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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.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