El Niño events cause fluctuations in temperature and rainfall patterns in the Pacific basin and beyond. These periodic variations that occur as a result of the interaction between the ocean and the atmosphere in the tropical Pacific region can affect ecosystems and human lives in far flung regions of the globe. This ocean-atmosphere phenomenon, known as the El Niño/Southern Oscillation (ENSO), affects climate-sensitive human activities such as agriculture and fisheries. The symbols on this map indicate the types of impacts associated with "warm" ENSO events, such as occurred in 1982-83 and during the early 1990s, in which ocean waters in the eastern tropical Pacific Ocean warm by only one to a few degrees above their normal conditions. That these relatively small changes cause such significant responses indicates how sensitive the climate, ecosystems, and society are to these natural fluctuations. The effects of a La Niña event (an anomalous cooling of the waters off the coast of South America) also can cause climatic and ecological disruptions; in some regions, the impact of La Niña is the opposite of that caused by El Niño. The figure is courtesy of the NOAA Office of Global Programs.