-Caveat Lector- ----Original Message Follows---- From: Mark Graffis <[EMAIL PROTECTED]> To: undisclosed-recipients:; Subject: Global Warming, more ominous signs Date: Sun, 10 Oct 1999 13:39:48 -0500 (CDT) [EMAIL PROTECTED] ***** New Scientist 9 October 1999 page 34 Weather Warning! El Nino seems to be changing from a minor nuisance to a climate monster. A chance find on a tropical beach suggests there's worse to come. Fred Pearce Some guys have all the luck. Two years ago Dam Schrag was in Indonesia diving for fun when he made his discovery. "We had a glorious day," he says. "After a morning dive when we saw a huge school of barracuda, we stopped for lunch. I took a walk down the beach, behind the mangrove swamp, and saw a massive coral head, incredibly well preserved. " He chiselled a piece out and headed for home. That fossilised coral, later dated at 125 000 years old, now looks as if it could transform our understanding of El Nino--the Pacific Ocean phenomenon that is the crucible of much of the world's climate. The find wasn't entirely serendipitous. Schrag, who is based at Harvard University' s Department of Earth and Planetary Sciences, had been looking for well-preserved fossil coral on four previous trips to Indonesia. The idea was to use growth rings in the coral to look for signs of El Nino in the distant past. Schrag' s earlier expeditions had yielded plenty of samples, but what he really needed was one with enough rings to record a series of El Ninos. He had been about to return home empty-handed yet again when he took his postprandial walk down the beach of Bunaken Island, a speck of old atoll off the Indonesian island of Sulawesi. "I guess I got really, really lucky," he says now. The coral he found was the first - and so far the only -piece located by researchers that is large enough and well enough preserved to give a good snapshot of ancient El Ninos. What's more, says Schrag, Sulawesi is the "bull's-eye" of El Nino. A record from there could tell us whether El Nino was different in the past, and how it might change in the future. It is an important issue. Until recently, climatologists looked on El Nino as an aberration in the tropical Pacific, of only passing interest to the outside 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 round the world. It has been appearing more frequently, with effects that last longer than ever. Its activity is unparalleled in the historical record. And yet nobody could be sure if this is a perfectly normal blip, or an alarming consequence of human-induced climate change. Schrag's coral could change all that. Along with other evidence now pouring in, it makes a strong case that the climate system is changing beyond all recognition. For climatologists, El Nino is the flywheel of the world's climate, a re-distributor of heat and energy that kicks in when the regular circulation systems cannot cope. In normal times, the winds and waters flow across the tropical Pacific from the Americas in the east to Indonesia in the west, driven by the Earth's rotation. In the tropical heat, the water warms as it goes. The result is the gradual accumulation of a pool of warm water around Indonesia that can be 40 centimetres higher and several degrees warmer than water on the other side of the ocean. This cannot last and, typically every three to seven years, this warm water breaks out and flows back across the surface of the ocean. As the pattern of ocean currents shifts, so do the wind and air pressure systems associated with it, and with them the weather. So the wet rainforest climate of Indonesia drenches the normally arid Pacific islands, and often reaches the coastal deserts of the Americas. Meanwhile, Indonesia and much of Australia dry out (see Map, p 39). But scientists have been uncertain about how far back El Nino goes. Reliable climate and ocean records cover only a century or so; delving further requires an alternative source of information. To this end, some researchers have been digging up the beds of old Andean lakes, in the expectation that they will bear the scars of the occasional El Nino-inspired floods that hit the normally arid region. In January, Donald Rodbell of Union College in Schenectady, New York, reported findings from a lake in southwest Ecuador, in which he dated sediments associated with occasional heavy flooding going back 15,000 years (Science, vol 283, p 516). For the first half of the period, the floods seemed to come only once every 15 years or less, he says. Then they speeded up quite abruptly to settle some 5000 years ago at an average return period of between two and Eight years -the classic El Nino pattern that has broadly held to the present. Some researchers interpret this as showing that El Nino started 6000 years ago. Others say that the 10 000 years before that were merely a quiet phase, caused perhaps by abnormal seasonal patterns which were in turn triggered by wobbles in the Earth's orbit. Others again say that the lake record may not be reliable, because local glaciers could have interfered with the El Nino "signal". Schrag's chunk of coral sidelines that debate by putting the date of the first recorded El Nino back by more than 100 000 years, to before the last ice age. In a paper due for publication shortly in Geophysical Research Letters, Schrag and his colleague Konrad Hughesh will reveal their analysis of the isotopic signature of the annual growth layers inside the Sulawesi coral, and use it to plot the pattern of the ancient El Ninos. When water evaporates, molecules containing the fighter isotope of oxygen - oxygen-16 -tend to evaporate slightly faster, leaving behind seawater that is enriched with oxygen-18. So in the Indonesian islands during El Ninos, when rainfall ceases and drought ravages the islands, both the seawater and the coral growing in those years contain more oxygen-18. By measuring the relative amount of oxygen-18 in his coral, Schrag has come up with a year-by-year El Nino record over the 65 years covered in its annual rings. According to Schrag, the pattern of El Nino events revealed in his 125 000-year old coral looks exactly like the modern period before 1976, but nothing like the post-1976 period. He has examined in detail the "return period" for El Ninos, both in the ancient coral and modern meteorological and coral records, and found that in the modern record prior to 1976 the dominant return period for El Nino was around six years. That was also the case in the 65-year time slice in his ancient coral. But the post-1976 record shows a peak return period at 3-5 years. The implication is that the cycling of El Nino was highly stable over hundreds of thousands of years, but has changed fundamentally in the past quarter-century. The crucial question is what lies behind this change. Has El Nino been disturbed by some external factor, such as global warming, or is it simply on a short-lived, exuberant joyride? Many oceanographers support the joyride theory. They point out that El Nino has always had decades when it is unusually quiet or busy or just plain w Elrd. Mark Cane of the Lamont-Doherty Earth Observatory at Columbia University in New York has compiled one of the most respected models of El Nino, one that has successfully predicted the onset of El Ninos. That model, he says, generates such fluctuations as part of the natural variability of El Nino, without introducing any outside element. Kevin Trenberth, head of climate analysis at the National Center for Atmospheric Research in Boulder, Colorado, thinks differently. Trenberth was one of the first researchers to spot the unusual state of the tropical Pacific after 1976, and he believes that the recent El Nino shenanigans could well be down to global warming. Global warming up to 1976 may have been modest enough for the "normal" climate system to cope with quite happily, he suggests. Only after the magnitude of the warming hit a threshold did it begin to trigger unusual effects in the El Ninos. One way to check this, says Schrag, is to look for signs of recent warming in the ocean. Together with Tom Guilderson from Lawrence Livermore National Laboratory in California, he has recently pointed out that the unique signature of the post-1976 El Ninos is down to a very specific warming of surface waters in the eastern Pacific during the cold season. Maximum sea temperatures in the area changed very little, but minima jumped from a typical 23.5 'C before 1976 to above 24.5 'C thereafter. This area of ocean is a constant battleground between warm waters at the surface and cold waters that well up from the deep. Most of the time the upwelling is dominant. But during El Ninos, when warm waters wash across the Pacific from the west, the upwelling is shut off. What seems to have happened is that this shutoff has become near permanent. In a paper in Science last year (vol 281, p 240), Guilderson and Schrag showed that since 1976 the thermocline--the boundary zone between surface and deep water that lies around 50 metres below the surface -has deepened by 10 metres or more. This dramatic change is reflected in a variation in the isotopic composition of the water. As the carbon-14 in seawater gradually decays, surface water can replenish its stocks from 'the atmosphere, but deep water cannot. So low levels of carbon-14 are an indicator of deep water. Since 1976, water at the surface in the eastern Pacific has been richer in carbon-14, showing that deep water is not welling up as much as before. Upwelling normally keeps the eastern Pacific cool, maintains the normal trade winds and so suppresses the outbreak of El Ninos. Reduce the upwelling and the system is permanently primed for an El Nino. Schrag concludes that the post-1976 change in the thermocline may be responsible for the increase in the frequency and intensity of El Nino events since then. Does any of this matter beyond the Pacific Ocean? As climatologists discover more and more about the workings of the oceans and atmosphere, they realise how central El Nino is to the functioning of the entire climate system. The Indian Ocean shows its own post-1976 shift. Analysis of weather statistics from the remote Chagos Archipelago by Charles Sheppard of the University of Warwick found that around the mid-1970s average air temperatures abruptly rose by a degree, while cloud cover shrank by 50 per cent. The islands' coral reefs, some of the largest and most pristine in the world, have been wrecked as a result. Catastrophic climate Is the shift in El Nino the long-sought "smoking gun" that will convict greenhouse gases of causing climatic mayhem ? Modelling studies provide a hint that it may be so. Mojib Latif and colleagues at the Max Planck Institute for Meteorology in Hamburg recently developed the first global climate model detailed enough to reproduce El Nino cycles. It successfully predicted the 1997-98 El Nino. And in April this year Latif reported in Nature (vol 398, p 694) that when they simulated global warming in the model it generated more frequent El Nino-like conditions. "Models are not proof of what will happen," Latif acknowledges. "But for the past 50 years, our model shows well what has actually happened." The model predicts that the average climate in the 21st century will become more like the typical El Nino conditions at the end of the 20th century. And Schrag' s coral results underline the growing feeling that some fundamental change is afoot. The suspicion is taking hold that sometime in the 1970s, a shift took place that stacked the odds in favour of events such as major El Ninos, and perhaps other catastrophic climate events too. Seen in isolation, the sea change of 1976 and the El Ninos of 1982-3 and 1998-9 were all quite plausible "natural" events. But all three of them within such a short space of time? That does sound as if greenhouse gases may have loaded nature's dice. Cane remains cautious. "If you ask me, as a scientist, if the unusual behaviour of El Nino lately has anything to do with greenhouse gases then the answer, at the level of confidence the customs of science demand, is a clear no. But if you ask me at a cocktail party if there is a causal link, I would say 'probably' ." To some extent, time will tell, says Schrag. "If pre-1976 conditions don't return soon that will tend to support the global warming hypothesis." Meanwhile, Schrag is unlikely to be joining Cane at many cocktail parties. He is off in search of new fossilised coral on the shores of Sulawesi. There is, he says, an urgent need to find corals from different times in the geological past, to bridge the huge gap in the history of El Nino between 125 000 years ago and the past century. "I am optimistic that over the next several years we will find enough to put together a much more complete story," he says. 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