The poles or the tropics? Who leads in the climatic dance?
As we have seen, researchers into the global history of climate, especially in the U.S., divide into two camps. One believes that the key drivers for past, and therefore probably future, climate change lie in the polar regions, especially the far North Atlantic. The other believes that the real action happens in the tropics.
The most outspoken advocate for the polar school is Wally Broecker, of Lamont-Doherty. As described in Chapter 23, he is the man behind the idea of the ocean conveyor, which begins in the far North Atlantic and which, he argues, is the great climatic amplifier. It has, he says, a simple on-off switch. It pushed the world into and out of ice ages; it modulates the effects of Bond's solar pulse, including its most recent manifestations in the medieval warm period and the little ice age; and it could be a big player in directing the consequences of global warming. Around Broecker is a whole school of researchers who have spent their careers investigating the dramatic climate events of the North Atlantic region, as recorded in the ice cores of Greenland.
The rival, tropical school has often looked to two characters. One, just down the corridor from Broecker at Lamont-Doherty, is Mark Cane, a leading modeler of El Nino, the biggest climate fluctuation in the tropics. The other is Lonnie Thompson, the man who decided thirty years ago to stop investigating polar ice cores and switch instead to drilling tropical glaciers. They argue that Broecker's ocean conveyor is at best a sideshow, relevant to the North Atlantic and the countries that border it, but not the great global amplifier it is claimed to be. For them, the important climatic levers must be in the planetary heat and hydrological engines around Earth's girth. The debate between the two schools has, at various stages, become quite personal. "It all came from one man: Wally Broecker," says Cane. "You were for him or against him. And I found myself against."
The polar people deploy their polar ice core data to show that climate change has been more dramatic and sudden in the far North, so that must be the cockpit of climate change. This is where the Gulf Stream turns turtle and drives the ocean conveyor; this is where ice melting and changes in freshwater flow can freeze the ocean virtually overnight and send temperatures tumbling by tens of degrees; this, above all, is where the great ice sheets of the ice ages formed and died. They have a point. There can be little doubt about the importance of ice formation to the ice ages. Virtually the whole world cooled then, and two thirds of that cooling was caused by the feedback of growing ice sheets and their ability to reflect solar radiation back into space. And nothing except a huge rush of meltwater from the receding ice caps could have plunged the world into the Younger Dryas, 12,800 years ago.
But that doesn't mean that the Arctic tells the whole story. What pulled the world out of the Younger Dryas, for instance—an event that happened even faster than its onset? And while big climate change during and at the close of the ice ages does seem to be associated with polar events, the evidence concerning climate change since is far less secure. Thompson argues that most of the global climatic shudders of the Holocene, such as events 5,500 and 4,200 years ago, must have been tropical in origin: "In climate models, you can only make such things happen in both the Northern and Southern Hemispheres by forcing events from the tropics, and I am convinced that is what is happening."
Hockey-stick author Mike Mann, though not a fully paid-up member of the tropical school, says: "I increasingly think that the tropical Pacific is the key player. When you see La Nina dominating the medieval warm period and El Nino taking hold in the little ice age, it begins to look like the tropics, rather than the North Atlantic, rule." The argument is that heat flows from the tropics are the true intermediaries between Bond's solar pulse and temperature fluctuations in the North Atlantic.
The tropical school also accuses the polar fraternity of being blinkered about what constitutes climate change. Besides overly focusing on events in North America and Europe, it stands accused of being overly concerned with temperature. In the tropics, the hydrological cycle matters more than the temperature. Megadroughts are as damaging as little ice ages, and the rains, rather than extra warmth, bring plenty. Witness the drying of the Sahara 5,500 years ago, and the importance of the vagaries of the Asian monsoon.
The tropical school doesn't stop there. Its adherents argue that many of the big climatic events in the Northern polar regions have their origins in the tropics. The tropics, by delivering warm water into the North Atlantic, are just as capable of flipping the switch of the ocean conveyor as is ice formation in the far North Atlantic. And if there is a tropical equivalent of Broecker's switch in the North Atlantic, they say, it is probably the warm water pool around Indonesia—an area they often call "the firebox." This is the greatest store and distribution point for heat on the surface of the planet, with a known propensity for threshold changes via the El Nino system. It is also the biggest generator of water vapor for the atmosphere, which is both a potent greenhouse gas and a driver of weather systems.
If this region can trigger short-term El Ninos that warm the whole planet, and La Ninas that cool it again, then might it not also trigger long-term climate changes? Might not events here have been important in turning a minor orbital wobble into the waxing and waning of the ice ages? The waning, certainly. For cores of ocean sediment recently taken from the tropical Pacific suggest that temperatures started to rise there a thousand or more years before the Northern ice sheets began to shrink.
But after some years of standoff, many protagonists in this debate are now seeking common ground. Not Broecker, of course. But Richard Alley, a polar man but also a fan of Thompson's, now thinks that the location of the climate system driver's seat may change with time. It is easy to imagine the power of ice and meltwater to hijack the world's climate during the glaciations, when a third of the Northern Hemisphere was covered with ice. But with less ice around in the interglacials, he concedes, the argument is less persuasive. And, with characteristic pithiness, he admits to past regional bias. "Suppose the North Atlantic circulation did shut down. Sure, Europe would care. They might have a midseason break in football in Britain.
Manchester United wouldn't be playing on Boxing Day. But in the Great Plains of the U.S. and in the Pacific Ocean, would it be so important?"
Meanwhile, on the tropical side, Cane admits: "I am less absolutist than I used to be." He agrees that his great enthusiasms, El Nino and the tropical Pacific, might not be behind everything. He still believes that the role of the ocean conveyor is hopelessly hyped, but he concedes the possible importance of the "sink or freeze" switch for sea ice in the North Atlantic. The divide between the polar and tropical schools is "a slightly false separation," says Peter deMenocal, of Lamont-Doherty. "You cannot at the end of the day change one bit without changing the other. They are all part of the same pattern, whether leading or following." Earth functions as an integrated system, not as a series of discrete levers.
That view seems to be confirmed by Steve Goldstein, of Columbia University, who has used analysis of a rare earth called neodymium, which has different isotopic ratios in different oceans, to reconstruct the order of events at the starts and ends of the ice ages. He argues that orbital changes, as expected, lead events. But the first feedback to respond is the ice-albedo feedback. It caused an initial cooling at the start of the last ice age that was most pronounced in the far North. Prompted by that initial cooling, the chemistry and biology of the oceans started to change, removing carbon dioxide from the atmosphere and accentuating the cooling further. Only then, some thousands of years later, did the ocean conveyor start to shut down. "The conveyor follows; it does not lead," he says. If his analysis is confirmed, it will be a blow to Broecker, but it will also confirm that both the tropics and the polar regions were deeply implicated in the elaborate dance that took the world into and out of the ice ages.
Paul Crutzen has been in the forefront of research in both spheres, helping crack the mysteries of the Antarctic ozone layer while making a strong case for the dynamic properties of the tropical heat engine. "Big planetary changes happen in both the tropics and the very high latitudes," he says. "The tropics are where the high temperatures drive a lot of the chemistry and dynamics of the atmosphere. And the polar regions are the homes of the big natural feedbacks that could accelerate climate change: things like melting ice and permafrost and alterations to ocean currents." That is probably as good a compromise statement as can be found right now. At the end of the day, the system is bigger than the individual parts.
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