Feedbacks are what determine thresholds, where one mode flips into another. It's happening right now:a North Atlantic Oscillation started in 1996. 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. All we would need to do is open a channel through the ice dam with explosives before dangerous levels of water built up. Indeed, were another climate flip to begin next year, we'd probably complain first about the drought, along with unusually cold winters in Europe. The sheet in 3 sheets to the wind crossword. If Europe had weather like Canada's, it could feed only one out of twenty-three present-day Europeans. Many ice sheets had already half melted, dumping a lot of fresh water into the ocean.
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. Computer models might not yet be able to predict what will happen if we tamper with downwelling sites, but this problem doesn't seem insoluble. Define 3 sheets to the wind. Coring old lake beds and examining the types of pollen trapped in sediment layers led to the discovery, early in the twentieth century, of the Younger Dryas. An abrupt cooling got started 8, 200 years ago, but it aborted within a century, and the temperature changes since then have been gradual in comparison. Implementing it might cost no more, in relative terms, than building a medieval cathedral. This tends to stagger the imagination, immediately conjuring up visions of terraforming on a science-fiction scale—and so we shake our heads and say, "Better to fight global warming by consuming less, " and so forth. These northern ice sheets were as high as Greenland's mountains, obstacles sufficient to force the jet stream to make a detour.
By 250, 000 years ago Homo erectushad died out, after a run of almost two million years. The North Atlantic Current is certainly something big, with the flow of about a hundred Amazon Rivers. A brief, large flood of fresh water might nudge us toward an abrupt cooling even if the dilution were insignificant when averaged over time. This El Niño-like shift in the atmospheric-circulation pattern over the North Atlantic, from the Azores to Greenland, often lasts a decade. They were formerly thought to be very gradual, with both air temperature and ice sheets changing in a slow, 100, 000-year cycle tied to changes in the earth's orbit around the sun. Water that evaporates leaves its salt behind; the resulting saltier water is heavier and thus sinks. But our current warm-up, which started about 15, 000 years ago, began abruptly, with the temperature rising sharply while most of the ice was still present. Three sheets to the wind synonym. Further investigation might lead to revisions in such mechanistic explanations, but the result of adding fresh water to the ocean surface is pretty standard physics. 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). 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.
The modern world is full of objects and systems that exhibit "bistable" modes, with thresholds for flipping. There used to be a tropical shortcut, an express route from Atlantic to Pacific, but continental drift connected North America to South America about three million years ago, damming up the easy route for disposing of excess salt. These carry the North Atlantic's excess salt southward from the bottom of the Atlantic, around the tip of Africa, through the Indian Ocean, and up around the Pacific Ocean. In discussing the ice ages there is a tendency to think of warm as good—and therefore of warming as better. Surface waters are flushed regularly, even in lakes. But we may be able to do something to delay an abrupt cooling. Those who will not reason. The discovery of abrupt climate changes has been spread out over the past fifteen years, and is well known to readers of major scientific journals such as Scienceand abruptness data are convincing.
Seawater is more complicated, because salt content also helps to determine whether water floats or sinks. Europe is an anomaly. Three scenarios for the next climatic phase might be called population crash, cheap fix, and muddling through. But sometimes a glacial surge will act like an avalanche that blocks a road, as happened when Alaska's Hubbard glacier surged into the Russell fjord in May of 1986. The cold, dry winds blowing eastward off Canada evaporate the surface waters of the North Atlantic Current, and leave behind all their salt. At the same time that the Labrador Sea gets a lessening of the strong winds that aid salt sinking, Europe gets particularly cold winters. 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. But we may not have centuries for acquiring wisdom, and it would be wise to compress our learning into the years immediately ahead. In late winter the heavy surface waters sink en masse. 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. Paleoclimatic records reveal that any notion we may once have had that the climate will remain the same unless pollution changes it is wishful thinking. 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. A remarkable amount of specious reasoning is often encountered when we contemplate reducing carbon-dioxide emissions.
Up to this point in the story none of the broad conclusions is particularly speculative. 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. This major change in ocean circulation, along with a climate that had already been slowly cooling for millions of years, led not only to ice accumulation most of the time but also to climatic instability, with flips every few thousand years or so. 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.
For a quarter century global-warming theorists have predicted that climate creep is going to occur and that we need to prevent greenhouse gases from warming things up, thereby raising the sea level, destroying habitats, intensifying storms, and forcing agricultural rearrangements. The last time an abrupt cooling occurred was in the midst of global warming. Glaciers pushing out into the ocean usually break off in chunks. We must look at arriving sunlight and departing light and heat, not merely regional shifts on earth, to account for changes in the temperature balance. A nice little Amazon-sized waterfall flows over the ridge that connects Spain with Morocco, 800 feet below the surface of the strait. Light switches abruptly change mode when nudged hard enough. Perhaps computer simulations will tell us that the only robust solutions are those that re-create the ocean currents of three million years ago, before the Isthmus of Panama closed off the express route for excess-salt disposal.
Like a half-beaten cake mix, with strands of egg still visible, the ocean has a lot of blobs and streams within it. This cold period, known as the Younger Dryas, is named for the pollen of a tundra flower that turned up in a lake bed in Denmark when it shouldn't have. 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. That, in turn, makes the air drier. In Broecker's view, failures of salt flushing cause a worldwide rearrangement of ocean currents, resulting in—and this is the speculative part—less evaporation from the tropics. 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. The same thing happens in the Labrador Sea between Canada and the southern tip of Greenland. The Atlantic would be even saltier if it didn't mix with the Pacific, in long, loopy currents.
Obviously, local failures can occur without catastrophe—it's a question of how often and how widespread the failures are—but the present state of decline is not very reassuring. 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. And in the absence of a flushing mechanism to sink cooled surface waters and send them southward in the Atlantic, additional warm waters do not flow as far north to replenish the supply.