Internal Climate Variability

INTERNAL OR NATURAL climate variability refers to variations over time in one or more measures of climate, resulting from natural causes. The distinction between climate variability and weather variability is not a matter of different timescales; rather, it is based on the fundamental distinction between climate and weather: weather refers to meteorological conditions at a specific time and location, whereas climate refers to any statistical characterizations (such as a long-term mean) of weather conditions. Thus, for example, a single measurement of diurnal temperature range measures a weather variation, whereas an estimate of a multi-year mean diurnal temperature range is a climate variation. An alternative definition is that natural climate variability is any variation in climate not resulting from human influences, such as increasing atmospheric greenhouse gases.

Natural climate variations occur on all timescales up to the age of Earth (for billions of years), and can be classified as either forced or unforced. Forced variations are caused by factors external to the climate system, including: Earth's rotation (resulting in the daily cycle); Earth's orbit (resulting in the seasonal cycle); large volcanic eruptions, which can result in more small particles in the stratosphere, lowering temperatures for a few years; variations in the Sun's inherent energy output, which may have caused, for example, the Little Ice Age during medieval times; variations in Earth's orbital parameters, which result in redistributions of incoming solar radiation, and trigger glacial/interglacial cycles; slow motions of continents (plate tectonics); and slow changes in atmospheric composition, particularly greenhouse gas concentrations, resulting from changes in the balance between natural sources and sinks of these gases.

Unforced variations are internally generated redistributions of energy within the system that occur without changes in external factors. Important modes of unforced climate variability include: the Madden-Julian oscillation, or variations in winds, cloudiness, and other phenomena in the tropics, with a timescale of 40–50 days; the El Nino/Southern Oscillation, a dominant mode of year-to-year tropical climate variability, characterized by changes in sea-surface temperatures in the tropical Pacific; the North Atlantic Oscillation, also known as the Arctic Oscillation and the Northern Annular Mode, variations in sea-level pressure in the Arctic, occurring simultaneously with variations of opposite sign at mid-latitudes in the Atlantic, reflecting North-South motions of atmospheric mass; and the Pacific Decadal Oscillation, changes in north Pacific climate, with a timescale of 20–30 years.