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Solar variations are fluctuations 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%  For a discussion of attribution of causes of current global warming see: Attribution of recent climate change
Research by Willie Soon and Sallie Baliunas presents evidence that variations in solar radiation produced the warming that "put the green in Greenland" and led to a "Little Ice Age". The IPCC's estimate of solar forcing since 1750 is available .
Douglass and Clader, Geophysical Research Letters, 2002 indicate that the climate response to forcings due to solar variations has been about twice that of simple radiation balancing, in agreement with the standard idea that some feedback mechanism is required to explain the influence of solar forcing found in ocean measurements and paleo data.
Solar variation in climate models
Climate models are computer simulations which are used to examine understanding of climate behavior. Some models use constant values for solar irradiance, while some include the heating effects of a variable Sun. A good simulation by GCMs of global mean temperature over the last 100 years requires both natural (solar; volcanic) and human (greenhouse gas) factors.
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  and show various results including Lean et al. (1995) . More recently Lean et al (GRL 2002, ) 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.
History of study of solar variations
The longest recorded aspect of solar variations are changes in sunspots. Shortly after astronomers began using the telescope in 1609, sunspots and their motions were observed. Initial study was focused on their nature and behavior. Although the physical aspects of sunspots was not identified until the 1900s, observations continued. Study was hampered during the 1600s and 1700s due to the low number of sunspots during what is now recognized as an extended period of low solar activity, this event named the Maunder Minimum. By the 1800s records of their numbers began to show variations in their numbers. For details about sunspots see the main article: Sunspot
Around 1900 connections between solar variations and weather on Earth began to be explored. Challenges are shown in the efforts of Charles Greeley Abbot, assigned by the Smithsonian Astrophysical Observatory to detect changes in the radiation of the Sun. His team had to begin by inventing instruments to measure solar radiation. Later, when he was head of the SAO, it established a solar station at Calama, Chile to complement its data from Mount Wilson Observatory. He detected 27 harmonic periods within the 273-month Hale cycles, including 7, 13, and 39 month patterns. He looked for connections to weather by means such as matching opposing solar trends during a month to opposing temperature and precipitation trends in cities.
Statistical studies of solar activity with weather and climate were particularly popular until the 1980s, when publications blossomed with studies of weather fronts and global meteorological patterns. Photos from space and weather satellites emphasized the importance of clouds and weather fronts. Climate studies and weather forecasting have been enhanced by increasing use of climate models, beginning with simple computer simulations and replacing "solar constant" values with more detailed solar variation as computing power increased and understanding of weather processes improves.
- ^ "Changing Sun, Changing Climate?." The Discovery of Global Warming. Accessed on February 21, 2005.
- ^ "11,000 Year Sunspot Number Reconstruction." Global Change Master Directory. Accessed on March 11, 2005.
- ^ Charles A. Perry and Kenneth J. Hsudagger (2000) "Geophysical, archaeological, and historical evidence support a solar-output model for climate change". PNAS 97 (23), 12433-12438. DOI:10.1073/pnas.230423297
- ^ "What the Sunspot Record Tells Us about Space Climate." Submitted to Solar Physics 2004/08/31. Accessed on March 11, 2005.
- ^ "SOLAR VARIABILITY: climatic change resulting from changes in the amount of solar energy reaching the upper atmosphere.." INTRODUCTION TO QUATERNARY ECOLOGY. Accessed on March 11, 2005.
- ^ Henrik Svensmark (1998) "Influence of Cosmic Rays on Earth's Climate". Physical Review Letters 81 (22), 5027-5030.
- ^ "Atmospheric Ionization and Clouds as Links Between Solar Activity and Climate." Tinsley, Brian A. and Fangqun Yu, in: Solar Variability and Its Effects on the Earth's Atmospheric and Climate System. Accessed on March 10, 2005.
- ^ E. Pallé, C.J. Butler, K. O’Brien (2004) "The possible connection between ionization in the atmosphere by cosmic rays and low level clouds". Journal of Atmospheric and Solar-Terrestrial Physics 66 (18), 1779-1720. DOI:10.1016/j.jastp.2004.07.041
- ^ Pallé, E. (2005) "Possible satellite perspective effects on the reported correlations between solar activity and clouds". Geophys. Res. Lett. 32 (3), L03802. DOI:10.1029/2004GL021167
- ^ "Variations in CO2 Growth Rate Associated with Solar Activity." Still Waiting for Greenhouse. Accessed on March 10, 2005.
- C. G. Abbot (1966) "SOLAR VARIATION, A WEATHER ELEMENT". Proc Natl Acad Sci U S A. 56 (6), 1627-1634.
- "The Sun and Climate." U.S. Geological Survey Fact Sheet 0095-00. Accessed on February 21, 2005.
- "The Sun's role in Climate Changes." Proc. of The International Conference on Global Warming and The Next Ice Age, 19-24 August, 2001, Halifax, Nova Scotia.. Accessed on February 21, 2005.
- Warren B. White, Judith Lean, Daniel R. Cayan and Michael D. Dettinger (1997) "Response of global upper ocean temperature to changing solar irradiance". J. Geophys. Res. 102 (C2), 3255-3266.
- Gerrit Lohmann, Norel Rimbu, Mihai Dima (2004). Climate signature of solar irradiance variations: analysis of long-term instrumental, historical, and proxy data. International Journal of Climatology 24(8), 1045-1056 - Abstract: http://www.palmod.uni-bremen.de/~gerrit/abstractSolar.html
- Solar Climatic Effects (Recent Influence) – Summary. Center for the Study of Carbon Dioxide and Global Change. 19 March 2003. http://www.co2science.org/subject/s/summaries/solarrecin.htm
- NOAA / NESDIS / NGDC (2002) Solar Variability Affecting Earth NOAA CD-ROM NGDC-05/01. This CD-ROM contains over 100 solar-terrestrial and related global data bases covering the period through April 1990. http://www.ngdc.noaa.gov/stp/CDROM/solar_variability.html
- S.K Solanski, M. Fligge (2001) Long-term changes in solar irradiance ESA SP-463, ESA Publications Division. http://www.astro.phys.ethz.ch/papers/fligge/solspa_2.pdf
- S.K. Solanki, M. Fligge (2000) Reconstruction of past solar irradiance Space Science Review 94, 127-138 http://www.astro.phys.ethz.ch/papers/fligge/solfli_rev.pdf
- George C. Reid (1995) The sun-climate question: Is there a real connection? Aeronomy Laboratory, NOAA/ERL, Boulder, Colorado. U.S. National Report to IUGG, 1991-1994 Rev. Geophys. Vol. 33 Suppl. http://www.agu.org/revgeophys/reid00/reid00.html
In a barotropic atmosphere the geostrophic wind is independent of height.
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