IS E L NIN O NOW A MAN-MAD E PHENOMENON ?
The devastating impact of Hurricane Mitch in Honduras on El Nino blamed
est through logging between 1964 and 1990, Honduras is still con tinuing to destroy its native upland forests at the rate of 80,000 hectares each year. The situation is even worse in Nicaragua: there, 150,000 hectares of forest are destroyed each year as a result of commercial timber extraction, the advancing agricultural fron tier, slash-and-burn farming and human-lit forest fires. The coun try has lost nearly 60 per cent of its forest cover in the last 50 years.
What could be a more obvious prelude to disaster? And were El Nino to become the norm that would play havoc with our systems of agriculture. We would experience torrents of unseasonal rain where before the weather had been dry and catastrophic drought where before we experienced summer monsoons.
El Nino and La Nina i n normal time s Until twenty years ago El Ninos would occur every four to seven years, last for one year, and give way to the opposing climate regime, which as a consequence has been dubbed La Nina. La Ninas therefore follow hard on the heel of El Ninos, but where El Nino brings drought, La Nina brings rain and vice versa. As far as the Pacific Ocean goes, El Nino goes hand in hand with an expanding region of warm waters, while La Nina presents the 'cool' side, with strong upwellings in tropical waters of cold cur rents from Antarctica. The more usual climate pattern occurs in the intervals between such oscillations. 'Normal' therefore means monsoon rains over South-east Asia, a rain-drenched Amazon, rains over arid Sahelia in Africa, some dousings of rain over Aus tralia and extremes of high pressure over the west coast of South America, south of the Equator. La Nina, as well as a 'normal' Pacific Ocean, bring rich fish harvests to the Peruvian fishermen and account for the mountains of 'guano' — bird manure — that have accumulated because of the gathering of millions of seabirds. In such years, the Trade Winds blow strong and drive the waters of the Pacific Ocean from East to West, so that by the time they have piled up against South-east Asia they have burdened
themselves with massive amounts of water vapour that are released as the air rises up over the opposing air currents pushing up from the Indian Ocean. The movement of water across the Pacific allows the cold waters of the Humboldt Current to surface along the South American coast south of the Equator. Loaded with vital nutrients, the Humboldt Current hosts a profusion of life.
When the normal regime gives way to El Nino, then the Hum boldt Current is held down by a thick cap of warm waters. With out the nutrients, the rich biological cycle is broken. Without the phytoplankton, the populations of the minute plankton-feeding zooplankton crash, and so on, to the fish and then to the seabirds. The difference is momentous. In a good year, such as in 1970, Peruvian fishermen took 12 million tons of anchovies from the sea. Three years later, an El Nino year, the catch plummeted to less than two million tons.
Causes of violen t El Ninos - global warming? The New Zealand climatologist Ken Trenberth, now at the Nation al Center for Atmospheric Research in the United States, was the first to pinpoint the four-to seven-year cycle of El Ninos. He there fore sees the sharp change in pattern of El Ninos since the mid-
The Ecologist, Vol. 29, No 2, March/April 1999