One of the subjects still poorly understood is the occurrence of droughts in areas of highly variable rainfall. In the early 1970s and again in the early 1980s the Sahel region of Africa suffered periods of severe drought, resulting in widespread famine and death. There have been many Sahelian droughts before, but the consequences of the recent droughts have been exacerbated by increased populations of people and grazing animals. The combination of drought and population growth results in desertification. It remains an unanswered scientific question as to whether the deterioration of the Sahel and other marginal lands is part of a long-term natural change or whether it is a result of human activities.
Some evidence for long-range interactions in the occurrence of droughts and other climatic regimes comes from studies of the ocean currents. It is known that the oceans are a major controlling influence on climate, but the processes involved remain the subject of active research. Some clues have been revealed by studies of El Niņo, a minor branch of the Pacific Equatorial Countercurrent that flows south along the coasts of Colombia and Ecuador where it meets the northward-flowing Peru Current. The cold Peru Current keeps rainfall along the coastal area of Peru very low but maintains a rich marine life, which in turn supports major bird populations and a fishing industry. In certain years El Niņo becomes much stronger, forcing the Peru Current to the south. Storms rake the coast, causing flooding and erosion. The sudden change in sea temperatures causes dramatic decreases in plankton production and, consequently, in fish and bird populations. Catastrophic El Niņo events occurred in 1925, 1933, 1939, 1944, 1958, and 1983. It is thought that the global changes associated with this last event included severe droughts in Australia and Central America and floods in the southwestern United States and Ecuador. Explanations of the El Niņo events have invoked both local and long-range interactions in the circulation of the Pacific winds and currents. The study of such dramatic events, enhanced by remote sensing and computer modeling, is a major stimulus to understanding the general circulation of the Earth's atmosphere and oceans.