Forced Climate Variability

STUDIES ON GLOBAL warming and climate change generally distinguish between internally generated and externally-forced climate variability. Internally generated variability is the result of processes within a system, while externally-forced variability is caused by some factors outside the system. A classic example of externally-forced climate variability is represented by the changes caused by variations in the amount and distribution of solar energy incidents on the Earth because of the differences in the solar luminosity or in the Earth's orbital parameters. The distinction between the two types of variability is not always so clear-cut, because it depends on how the boundaries of the system under examination are defined. For example, when studying or modeling the atmosphere in isolation from the rest of the climate system, changes in sea surface temperatures would be termed external forcing. Yet, in a coupled ocean/atmosphere model, variability induced in the atmosphere by variations in sea surface temperatures would be internally generated variability. There is still considerable debate concerning the extent of internal climate variability.

Internal climate variability not forced by external agents occurs on all time-scales, from weeks, to centuries and millennia. Slow climate components, such as oceans, have particularly important roles on decadal and century timescales because they complement high-frequency weather variability and work together with faster components. Thus, climate is capable of producing long timescale internal variations of considerable magnitude without any external influences. Externally-forced climate variations may be caused in changes by natural forcing factors, such as solar radiation or volcanic aerosols, or changes in anthropogenic forcing factors, such as increasing concentrations of greenhouse gases or sulfate aerosols. Therefore, the response to anthropogenic changes in climate forcing is set against the background of natural internal and externally-forced climate variability that can take place on similar temporal and spatial scales.

The action of natural climate variability means that the detection and attribution of anthropogenic climate change should demonstrate that an observed change is significantly different statistically than can be explained by natural internal variability. However, although a particular change in climate is detected, this does not necessarily mean that its causes are understood. In order to define more precisely the adaptation and mitigation strategies needed to deal with the potential impacts of expected climate change, a deeper understanding of the intrinsic and externally-

forced variability of the climate system is needed to detect, attribute, and predict global and regional climate change provoked by natural and anthropogenic causes.

Examples of externally-forced climate variability include variations in solar output and volcanic eruptions that cause a general cooling of climate by injecting aerosols into the stratosphere. Human activities can also produce changes in the composition of the atmosphere. The extent of this externally-forced variability depends on the extent of the forcing and the sensitivity of the climate system to the forcing. When the forcing is gradual, as in the case of the increase in the Sun's emissions over the Earth's life span, all the different components of the system retain their equilibrium. In contrast, forcing that produces immediate changes or consists of a short-lived impulse, such as volcanic eruptions, provoke a transient reaction with multiple timescales.

The intensity of solar radiation varies over a large range of timescales and the extent of these variations depends on wavelengths. Sunspots are cool patches that break up the convective movement of cells in the photosphere. They take place in conjunction with strong magnetic fields and their lifetimes range from a day to a few months. In contrast, fac-ulae are hot spots in the pattern of convective cells that often accompany sunspots and strong magnetic fields. Their lifetimes are comparable to those of the sunspots. Flares are intense emissions of ultraviolet and x-ray radiations and high-energy particles that the sun's outer atmosphere emanates within its active regions. Their main features are strong magnetic fields, violent motions and a lifespan of an hour. During the active phase of the 11-year solar cycle, there are numerous sunspots, faculae, and flares, appearing first at higher latitudes, and later at lower ones as well. During the quiet phase of the solar cycle very few of these disturbances occur.

The impacts on climate due to volcanic eruptions are mainly represented by the effects of sulfate aerosols formed from SO2 emissions. Yet, traces of these aerosols remain in the troposphere for only a few weeks. Thus, only major eruptions, such as the 1991 Mt. Pinatubo eruption in the Philippines, penetrate the lower stratosphere and have a significant impact on global climate. Aerosol clouds generated by major volcanic eruptions can reduce the flux density of direct solar radiation incident on the Earth's surface. This reduction in total incident solar radiation produces lower global-mean surface air temperature following major volcanic eruptions than in the long-term average. The duration of the cooling of the Earth following major volcanic eruptions appears to be longer than the one to two-year duration of aerosols in the stratosphere. This is due to the interaction of the ocean mixed layer and its large heat capacity. An additional period of a year or two is necessary for the layer to regain the heat lost due to the air-sea temperature imbalance while the aerosols were in the atmosphere. The global-mean surface air temperature returns to its pre-eruptions level only when the equilibrium of the ocean is restored. In addition to these forcings, there are others that are associated with human-induced activities such as the burning of fossil fuels that result in a buildup of carbon dioxide in the atmosphere and the consequent greenhouse warming.

SEE ALSo: Climate Forcing; Volcanism.

BIBLIoGRAPHY. Kerry Emanuel, J.A. Layzer, and W.R. Moomaw, What We Know About Climate Change (Massachusetts Institute of Technology Press, 2007); J.M. Wallace and P.V. Hobbs, Atmospheric Science (Academic Press, 2006).

Luca Prono University of Nottingham

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