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I . WHAT' S HAPPENIN G TO GLOBA L CLIMATE ?

1970s as evidence that a switch in climate has begun to take place. Nothing like it, he says, has been seen for at least 2,000 years, and he believes the change in behaviour of the Pacific Ocean to be a consequence of global warming.3

The general belief is that the Southern Pacific Ocean acts like a capacitor, mopping up energy in the form of heat, until a point is reached when the system overloads and dumps the energy in one dramatic moment. Indeed, as they warm, the surface waters of the central Pacific can move hundreds, i f not thousands, of miles east­ wards. Although the mechanism is not clear, the extra heat going into the oceans because of global warming appears to have tipped the balance to more frequent El Ninos as well as more severe ones. El Ninos therefore reflect an instability in the climate system and in that respect, signal that climate is changing dramatically.

The amount of moisture in the climate system is a good indica­ tor of the additional heating that the Earth is now undergoing. And more water vapour in the atmosphere means "a significant increase in the energy available to drive storms and associated weather fronts", according to scientists at the US Global Change Research Program. Meanwhile, atmospheric moisture has grown by five per cent per decade since 1973 over the United States and by 10 per cent over temperate regions of the northern hemisphere over the past century. Some climate simulations show El Nino-like conditions developing over the Pacific as carbon dioxide levels in the atmos­ phere double: they add weight to the notion that global warming is having a major impact on climate and weather patterns.4

The volcani c catalys t Global warming on its own, however, may not be enough to cause an El Nino. One idea that has been gaining ground is that some of that kick-start energy comes from volcanic activity in the ocean bottom. Oceanographers have now discovered large flows of magma from the mid-ocean Pacific ridge that over a period of approximately five years could release a good proportion — per­ haps as much as ten per cent — of the heat that is normally asso­ ciated with changes in the sea surface that go with El Ninos. Even more extraordinary is the correlation between volcanic eruptions that send debris and gases into the atmosphere. Major volcanic eruptions, such as El Chichon in Mexico in 1982 and Pinatubo in the Philippines in 1991, prevented as much as ten per cent of sun­ light getting down to the Earth's surface over the northern Trop­ ics. According to Paul Handler and Karen Andsager, of the University of Illinois, that cooling will have led to substantial shifts in the amount of air building up over the Eurasian continent

Climatic Dislocation in Colombia. The 1990s have been marked by the phenomenon of El Nino, which used to occur once every five years and now makes itself felt every three months. The radical change in its frequency has had a major effect on the quantity and distribution of water all over Colombia, causing a decline in the yield of crops, in cattleraising and bringing about problems in the production of hydroelectricity, with severe repercussions for the economy.

"Even though Colombia has been well aware of the existence of El Nino and La Nina," says meteorologist Max Henriquez, "it has taken us completely by surprise through lasting so long. Every time there is drought, however intense, it has an impact on ecosystems. Given the new frequency of such events, there is no time for recovery. As a result, we could experience serious deterioration of biodiversity in the west of the country, in what is one of the world's richest habitats, the biogeographic region known as the Choco."

Equally, the snow-clad peaks in Colombia, as elsewhere in the world, are also experiencing a rapid shrinking in snow both in area and depth brought about by global climate change. Although volcanic activity has brought about some melting, that does not account for Colombian glaciers having lost almost 40 per cent of their cover over the past forty years: their total disappearance seems increasingly likely. In general the country is suffering from frequent and unexpected floods, while the prolonged droughts are causing a series of forest fires in different regions of the country, as well as famine and disease.

By Monica del Pilar Uribe Marin, a Colombian journalist.

during the winter months. As a result, the air mass that normally feeds the Trade Winds is much weaker and conditions are set for an impending El Nino. On the basis of such a scenario, volcanic eruptions are most effective in bringing about an El Nino when they inject their debris over the low latitudes of the northern hemi­ sphere, just as El Chichon did in 1982. The correlation held for 1997, which saw the volcanic eruptions on the Caribbean island of Montserrat — in the right place and at the right time.5

Deforestation and El Nino But whereas volcanoes and the four-to seven-year El Nino cycle are all natural phenomena, we have now introduced another play­ er in the process, aside from global warming. The massive and

continuing destruction of tropical forests may be responsible for the abrupt change in the behaviour of El Ninos. Tropical forests, particularly when intact, are responsible for prodigious releases of energy in the form of water vapour into the atmos­ phere — equivalent to the energy that would be released by exploding some 5-6 million atomic bombs every day just over the Amazon Basin.6 That energy is then trans­ ferred in the global circulations from the Equator up into the higher lati­ tudes and is crucial for the move­ ment of air masses. Those same air masses form the 'highs' and 'lows' that provide the basis of global weather. The tropical forests of the world, including those of the Ama­ zon, of Central Africa and of Indone­ sia, lie judiciously at the point along the Equator where tropical thunder-

66

The Ecologist, Vol. 29, No 2, March/April 1999 IS E L NIN O NO W A MAN-MAD E PHENOMENON ?

storms develop. But they are not just fortuitous recipients of rain: they actually generate rain, first by pumping water vapour into the atmosphere through transpiration and second by releasing volatile hydrocarbons, such as isoprene, that act as cloud-condensation nuclei. Consequently, as much as three-quarters of all the rain that falls over the rainforest in the humid tropics gets returned to the atmosphere by means of evapo-transpiration. Given the crucial role of tropical forests in re­ distributing the energy that falls over the Equator and given our destructive obsession with chopping them all down, we could well be in for a spate of exceedingly destructive switches between El Nihos and La Ninas.

Hence, heated air masses rise above the forests and become rivers of air which cross the Pacific from west to east. They then cool down and descend where the waters are coolest, towards the American coasts, where they feed the trade winds. Rainforests therefore appear to act as thermal machines and, above all, as reg­ ulators of atmospheric and oceanic systems which control the cli­ mate.

Even though the El Nino/Southern Oscillation has existed as a phenomenon for longer than history, the wholesale destruction of tropical forests over the past forty years will have seriously jeop­ ardised the efficiency with which energy gets transferred from the Equator to the higher latitudes. Other climatologists, Ann Hender­ son-Sellers for example, have begun modelling rainforests into the climate system. Their models indicate that rainforest destruction is having a significant impact on the jet-streams that wedge their way between the various atmospheric circulation cells. By shifting

the air masses of the major circulation systems both south and north, east and west, the jet-streams have a profound effect on regional climate. Forest destruction in the tropics is therefore changing climate and is sending weather systems spiralling off in new and unpredicted directions.7

Given the crucial role of tropical forests in re-distributing the energy that falls over the Equator and given our destructive obses­ sion with chopping them all down, we could well be in for a spate of exceedingly destructive switches between El Ninos and La Ninas. A spate of powerful El Nihos would play havoc with trop­ ical agriculture, with vast areas of the Tropics becoming irre­ versibly desertified through a successive drying out. Yet that is where we are heading. The 1997/98 El Nino is a warning: we can­ not afford to destroy more forests; nor can we afford to pump great volumes of greenhouse gases into the atmosphere. As a warning to humanity of the dangers of aberrant behaviour, perhaps El Nino was aptly named.

Alain-Claude Galtie is a French ecologist and is on the board of editors of Silence magazine.

References: 1. Stephen Zebiak and his colleague, Mark Cane, at the Lamont-Doherty Earth

Observatory at Columbia University, New York, are considered among the top experts on El Nino. Their models led them to believe that 1998 would be the year of El Nino and not 1997 (New Scientist, May 31st, 1997). 2. Fred Pearce, Quick Change, New Scientist, 14 November, p. 15, 1998. 3. Ken Trenberth, quoted in University Cooperation in Atmospheric Research, edited

by Carol Rasmussen, Winter 1997. 4. ibid 5. Paul Handler & Karen Andsager. El Nino, Volcanism and Global Climate. Human

Ecology, Vol. 22, No.l , 1994. 6. Eneas Salati, The Forest and the hydrological cycle, in The Geophysiology of

Amazonia, ed. Robert Dickinson, Wiley, 1987. 7. H. Zhang, K. McGuffie and A. Henderson-Sellers, Journal of Climate Vol. 9, 1996.

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The Ecologist, Vol. 29, No 2, March/April 1999

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