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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