in the afternoon, but it’s very possible to know what the average August or January temperatures will be. One is weather, the other is climate.

Climate models are also amazing tools for figuring out how our complex world actually works. Suppose that it is an observed fact that summer rainfall is declining in Georgia, but this phenomenon simply won’t show up in a climate model’s simulations no matter how many times it is run. Puzzled, its programmers realize that something is missing and wonder what it might be. Into the model goes a hypothesis—say, loss of forest (trees pump enormous volumes of water vapor back to the atmosphere), because many trees have been removed to build Atlanta suburbs. Does the model now correctly simulate the measured rainfall decline? If so, congratulations—new scientific understanding has been won about how rainfall works in Georgia, and the climate model has been made more realistic. If not, on to test the next hypothesis down the list. Eventually the missing bit of physics is discovered, the model is improved, and its creators move on to ponder its next little failure.

At their core, climate modelers seek to understand how the atmosphere functions, and how it responds to changing drivers. By studying when and where the models break down, we improve scientific understanding of how the real world works, and our models become more accurate. After more than fifty years of trial and error, they have now evolved far beyond their primitive ancestors of the 1960s. We’ve learned a great deal about how Earth’s climate system actually operates. In today’s generation of models, complicated things like El Niño and the Hadley Circulation emerge organically without programmers having to “add” them at all. That is very encouraging, because it tells us the models’ assumptions and physics
⁴⁸⁶
are realistic and working correctly.

The big push now is to hone down climate model spatial resolutions (i.e., the “pixel size” of their simulations) from hundreds of kilometers, useful for broad-scale projections like the ones presented in this book, to kilometers, which is what local planners need. But even at the coarser spatial scale of today’s generation of models, many important conclusions about our future are now well vetted and uncontroversial. All of the megatrends discussed so far—rising global average temperature, the amplified warming in the Arctic, rising winter precipitation around the northern high latitudes—fall within this uncontroversial category.

More troublesome are the short-sellers and inside traders of natural climatic variability. Volcanoes, wildfires, and sunspot cycles are just a few of many phenomena imprinting their own natural variations over the underlying greenhouse gas signal. But now these volatile (and fairly common) phenomena, too, are being added to climate models and tested.

Where climate models suffer most is in capturing rare events lying totally outside of our modern experience. Most weather stations are less than a century old; the satellite data era began only in the 1960s and ’70s. These records are far too short to illuminate the full range of our Earth’s twitchy behavior. Shifting oceans and ice sheets are key drivers of climate yet contain toggles and circuits with longer patience than our short instrumental records. They add boosts, buffers, and dips to the overall greenhouse effect, so we must understand them as well.

Unfortunately, a naturally twitchy climate makes the steady, predictable push from anthropogenic greenhouse gases more dangerous, not less. From the geological past we know the Earth’s climate has not always been so quiet as it is now. Therefore, through greenhouse loading we are applying a persistent pressure to a system prone to sudden jumps in ways we don’t fully understand. Imagine a wildcat quietly sleeping on your porch—it looks peaceful but is by nature an ill-tempered, unpredictable beast that might spring into a flurry of teeth and claws in an instant. Greenhouse gases are your knuckles pressing inexorably into its soft slumbering belly; the global ecosystem is your exposed hand and arm.

Rare or threshold behaviors—like a permanent reorganization of rainfall patterns, accelerated sea-level rise, or a giant burp of greenhouse gas from the ground—all pose legitimate threats to the world. We know they are plausible but, unlike greenhouse gas forcing, don’t know yet how probable. But their behaviors, too, must be added to climate models somehow. Just because something seems unlikely doesn’t mean it won’t happen, or that its impacts are not potentially enormous if it does. These are the climate genies, and we are just beginning to discern the outline of their various sleeping forms. To find them at all, we must turn to the prehistoric past.

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,
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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.
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Even my thesis advisor—who was pretty famous himself, having written the paper putting together the theory of plate tectonics
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—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.
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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.
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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).
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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.”
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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 southern 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.
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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.
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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.
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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.