Then the Isthmus of Panama formed, so that all the warm Atlantic water that used to flow into the Pacific began pushing north, strengthening what we now call the Gulf Stream. Wouldn't you think that sending more tropical water north would warm up the Arctic?
But no. Paradoxically, the Arctic developed glaciers. The Ice Age was launched_and possibly triggered the evolution of Homo sapiens, according to Hopkins geologist Steven Stanley. Stanley also thinks he knows why the Ice Age began when it did, despite--or because of?--the Isthmus of Panama.
What do you think? Before you read on, you might care to ponder. The captions and art below contain the key facts.
The puzzle of what triggered the Ice Age is one that geological theorists have been gnawing on for decades, with no satisfactory answer. "We were shrugging our shoulders," says Stanley.
A few surmised that maybe the enhanced Gulf Stream worked like a hot tub on a cold day, sending more moisture into the atmosphere. The moisture would come down as snow, which would turn into glaciers. Well, maybe. Stanley was unconvinced because, as he points out, "We are still in the Ice Age. The Greenland glacier is still there. So we have to think of glaciers as symptoms, not a cause."
Most speculations boiled down to the notion that something, some mysterious something, must have shifted in the global "greenhouse," the envelope of gas that holds in Earth's warmth. So the Ice Age would be a case of greenhouse cooling.
And there the matter lay, at least as far as Steve Stanley was concerned. He was busy thinking about mollusks and punctuated evolution.
Last spring, however, the greenhouse explanation went out the window when Stanley heard Maureen Raymo of M.I.T. speak at a scholarly conference. Raymo had calculated CO2 levels in the Pliocene's oceans, and therefore in its atmosphere, from plankton skeletons layered in deep ocean sediment. She had found that, in fact, atmospheric levels of CO2--i.e., of greenhouse gas--were not significantly different before the Ice Age began.
"I was riveted," says Stanley. "And that's what got me thinking." What keeps Earth cool now? he asked himself--and his mind's eye fell on the Arctic Ocean, the water at the top of the world.
Compared with air, water has a very high capacity to absorb and store heat, Stanley explains. "It takes a whole lot of heat to heat water, compared with the amount it takes to heat air," and the same with draining heat away. Water temperatures have much more impact on air temperatures than vice versa.
That being so, Stanley thought the Arctic Ocean "the likely control of what's going on today." The location is right, and it is impeccably icy.
Yet before the Ice Age, the Arctic was warmer. "So you say, if we know the mechanism that is keeping it cold today, and if we can show it was set in place at the time the Ice Age began, then we have a potential explanation for the Ice Age.... Well, what is holding the cold in place is this big conveyor belt in the [Atlantic]."
The top of the conveyor belt is the Gulf Stream, ferrying heat northeastward. You can think of it as a river on the surface of the sea, so well-defined at the start that you can see it, muddy green against the ocean's navy. It is bordered with swirling clumps of seaweed and debris, and its warmth is palpable.
But the closer this warm water comes to the Arctic, the more it spreads out and cools, and the more it cools the denser and heavier it becomes. Finally it sinks, due west of Norway, which starts the conveyor belt on its way south. It goes all the way past the tip of Africa, flowing on the chill plateau of the deep sea waters.
"So those warm Atlantic waters from the Gulf Stream never get to the Arctic," says Stanley. "The sinking deprives the Arctic Ocean of the warmth of the Gulf Stream." He argues that's why the Arctic Ocean stays icy all year round, making the Arctic a self-perpetuating natural refrigerator for the northern half of the globe.
Now, here's the crux of the matter: the waters in the northern Pacific, up where Alaska meets Siberia, get just as cold as the North Atlantic--but little water sinks. The Pacific has no conveyor belt, because it is less dense, less salty: 32 parts per thousand in the Bering Sea, vs. some 35 ppt where the Atlantic chills and sinks.
Portentous question: Does the Atlantic's high salt content have anything to do with the Isthmus of Panama?
Yes, it does. What happens today is that the equatorial trade winds, being dry, suck up water from the Atlantic. (That's pure water: vapor can't carry salt any more than steam from a pot can carry rice.) Then, says Stanley, where the trade winds "cross the Isthmus of Panama, they dump a lot of that fresh water as rain in the Pacific." The Atlantic becomes more concentrated--more salty. Water in the Caribbean has a salinity of 36.5 ppt, as opposed to 34 ppt on the Pacific side of the Isthmus.
Before the Isthmus arose, contrariwise, the trade winds kept pushing Atlantic water west, through the gap between North and South America, so the oceans mixed and the Atlantic did not accumulate extra salt. And at a lower salinity, even a weakish Gulf Stream could get north to warm the Arctic Ocean.
When you put the whole picture together, it almost sounds like a nursery rhyme: Once the Isthmus arose, the Atlantic was saltier. The saltier the Atlantic, the sooner the Gulf Stream waters sank. The sooner the sinking, the less warmth reached the Arctic Ocean. The less warmth reached the Ocean, the colder it grew. The colder the Ocean, the colder the air, and glaciers formed. So we live in the Ice Age that salt built--about 2 to 3 parts per thousand of salt, to be precise.
Case complete, Stanley believes, and fossils from the seabed tell the same story. Creatures of the temperate zone migrated through the Arctic Ocean as of 3.5 Ma (million years ago). Then came a dramatic cooling, reaching down to the latitude of the English Channel just before 3 Ma. The deep ocean conveyor belt developed in about 3.1 Ma, pushing drifts of sediment off the tip of Greenland. Even the growing saltiness of the Caribbean can be dated, from a marked change in the bottom-living foraminifera just before 3 Ma.
All of which correlates nicely with the rising of the Isthmus of Panama. This event took place between 3.5 and 3 Ma, judging from the arrival of nearshore life forms.
Stanley's theory was published in the November issue of the Journal of Paleontology; it was his address as the outgoing president of the Paleontological Society.
So far, his colleagues' reactions are mixed, while he himself has two further thoughts. One, he finds himself musing over his earlier idea that the Ice Age had triggered the evolution of Homo sapiens by shrinking the tropical rain forests. Africa dried out, turning forest into savannah and forcing Australopithecus out into the open.
The challenge put a premium on protohuman resource and sagacity, no doubt, but more than that: As ground-dwellers, Australopithecus babies no longer needed to be born mature enough to hang on while mom swung through the trees. Their moms could carry them--making possible a long infancy and the development of Homo's large brain.
At least, so Stanley speculates, and he finds it strange. "People think our genus was inevitable," he says. "But it was dictated by plate tectonics [producing the Isthmus] in one little part of the world, far across the ocean from where the genus Homo evolved. It's kind of mind-boggling."
Stanley also wonders about implications for the globe's future climate. "In general," he says, "it illustrates how fragile the whole situation is. If this tiny neck of land could rise up and have such a tremendous effect, if a few parts per thousand of salt could matter so much... you have to wonder what other, even smaller changes" will do. "If we are creating significant global warming, there could be some threshold, some feature of the Earth's surface or the oceans... Some threshold could be reached that pushes us through to a new domain."
Elise Hancock is the senior editor of this magazine.
In the Pliocene four million years ago, the Northern Hemisphere basked in relative warmth. There were no large glaciers in the Arctic, no perennial northern cold to send down wintry blasts. The Arctic Ocean was warmed by Gulf Stream waters and ringed by trees, while temperatures off the Virginia coast were subtropical.
The tropics were much as they are today, sun-warmed to temperatures ranging upward from 80øF and gusted by the trade winds. These hot, arid winds are driven ever westward by the spinning of the Earth, and in the Pliocene they pushed a powerful warm current all the way around the globe. They also pick up moisture from the oceans and carry it west.
The subtropics of the Pliocene were somewhat different from today. For example, the Sahara was smaller. North and south of the African rainforest lay broad expanses of woodland, where humanity's ancestor, Australopithecus, made his home.
The Americas were not yet joined.
Today, in the modern Ice Age, not only have temperatures in the Arctic dropped perhaps 15 to 20øF, but the climate of the Northern Hemisphere is more extreme. Montana, for instance, can range from 90 to -40øF. Florida gets frost.
What keeps it that way is the cold of the Arctic region, carried south by prevailing winds. Each winter, as the sun moves south, the Pole is left to the dark and chill of space. Air temperatures drop as low as -90øF in Siberia, and the chill can be felt all the way to Chattanooga. Even when the sun returns north, today's Arctic never fully thaws.
Ocean currents, too, have altered since the Pliocene. In the Atlantic, the Gulf Stream still pushes tropical water north, and the Coriolis effect still gyres all that sun-warm water clockwise, toward Europe and points north. But now it never reaches the Arctic Ocean. Instead, just due west of Norway, the water cools and sinks. It forms a subsurface conveyor belt that ferries icy water all the way to the Southern Hemisphere.
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