Interacting parts of a single global system of climate fluctuations, ENSO is the most prominent known source of interannual variability in weather and climate around the world, though not all areas are affected. The Southern Oscillation (SO) is a global scale seesaw in atmospheric pressure between Indonesia and North Australia and the southeast Pacific. In major warm events El Niño warming extends over much of the tropical Pacific and becomes clearly linked to the SO pattern. Many of the countries most affected by ENSO events are developing countries that depend on their agricultural and fishery sectors as a major source of food supply, employment, and foreign exchange. New capabilities to predict the onset of ENSO events can have a global impact. While ENSO is a natural part of Earth's climate system, whether its intensity or frequency may change as a result of global warming is an important concern. See also El Niño events and Southern Oscillation.