Solar variations are changes in the amount of solar radiation emitted by the Sun. There are periodic components to these variations, the principal one being the 11-year solar cycle (or sunspot cycle), as well as aperiodic fluctuations. Solar activity has been measured via satellites during recent decades and through 'proxy' variables in prior times. Climate scientists are interested in understanding what, if any, effect variations in solar activity have on the Earth. Effects on the earth caused by solar activity are called "solar forcing".

The variations in total solar irradiance (TSI) 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 three 11-year sunspot cycles) by approximately 0.1%[1][2] or about 1.3 W/m² peak-to-trough during the 11 year sunspot cycle. The amount of solar radiation received at the outer surface of Earth's atmosphere varied little from an average value of 1,366 watts per square meter (W/m²).[3] There are no direct measurements of the longer-term variation and interpretations of proxy measures of variations differ; recent results suggest about 0.1% variation over the last 2,000 years,[4] although other sources suggest a 0.2% increase in solar irradiance since 1675.[5] The combination of solar variation and volcanic effects has very likely been the cause of some climate change, for example during the Maunder Minimum.

A 2006 study and review of existing literature, published in Nature, determined that there has been no net increase in solar brightness since the mid 1970s, and that changes in solar output within the past 400 years are unlikely to have played a major part in global warming.[6] However, the same report cautions that "Apart from solar brightness, more subtle influences on climate from cosmic rays or the Sun's ultraviolet radiation cannot be excluded, say the authors. They also add that these influences cannot be confirmed because physical models for such effects are still too poorly developed."

Sources and CitationsEdit

  1. Solar Forcing of Climate. Climate Change 2001: Working Group I: The Scientific Basis. Retrieved on 2005 March 10.
  2. Weart, Spencer (2006), "Changing Sun, Changing Climate?", in Weart, Spencer, The Discovery of Global Warming, American Institute of Physics, <> (retrieved on 2007-04-14) 
  3. Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present. Physikalisch-Meteorologisches Observatorium Davos (PMOD). Retrieved on 2005 October 5.
  4. North, Gerald R.; Franco Biondi & Peter Bloomfield et al., eds. (2006), "Climate Forcings and Climate Models", Surface Temperature Reconstructions for the Last 2,000 Years, National Academies Press, ISBN 0-309-10225-1, <> (retrieved on 2007-04-19) 
  5. Lean, J. (2000), "Evolution of the Sun's Spectral Irradiance Since the Maunder Minimum", Geophysical Research Letters 27 (16): 2425–2428, DOI:10.1029/2000GL000043, <> (retrieved on 2008-02-01) 
  6. Foukal, Peter; et al. (2006). "Variations in solar luminosity and their effect on the Earth's climate". Nature 443 (7108): 161–166. DOI:10.1038/nature05072. 

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