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Solar variation refers to fluctuation in the amount of energy emitted by the Sun. Small variations have been measured from satellites during recent decades. Of interest to climate scientists is whether these variations have a significant effect on the temperature of the earth's atmosphere. The amount of solar radiation emitted at the surface does not change much (see solar constant) from an average value of 1366 W/m˛. The variations in total output are so slight (as a percentage of total output) that they remained at or below the threshold of detectability until the satellite era, although the small fraction in ultra-violet wavelengths varies by a few percent. Total solar output is now measured to vary (over the last two 11-year sunspot cycles) by less that 0.1% [1] [2] or about 1 W/m˛ peak-to-trough of the 11 year sunspot cycle. There are no direct measurements of the longer-term variation and interpretations of proxy measures of variations differ. Nonetheless, some theorise that solar variation is the primary cause of climate change.
1 Sunspots and Solar luminosity
Sunspots are relatively dark areas on the surface of the Sun and are thus cooler than its average surface. The number of sunspots correlates with the intensity of solar radiation. Since sunspots are dark it is natural to assume that more sunspots means less solar radiation. However the surrounding areas are brighter and the overall effect is that more sunspots means a brighter sun. The variation is small (of the order of 1 W/m˛ or 0.1% of the total) and was only established once satellite measurements of solar variation became available in the 1980s. Various studies have been made using sunspot number (for which records extend over hundreds of years) as a proxy for solar output (for which good records only extend for a few decades). Also, comparisons between ground instruments, high-altitude instruments, and instruments in orbit have been used to calibrate ground instruments. Researchers have combined present readings and factors to adjust historical data. Also used have been proxy data, such as measurements of cosmic ray isotopes to infer solar magnetic activity and thus the likely brightness.
There is currently no clear agreement as to the likely magnitude of long-term (last hundred or more years) solar variation. The IPCC discuss this in section 6.11 of the TAR [3] and show various results including Lean et al. (1995) [4]. More recently Lean et al (GRL 2002, [5]) say:
- Our simulation suggests that secular changes in terrestrial proxies of solar activity (such as the 14C and 10Be cosmogenic isotopes and the aa geomagnetic index) can occur in the absence of long-term (i.e., secular) solar irradiance changes. ...this suggests that total solar irradiance may also lack significant secular trends. ...Solar radiative forcing of climate is reduced by a factor of 5 when the background component is omitted from historical reconstructions of total solar irradiance ...This suggest that general circulation model (GCM) simulations of twentieth century warming may overestimate the role of solar irradiance variability. ...There is, however, growing empirical evidence for the Sun's role in climate change on multiple time scales including the 11-year cycle ...Climate response to solar variability may involve amplification of climate modes which the GCMs do not typically include. ...In this way, long-term climate change may appear to track the amplitude of the solar activity cycles because the stochastic response increases with the cycle amplitude, not because there is an actual secular irradiance change.
2 Solar cycles
Solar cycles are cyclic changes in behavior of the Sun. Most obvious is a gradual increase and decrease of the number of sunspots over a period of about 11 years, called the Schwabe cycle. This seems to be due to a shedding of entangled magnetic fields. The Sun's surface is also the most active when there are more sunspots, although the luminosity does not change much due to an increase in bright spots ( faculae). Other patterns detected are the Hale cycle (22 years) and the Gleissberg cycle (70-100 years).
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