El Nino and Human Health

Article excerpt

Voir page 1133 le resume en francais. En la pagina 1134 figura un resumen en espanol.


El Nino is a complex climatic phenomenon that is, perhaps unfairly, associated with death and destruction. It increases the risk of extreme weather but also has more subtle effects on human health. This article reviews those studies that have examined the relationship between the quasi-periodic climate pattern and disease risk. The attribution of any single epidemic to El Nino is difficult since the climate phenomenon is not the same as extreme weather. The relationships between extreme weather and deaths, injuries and subsequent outbreaks of disease have been well described. Particularly during disasters, initial outbreaks may be compounded by the displacement of populations and failures in health infrastructures. Many recent events have shown how easily health services are overwhelmed by atypical weather conditions. Between October 1999 and March 2000 there were major disasters associated with La Nina conditions in India, Mozambique, and Venezuela. Given the context of global climate change, which poses an increasingly certain threat (1), a better understanding of the relationships between weather, climate and human health is necessary.

Climate variability and ENSO

El Nino refers to the exceptionally marked and prolonged warm periods that occur in the Pacific Ocean around the equator (2, 3). They are linked to weather changes all over the world. The cycle of warming and cooling of the East Pacific is closely mirrored by air pressure deviations over the East and West Pacific, called the Southern Oscillation. When pressure rises in the East Pacific it usually falls in the West. Walker pioneered the use of statistical methods to link weather anomalies with the Southern Oscillation, including the Asian monsoon, and was one of the first to explore the possibility of seasonal forecasting (4). In the 1960s the link was made between the atmospheric Southern Oscillation and the oceanic El Nino, now referred to as the El Nino -- Southern Oscillation (ENSO) (see Box 1). The broader definition of El Nino has sometimes been used interchangeably with ENSO because the latter term is less well known. The two extremes of ENSO are El Nino (a warm event) and La Nina (a cold event). Global weather patterns associated with La Nina are generally less pronounced and, in some areas, tend to be the opposite of those associated with El Nino.

Box 1. The discovery and investigation of El Nino

1860s        First recorded use of the term "El Nino" in Peru to
             describe periodic warming of coastal waters

1910-1930s   Gilbert Walker documents the Southern Oscillation and
             its linkages with the Asian monsoon

1960s        Jacob Bjerknes links El Nino in the Pacific Ocean
             with the Southern Oscillation in the atmosphere

1981-1982    First international efforts to reduce the impact of
             El Nino and improve forecasting

1985-1987    First global climate models to simulate ENSO

1997         United Nations Interagency Task Force on El Nino

1997         Many models successfully forecast the onset of El Nino
             but fail to forecast its magnitude

El Nino begins when the prevailing easterly winds weaken over the Pacific. The major rain zone is shifted eastwards towards the central Pacific, causing a prolonged dry period in northern Australia, Indonesia, and the Philippines. The Pacific Ocean is warmer along the western coast of South America and during El Nino the upwelling along this coast is reduced, i.e. the nutrient-rich cold waters from the deep ocean no longer appear at the surface. Because of this there is a decline in the production of plankton, which has repercussions for populations of fish and sea birds. Sea levels on either side of the Pacific are also affected. The behaviour of the ocean and that of the atmosphere reinforce each other until El Nino is fully established. …