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.
Feedbacks
Both forced and unforced modes of natural climate variability are affected by feedbacks: responses of the climate system that either amplify or dampen the underlying variability. Glacial/interglacial cycles, for example, are exaggerated by ice-albedo feedback, in which land ice sheets (which result from cooling temperatures) reflect sunlight into space and thereby amplify cooling. Natural climate variability can be measured via traditional meteorological measurements, or different types of geological (proxy) measurements. Both approaches have limitations: the instrumental record (weather station measurements) is too short to characterize variability on many timescales of interest, while proxy measurements are typically too sparse to yield reliable estimates of variability over large regions (such as continents).
In principle, computer models of climate can be used to help understand natural variability, and simulations of intra-seasonal and intra-annual variability have improved in recent years. However, computational limitations prevent adequate simulation of millennial- and longer-timescale variability, and evaluation of simulations on these timescales is difficult. The character of natural climate variability has itself varied over time. In particular, records derived from ice cores show that the past approximately 10,000 years (the Holocene) have been unusually stable compared to the rest of the most recent 400,000 years; this is thought to have been a significant factor in the development of agricultural societies.
Natural climate variability on timescales of decades to a century complicates "detection of anthropogenic climate change," the search for signals of human influences in the observed climate record since the late 19th century. Because there is no a priori way to attribute an observed climate trend to either natural variability or human influences, a common approach has been to determine if an observed trend is too rapid to result from natural variability alone. While simple in principle, this approach is complicated by the difficulty of characterizing natural variability on the time-scale of about a century.
