It is a recurrent phenomenon with an average return period of four and a half years, but can recur as little as two or as much as ten years apart. Such events have occurred for millennia, and can be expected to continue to occur in the future. However, it was the 1982-1983 event that first sparked serious interest. Nor have we seen all of the ways that they can affect society and ecosystems. Droughts can have devastating impacts on communities, affecting food and energy production and, in arid zones, the very survival of humans and animals.
Floods can destroy property and kill people.
In January 1998, in northern Peru and Kenya, for example, heavy rain had serious consequences. Timely warning of such extreme events can enable civil defence experts to plan for evacuation of at-risk populations. If reservoir managers are given a reliable forecast of seasonally above average rainfall, they can lower reservoir water levels to make storage room for the expected excess water. Researchers are just beginning to realize that they should focus more attention on the cold part of the cycle. Such studies are currently being undertaken in Australia, Costa Rica, Japan, North America, Panama, Thailand and Viet Nam.