Probing the hidden life of El Nino
Some researchers have a way of combining business with pleasure. Not for Dan Schrag, the Harvard geochemist, the arduous journeys into thin, cold air on tropical glaciers. Back in 1997, he was on his fourth trip to the paradise islands of the East Indies in search of ancient coral. One day, he was sauntering along a beach on Bunaken Island, a speck of old atoll off the Indonesian island of Sulawesi. "We had had a glorious dive, during which we saw a huge school of barracuda," he remembers. "We stopped for lunch, and I took a walk down the beach, behind the mangrove swamp. It was the last day of the trip. We had failed to find anything useful, and I was preparing to go home. Then I saw this massive coral head on the beach, incredibly well preserved." He chiseled out a piece and headed for the plane.
Back in the lab at Harvard, Schrag discovered that this fossilized piece of coral was 125,000 years old and contained sixty-five years' worth of growth rings that gave a brief window on the climate of the western Pacific back before the last ice age. It was a "fantastic discovery," he says. "I guess I got really, really lucky." The coral he had found was the first piece ever located that was large enough and well enough preserved to give a good snapshot of ancient El Ninos. What's more, says Schrag, it came from a region that is in the "bull's-eye" of El Nino, in the heart of Indonesia. His preprandial discovery is helping transform our understanding of El Nino's place in the climate system.
Until recently, climatologists looked on El Nino as a minor aberration in the tropical Pacific, of only passing interest to the wider world. But in the past two decades it has become the fifth horseman of the Apocalypse, a bringer of devastating floods, fires, and famine from Ethiopia to Indonesia to Ecuador, and a sender of weird weather around the world. It has been appearing more frequently, and with effects that are much more violent and last longer. Its current level of activity is unparalleled in the historical record. Yet the historical record doesn't go back far, so nobody has been sure whether this is a perfectly normal upturn or an alarming consequence of global climate change. Schrag's coral has helped provide some answers. It makes a strong case that global warming is already having a profound effect on what climatologists are coming to regard as the flywheel of the world's climate.
El Nino is a periodic reversal of ocean currents, winds, and weather systems that stretches across the equatorial Pacific Ocean, halfway around the planet at its widest girth. It is a redistributor of heat and energy in the hottest part of the world's oceans, which kicks in when the regular circulation systems can no longer cope. In normal times, the winds and surface waters of the tropical Pacific, driven by Earth's rotation, flow from the Americas in the East to Indonesia in the West. In the tropical heat, the water warms as it goes. The result is the gradual accumulation of a pool of hot water on the ocean surface around Indonesia. This pool can be up to I3°F warmer than the water on the other side of the ocean, and can contain more heat energy than the entire atmosphere. All that heat generates storm clouds that keep the rainforests of Southeast Asia wet.
But the constant flow to the west also piles up water. Trapped against the Indonesian archipelago, the warm pool can rise as much as 15 inches above sea levels farther east. Clearly, this state of affairs cannot last. And every few years, usually when the winds slacken, this raised pool of warm water breaks out and flows back across the surface of the ocean, right along the equator. As the warm water moves east, the wind and weather systems that it creates follow.
Deprived of their storm-generating weather systems, Indonesia and a wide area of the western Pacific, including much of Australia, dry out. There are forest and bush fires, and crops shrivel in the fields. Meanwhile, the displaced wet and stormy rainforest climate drenches normally arid Pacific islands, and often reaches the coastal deserts of the Americas. Ripples from this vast movement of heat and moisture spread around the globe. They move west through the Indian Ocean, disrupting the Indian monsoon and causing rains or drought in Africa, depending on the season. They move east. Beyond the flooding on the Pacific shores of the Americas, El Nino brings drought in the Amazon rainforest. Its hidden hand alters flow down the River Nile, triggers rains in the hills of Palestine, and damps down hurricane formation in the North Atlantic.
Typically, an individual El Nino event lasts twelve to eighteen months. After it has abated, the system often goes into sharp reverse, with exceptionally wet conditions in Indonesia and fierce drought further east. This is called La Nina. Together, El Nino and its sister constitute a vast oscillation of ocean and atmosphere that in recent times has been the most intense fluctuation in the world's climate system.
Scientists first became aware of the oscillation we now call El Nino in the nineteenth century. But they have been uncertain about how far back El Nino goes. Is it a permanent feature of the climate system, or a minor and occasional aberration? Does it have long-term variability tied to global climate changes? Does the Pacific get "stuck" in either a permanent El Nino or a permanent La Nina?
Reliable climate and ocean records cover only a couple of centuries or so. Delving further requires alternative sources of information. To this end, Donald Rodbell, of Union College, in Schenectady, New York, dug up the bed of a lake in southern Ecuador to chart its past flood levels, in the expectation that, as today, floods would be a feature of El Nino episodes. In 1998, he published a remarkable 12,000-year record of the lake's floods. For the first half of the period, they came roughly once every fifteen years, suggesting a near-dormant El Nino. Then they speeded up quite abruptly, to settle at an average return period of about six years—the classic El Nino pattern until recently. This pattern has been confirmed by Lonnie Thompson's ice cores from nearby glaciers.
The change in the flood pattern also seems to coincide with the same precession shift in Earth's tilt that caused the desertification of the Sahara and the advance of tropical glaciers spotted by Thompson. Rodbell's record was a major breakthrough, implicating El Nino as a key driver of the global climate system. El Nino was no longer just a short-term cycle played out over a few months in one ocean: it had global and long-term meaning. Then came Schrag's chunk of coral.
Through isotopic analysis, Schrag extracted an El Nino signal from his piece of jetsam. When water evaporates, molecules containing the lighter isotope of oxygen—oxygen-16—evaporate slightly faster, leaving behind seawater that is rich in the heavier oxygen-18. When it rains, the oxygen-16 is returned. So in the Indonesian islands during El Ninos, when rainfall ceases, both the seawater and the coral growth in those years contain more oxygen-18. Schrag measured the ratio of the two oxygen isotopes in the sixty-five annual growth rings in his ancient chunk of coral. He found two things of importance: First, there had indeed been an El Nino cycle back then. That pushed the longevity of the phenomenon back to before the last ice age, further establishing it as a permanent feature of the climate system. And second, the El Nino cycle looked exactly like that of the modern period from the mid- 1800s to the mid-1970s, in which El Nino returned, on average, about every six years. This underlined the idea that six years is the natural length of the cycle—and made the post-1976 period, during which El Nino has developed a return period averaging 3.5 years, appear increasingly unusual.
This sense that El Nino may have changed in some fundamental way in the past thirty years has been reinforced by another change. The earliest records of the El Nino phenomenon are from the Pacific shores of South America, where a cold ocean current normally works its way north, bringing waters rich in nutrients that sustain one of the world's largest fisheries, off Peru. But during El Ninos, the flood of warm water from the west overrides this cold current for a while, and the fish disappear. That has been the classic pattern. But since 1976, the underlying state has changed. The cold current has been pushed to ever-greater depths, even during normal times. The ocean system appears to have become stuck in a quasi-El Nifio state.
What lies behind these recent changes? Some say that El Nino is simply on a short-lived, exuberant joyride. They point out that there have always been decades when it is unusually quiet or busy or just plain weird. But Schrag thinks this is unlikely to explain recent events. Publishing his Sulawesi findings, he said: "In 1982—83 we experience the most severe El Nino of the 20th century. According to previous records you wouldn't expect another that powerful for a hundred years. But 15 years later, in 1997-98, we have one even larger." And since then, in 2002 and 2004, there have been two more significant El Ninos—not as large as those before, but turning up with ever-greater frequency.
Kevin Trenberth, the head of climate analysis at NCAR, was one of the first researchers to claim that the Pacific entered an unusual state after 1976. He believes that the recent spate of strong and frequent El Ninos could well be due to the hand of man. It looks as if global warming, which gathered real pace only in the 1970s, is generating so much warming in the tropical Pacific that the old flywheel pattern in which occasional El Ninos distribute the heat that accumulates around Indonesia is not sufficient to handle the amount of energy in the system.
Modelers have been testing this theory, with interesting results. Mojib Latif, at the Max Planck Institute for Meteorology, in Hamburg, developed the first global climate model that was detailed enough to reproduce El Nino. His model predicts that the average climate in the twenty-first century will be more like the typical El Nino conditions of the twentieth century. Cold La
Nina events will still happen occasionally, and may even be more intense. But they will become the breakout events.
It would be wrong to suggest that science has somehow cracked the enigma of El Nino. There are still many mysteries. Certainly the idea that a strong El Nino is necessarily associated with warm times could be a gross simplification. Schrag's coral, along with other evidence, suggests that El Nino kept going right through the last ice age, when, even in the western Pacific, temperatures were several degrees lower than they are today. There is even some suggestion that El Ninos were more common in the colder phases of the ice age, whereas La Nina held sway during the warmer periods. Likewise, the warm early Holocene era, before 6,000 years ago, saw El Nino largely in abeyance. It recovered during a cooler period.
Clearly, El Nino is not a simple planetary thermostat. But its operation in the past may have had more to do with changes in solar radiation that were reflected in alterations to the tropical hydro logical cycle than with temperature. It is possible to imagine a climate system in which those changes triggered different temperature signals at different times. So efforts to tie past El Ninos to temperature trends may not provide a good guide to what happens in a world of pumped-up greenhouse gas concentrations.
But what is becoming clear is that El Nino is a phenomenon that influences basic planetary processes such as the transfer of heat and moisture in huge swaths of the tropics. That it has big swings that operate on timescales varying from months to thousands of years. That it leaves its calling card in different ways right around the planet. And that its variability seems to be keyed into critical external drivers of past climate such as the precession and Bond's 1,500-year solar cycles. You would not bet against its playing an equally important role in moderating or amplifying global warming caused by greenhouse gases. What is not yet clear is which way it will jump.
Perhaps scientists should put aside their models and search for some wisdom on El Nino from Peruvian farmers, who have grown potatoes high in the Andes for thousands of years. Throughout that time, El Ninos have become stronger and weaker, more frequent and less frequent, and have influenced potato growing all the while. For many centuries now, farmers have gathered in mid-June (the Southern winter) to gaze up at the night sky in the Andes and observe the eleven-star constellation known as Pleiades, or the Seven Sisters. If the stars are bright, they set to planting quickly, confident that there will be good rains and a healthy harvest.
For years this folklore was dismissed by agriculturalists as mum bo-jumbo—until Mark Cane, one of the world's foremost modelers of El Nino, heard about it from a guide while traveling in the Andes. Intrigued, he checked meteorological records, and discovered that typically about six months before an El Nino, thin, high, and almost invisible clouds form above the Andes. These dim the brightness of the constellation. So a dim constellation means a dry spring, while clear skies and a bright constellation mean good rains.
The farmers had thus perfected many hundreds of years ago what climate modelers like Cane have only fitfully managed in the past twenty years—a way of forecasting El Nino. Cane says the Peruvian potato farmers' forecast is better than his. "It's a brilliant scheme, really quite a feat. I still wonder how they possibly worked it out." Perhaps, he muses, the Peruvian potato farmers have had the key to the world's climate all along.
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