The world's climate varies all on its own, just as the weather does. It varies on the decadal scale, the century scale and the millennial scale. Modern measurements are shedding light on a variety of natural decadal cycles:
El Niño–Southern Oscillation (ENSO) combines El Niño, the irregular oceanic cycle affecting the sea surface temperature in the equatorial Pacific, and the corresponding Southern Oscillation in the Pacific trade winds. ENSO most strongly affects the northwest coast of South America, but it affects many other parts of the world. ENSO fluctuates over periods of several months to a year or two.
The Pacific Decadal Oscillation (PDO) is a slower pattern of winds and sea temperature that affects the northern Pacific and North American west coast. It goes between "cool" and "warm" phases over periods of several decades.
The North Atlantic Oscillation (NAO) or Arctic Oscillation (AO) affects weather in eastern North America and Europe. A similar cycle, the Antarctic Oscillation, affects the southern polar region. Both cycles are widely variable, lasting from weeks to decades.
Other major climate variations involve precipitation: drought, monsoons, and growth or shrinkage of lakes and glaciers. Research continues into the relationships of these events with the major oscillations and other factors. The goal is to explain these natural variations and subtract them from the long-term climate record to see what's left.
Internal climate variations are generally not well known. ENSO, PDO and NAO were first recognized only a few decades ago, and the data does not go very far back in time. Again, as with the climate forcings discussed on the previous page, we have to rely on proxy indicators to study these in the past.