New Scientist
October 9, 1999
Weather warning
By Fred Pearce
HIGHLIGHT: El Nino seems to be changing from a minor nuisance to
a
climatemonster. A chance find on a tropical beach suggests
there's worse
to come,saysFred Pearce.
SOME guys have all the luck. Two years ago Dan 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 redistributor 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 .
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 Hughen 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 lighter 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 is 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 th joyride theory.
They point out that El Nino has always had decades when it is
unusually quiet or busy or just plain weird. 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 degrees C before 1976 to above
24.5 degrees 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 shut-off 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.
Happy diving,
Dan.
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