Fortunately, big parallel computers have proved useful for both global climate modeling and detailed modeling of ocean circulation. An abrupt cooling could happen now, and the world might not warm up again for a long time: it looks as if the last warm period, having lasted 13, 000 years, came to an end with an abrupt, prolonged cooling. Just as an El Niño produces a hotter Equator in the Pacific Ocean and generates more atmospheric convection, so there might be a subnormal mode that decreases heat, convection, and evaporation. Were fjord floods causing flushing to fail, because the downwelling sites were fairly close to the fjords, it is obvious that we could solve the problem. It was initially hoped that the abrupt warmings and coolings were just an oddity of Greenland's weather—but they have now been detected on a worldwide scale, and at about the same time. Three sheets in the wind meaning. Unlike most ocean currents, the North Atlantic Current has a return loop that runs deep beneath the ocean surface.
One is diminished wind chill, when winds aren't as strong as usual, or as cold, or as dry—as is the case in the Labrador Sea during the North Atlantic Oscillation. They are utterly unlike the changes that one would expect from accumulating carbon dioxide or the setting adrift of ice shelves from Antarctica. Fatalism, in other words, might well be foolish. Keeping the present climate from falling back into the low state will in any case be a lot easier than trying to reverse such a change after it has occurred. To the long list of predicted consequences of global warming—stronger storms, methane release, habitat changes, ice-sheet melting, rising seas, stronger El Niños, killer heat waves—we must now add an abrupt, catastrophic cooling. For Europe to be as agriculturally productive as it is (it supports more than twice the population of the United States and Canada), all those cold, dry winds that blow eastward across the North Atlantic from Canada must somehow be warmed up. The populous parts of the United States and Canada are mostly between the latitudes of 30° and 45°, whereas the populous parts of Europe are ten to fifteen degrees farther north. Another sat on Hudson's Bay, and reached as far west as the foothills of the Rocky Mountains—where it pushed, head to head, against ice coming down from the Rockies. The sheet in 3 sheets to the wind crossword. There is also a great deal of unsalted water in Greenland's glaciers, just uphill from the major salt sinks. Alas, further warming might well kick us out of the "high state. "
Suppose we had reports that winter salt flushing was confined to certain areas, that abrupt shifts in the past were associated with localized flushing failures, andthat one computer model after another suggested a solution that was likely to work even under a wide range of weather extremes. Futurists have learned to bracket the future with alternative scenarios, each of which captures important features that cluster together, each of which is compact enough to be seen as a narrative on a human scale. We are near the end of a warm period in any event; ice ages return even without human influences on climate. That increased quantities of greenhouse gases will lead to global warming is as solid a scientific prediction as can be found, but other things influence climate too, and some people try to escape confronting the consequences of our pumping more and more greenhouse gases into the atmosphere by supposing that something will come along miraculously to counteract them. Meaning of three sheets to the wind. It, too, has a salty waterfall, which pours the hypersaline bottom waters of the Nordic Seas (the Greenland Sea and the Norwegian Sea) south into the lower levels of the North Atlantic Ocean. Even the tropics cool down by about nine degrees during an abrupt cooling, and it is hard to imagine what in the past could have disturbed the whole earth's climate on this scale.
By 1961 the oceanographer Henry Stommel, of the Woods Hole Oceanographic Institution, in Massachusetts, was beginning to worry that these warming currents might stop flowing if too much fresh water was added to the surface of the northern seas. The fact that excess salt is flushed from surface waters has global implications, some of them recognized two centuries ago. The Great Salinity Anomaly, a pool of semi-salty water derived from about 500 times as much unsalted water as that released by Russell Lake, was tracked from 1968 to 1982 as it moved south from Greenland's east coast. Such a conveyor is needed because the Atlantic is saltier than the Pacific (the Pacific has twice as much water with which to dilute the salt carried in from rivers). Scientists have known for some time that the previous warm period started 130, 000 years ago and ended 117, 000 years ago, with the return of cold temperatures that led to an ice age. Implementing it might cost no more, in relative terms, than building a medieval cathedral. Instead we would try one thing after another, creating a patchwork of solutions that might hold for another few decades, allowing the search for a better stabilizing mechanism to continue. And it sometimes changes its route dramatically, much as a bus route can be truncated into a shorter loop. Because water vapor is the most powerful greenhouse gas, this decrease in average humidity would cool things globally.
We may not have centuries to spare, but any economy in which two percent of the population produces all the food, as is the case in the United States today, has lots of resources and many options for reordering priorities. I call the colder one the "low state. " Things had been warming up, and half the ice sheets covering Europe and Canada had already melted. What could possibly halt the salt-conveyor belt that brings tropical heat so much farther north and limits the formation of ice sheets? That's how our warm period might end too. But to address how all these nonlinear mechanisms fit together—and what we might do to stabilize the climate—will require some speculation. The high state of climate seems to involve ocean currents that deliver an extraordinary amount of heat to the vicinity of Iceland and Norway. The last abrupt cooling, the Younger Dryas, drastically altered Europe's climate as far east as Ukraine. This El Niño-like shift in the atmospheric-circulation pattern over the North Atlantic, from the Azores to Greenland, often lasts a decade.
When this happens, something big, with worldwide connections, must be switching into a new mode of operation. Out of the sea of undulating white clouds mountain peaks stick up like islands. Canada lacks Europe's winter warmth and rainfall, because it has no equivalent of the North Atlantic Current to preheat its eastbound weather systems. Broecker has written, "If you wanted to cool the planet by 5°C [9°F] and could magically alter the water-vapor content of the atmosphere, a 30 percent decrease would do the job. Ours is now a brain able to anticipate outcomes well enough to practice ethical behavior, able to head off disasters in the making by extrapolating trends. Our goal must be to stabilize the climate in its favorable mode and ensure that enough equatorial heat continues to flow into the waters around Greenland and Norway. Any abrupt switch in climate would also disrupt food-supply routes. Up to this point in the story none of the broad conclusions is particularly speculative. Sudden onset, sudden recovery—this is why I use the word "flip-flop" to describe these climate changes.
Like bus routes or conveyor belts, ocean currents must have a return loop. We might create a rain shadow, seeding clouds so that they dropped their unsalted water well upwind of a given year's critical flushing sites—a strategy that might be particularly important in view of the increased rainfall expected from global warming. This produces a heat bonus of perhaps 30 percent beyond the heat provided by direct sunlight to these seas, accounting for the mild winters downwind, in northern Europe.