Twice a year they sink, carrying their load of atmospheric gases downward. Meaning of 3 sheets to the wind. Whole sections of a glacier, lifted up by the tides, may snap off at the "hinge" and become icebergs. Europe is an anomaly. Surprisingly, it may prove possible to prevent flip-flops in the climate—even by means of low-tech schemes. Change arising from some sources, such as volcanic eruptions, can be abrupt—but the climate doesn't flip back just as quickly centuries later.
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. A muddle-through scenario assumes that we would mobilize our scientific and technological resources well in advance of any abrupt cooling problem, but that the solution wouldn't be simple. I call the colder one the "low state. " 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. This salty waterfall is more like thirty Amazon Rivers combined. The expression three sheets to the wind. Europe's climate could become more like Siberia's. Ancient lakes near the Pacific coast of the United States, it turned out, show a shift to cold-weather plant species at roughly the time when the Younger Dryas was changing German pine forests into scrublands like those of modern Siberia. These northern ice sheets were as high as Greenland's mountains, obstacles sufficient to force the jet stream to make a detour. "Southerly" Rome lies near the same latitude, 42°N, as "northerly" Chicago—and the most northerly major city in Asia is Beijing, near 40°.
A gentle pull on a trigger may be ineffective, but there comes a pressure that will suddenly fire the gun. Water is densest at about 39°F (a typical refrigerator setting—anything that you take out of the refrigerator, whether you place it on the kitchen counter or move it to the freezer, is going to expand a little). Water falling as snow on Greenland carries an isotopic "fingerprint" of what the temperature was like en route. We could go back to ice-age temperatures within a decade—and judging from recent discoveries, an abrupt cooling could be triggered by our current global-warming trend. What is 3 sheets to the wind. A lake formed, rising higher and higher—up to the height of an eight-story building. The job is done by warm water flowing north from the tropics, as the eastbound Gulf Stream merges into the North Atlantic Current. Greenland looks like that, even on a cloudless day—but the great white mass between the occasional punctuations is an ice sheet. The fjords of Greenland offer some dramatic examples of the possibilities for freshwater floods.
The same thing happens in the Labrador Sea between Canada and the southern tip of Greenland. When that annual flushing fails for some years, the conveyor belt stops moving and so heat stops flowing so far north—and apparently we're popped back into the low state. The scale of the response will be far beyond the bounds of regulation—more like when excess warming triggers fire extinguishers in the ceiling, ruining the contents of the room while cooling them down. There is, increasingly, international cooperation in response to catastrophe—but no country is going to be able to rely on a stored agricultural surplus for even a year, and any country will be reluctant to give away part of its surplus. 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. Nothing like this happens in the Pacific Ocean, but the Pacific is nonetheless affected, because the sink in the Nordic Seas is part of a vast worldwide salt-conveyor belt.
Though some abrupt coolings are likely to have been associated with events in the Canadian ice sheet, the abrupt cooling in the previous warm period, 122, 000 years ago, which has now been detected even in the tropics, shows that flips are not restricted to icy periods; they can also interrupt warm periods like the present one. They even show the flips. There seems to be no way of escaping the conclusion that global climate flips occur frequently and abruptly. If Europe had weather like Canada's, it could feed only one out of twenty-three present-day Europeans. Fjords are long, narrow canyons, little arms of the sea reaching many miles inland; they were carved by great glaciers when the sea level was lower. It's also clear that sufficient global warming could trigger an abrupt cooling in at least two ways—by increasing high-latitude rainfall or by melting Greenland's ice, both of which could put enough fresh water into the ocean surface to suppress flushing. Retained heat eventually melts the ice, in a cycle that recurs about every five years. 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. We need heat in the right places, such as the Greenland Sea, and not in others right next door, such as Greenland itself. 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. Although I don't consider this scenario to be the most likely one, it is possible that solutions could turn out to be cheap and easy, and that another abrupt cooling isn't inevitable. When there has been a lot of evaporation, surface waters are saltier than usual. By 125, 000 years ago Homo sapienshad evolved from our ancestor species—so the whiplash climate changes of the last ice age affected people much like us. 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.
But the ice ages aren't what they used to be. In late winter the heavy surface waters sink en masse. 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. Thermostats tend to activate heating or cooling mechanisms abruptly—also an example of a system that pushes back. By 1971-1972 the semi-salty blob was off Newfoundland. Within the ice sheets of Greenland are annual layers that provide a record of the gases present in the atmosphere and indicate the changes in air temperature over the past 250, 000 years—the period of the last two major ice ages. Oslo is nearly at 60°N, as are Stockholm, Helsinki, and St. Petersburg; continue due east and you'll encounter Anchorage.
But we can't assume that anything like this will counteract our longer-term flurry of carbon-dioxide emissions. 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.