Along the west coast of South America, near the end of each calendar year, a warm current of nutrient-poor tropical water moves southward, replacing the cold, nutrient-rich surface water. Because this condition frequently occurs around Christmas, local residents call it El Niño (Spanish for child), referring to the Christ child. In most years the warming lasts for only a few weeks. However, when El Niño conditions last for many months, the economic results can be catastrophic. It is this extended episode of extremely warm water that scientists now refer to as El Niño.

During a severe El Niño, large numbers of fish and marine plants may die. Decomposition of the dead material depletes the water’s oxygen supply, which leads to the bacterial production of huge amounts of smelly hydrogen sulfide. A greatly reduced fish (especially anchovy) harvest affects the world’s fishmeal supply, leading to higher prices for poultry and other animals that normally are fed fishmeal. Anchovies and sardines are also major food sources for marine mammals, such as sea lions and seals. When the food source is in short supply, these animals travel farther from their homes in search of food. Not only do many sea lions and seals starve, but also a large proportion of the infant animals die. The major El Niño event of 1997–1998 indirectly caused 2,100 human deaths and $33 billion in damage globally.