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Solar variation is the change in the amount of radiation emitted by the Sun (see Solar radiation) and in its spectral distribution over years to millennia. These variations have periodic components, the main one being the approximately 11-year solar cycle (or sunspot cycle). The changes also have aperiodic fluctuations. In recent decades, solar activity has been measured by satellites, while before it was estimated using 'proxy' variables. Scientists studying climate change are interested in understanding the effects of variations in the total and spectral solar irradiance on Earth and its climate.
Variations in total solar irradiance were too small to detect with technology available before the satellite era, although the small fraction in ultra-violet light has recently been found to vary significantly more than previously thought over the course of a solar cycle. Total solar output is now measured to vary (over the last three 11-year sunspot cycles) by approximately 0.1%, or about 1.3 Watts per square meter (W/m2) peak-to-trough from solar maximum to solar minimum during the 11-year sunspot cycle. The amount of solar radiation received at the outer surface of Earth's atmosphere averages 1366 W/m2. There are no direct measurements of the longer-term variation, and interpretations of proxy measures of variations differ. The intensity of solar radiation reaching Earth has been relatively constant through the last 2000 years, with variations estimated at around 0.1–0.2%. Solar variation, together with volcanic activity are hypothesized to have contributed to climate change, for example during the Maunder Minimum. However, changes in solar brightness are too weak to explain recent climate change.
History of study into solar variations 
The longest recorded aspect of solar variations are changes in sunspots. The first record of sunspots dates to around 800 BC in China and the oldest surviving drawing of a sunspot dates to 1128. In 1610, astronomers began using the telescope to make observations of sunspots and their motions. Initial study was focused on their nature and behavior. Although the physical aspects of sunspots were not identified until the 20th century, observations continued. Study was hampered during the 17th century due to the low number of sunspots during what is now recognized as an extended period of low solar activity, known as the Maunder Minimum. By the 19th century, there was a long enough record of sunspot numbers to infer periodic cycles in sunspot activity. In 1845, Princeton University professors Joseph Henry and Stephen Alexander observed the Sun with a thermopile and determined that sunspots emitted less radiation than surrounding areas of the Sun. The emission of higher than average amounts of radiation later were observed from the solar faculae.
Around 1900, researchers began to explore connections between solar variations and weather on Earth. Of particular note is the work of Charles Greeley Abbot. Abbot was assigned by the Smithsonian Astrophysical Observatory (SAO) to detect changes in the radiation of the Sun. His team had to begin by inventing instruments to measure solar radiation. Later, when Abbot 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. With the advent of dendrochronology, scientists such as Waldo S. Glock attempted to connect variation in tree growth to periodic solar variations in the extant record and infer long-term secular variability in the solar constant from similar variations in millennial-scale chronologies.
Statistical studies that correlate weather and climate with solar activity have been popular for centuries, dating back at least to 1801, when William Herschel noted an apparent connection between wheat prices and sunspot records. They now often involve high-density global datasets compiled from surface networks and weather satellite observations and/or the forcing of climate models with synthetic or observed solar variability to investigate the detailed processes by which the effects of solar variations propagate through the Earth's climate system.
Solar activity and irradiance measurement 
Direct irradiance measurements have only been available during the last three cycles and are based on a composite of many different observing satellites. However, the correlation between irradiance measurements and other proxies of solar activity make it reasonable to estimate past solar activity. Most important among these proxies is the record of sunspot observations that has been recorded since ~1610. Since sunspots and associated faculae are directly responsible for small changes in the brightness of the sun, they are closely correlated to changes in solar output. Direct measurements of radio emissions from the Sun at 10.7 cm also provide a proxy of solar activity that can be measured from the ground since the Earth's atmosphere is transparent at this wavelength. Lastly, solar flares are a type of solar activity that can impact human life on Earth by affecting electrical systems, especially satellites. Flares usually occur in the presence of sunspots, and hence the two are correlated, but flares themselves make only tiny perturbations of the solar luminosity.
Recently it has been claimed that the total solar irradiance is varying in ways that are not duplicated by changes in sunspot observations or radio emissions, though Willson, DeWitte, and others have pointed out that these shifts in irradiance may be no more than the result of calibration problems in the measuring satellites. These speculations also admit the possibility that a small long-term trend might exist in solar irradiance.
Sunspots are relatively dark areas on the radiating 'surface' (photosphere) of the Sun where intense magnetic activity inhibits convection and cools the photosphere. Faculae are slightly brighter areas that form around sunspot groups as the flow of energy to the photosphere is re-established and both the normal flow and the sunspot-blocked energy elevate the radiating 'surface' temperature. Scientists have speculated on possible relationships between sunspots and solar luminosity since the historical sunspot area record began in the 17th century. Correlations are now known to exist with decreases in luminosity caused by sunspots (generally < - 0.3%) and increases (generally < + 0.05%) caused both by faculae that are associated with active regions as well as the magnetically active 'bright network'.
Modulation of the solar luminosity by magnetically active regions was confirmed by satellite measurements of total solar irradiance (TSI) by the ACRIM1 experiment on the Solar Maximum Mission (launched in 1980). The modulations were later confirmed in the results of the ERB experiment launched on the Nimbus 7 satellite in 1978, and satellite observation of solar irradiance continues today with ACRIM-3 and other satellite measurements. Sunspots in magnetically active regions are cooler and 'darker' than the average photosphere and cause temporary decreases in TSI of as much as 0.3%. Faculae in magnetically active regions are hotter and 'brighter' than the average photosphere and cause temporary increases in TSI.
The net effect during periods of enhanced solar magnetic activity is increased radiant output of the sun because faculae are larger and persist longer than sunspots. Conversely, periods of lower solar magnetic activity and fewer sunspots (such as the Maunder Minimum) may correlate with times of lower terrestrial irradiance from the sun.
There had been some suggestion that variations in the solar diameter might also cause significant variations in output. But recent work, mostly from the Michelson Doppler Imager instrument on SOHO, shows these changes to be small, about 0.001%, much less than the effect of magnetic activity changes (Dziembowski et al., 2001).
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, ground instruments have been calibrated by comparison with high-altitude and orbital instruments. Researchers have combined present readings and factors to adjust historical data. Other proxy data – such as the abundance of cosmogenic isotopes – have been used to infer solar magnetic activity and thus likely brightness. Sunspot activity has been measured using the Wolf number for about 300 years. This index (also known as the Zürich number) uses both the number of sunspots and the number of groups of sunspots to compensate for variations in measurement. A 2003 study by Ilya Usoskin of the University of Oulu, Finland found that sunspots had been more frequent since the 1940s than in the previous 1150 years.
Sunspot numbers over the past 11,400 years have been reconstructed using dendrochronologically dated radiocarbon concentrations. The level of solar activity during the past 70 years is exceptional – the last period of similar magnitude occurred around 9,000 years ago (during the warm Boreal period). The Sun was at a similarly high level of magnetic activity for only ~10% of the past 11,400 years, and almost all of the earlier high-activity periods were shorter than the present episode.
|Oort minimum (see Medieval Warm Period)||1040||1080|
|Medieval maximum (see Medieval Warm Period)||1100||1250|
A list of historical Grand minima of solar activity includes also Grand minima ca. 690 AD, 360 BC, 770 BC, 1390 BC, 2860 BC, 3340 BC, 3500 BC, 3630 BC, 3940 BC, 4230 BC, 4330 BC, 5260 BC, 5460 BC, 5620 BC, 5710 BC, 5990 BC, 6220 BC, 6400 BC, 7040 BC, 7310 BC, 7520 BC, 8220 BC, 9170 BC.
Solar cycles 
The sun undergoes various quasi-periodic changes, the principal one referred to as the solar cycle has an 11-year quasi-period. Only the 11 and closely related 22-year cycles are clear in the observations.
- 11 years: Most obvious is a gradual increase and more rapid decrease of the number of sunspots over a period ranging from 9 to 12 years, called the Schwabe cycle, named after Heinrich Schwabe. Differential rotation of the sun's convection zone (as a function of latitude) consolidates magnetic flux tubes, increases their magnetic field strength and makes them buoyant (see Babcock Model). As they rise through the solar atmosphere they partially block the convective flow of energy, cooling their region of the photosphere, causing 'sunspots'. The Sun's apparent surface, the photosphere, radiates more actively when there are more sunspots. Satellite monitoring of solar luminosity since 1980 has shown there is a direct relationship between the solar activity (sunspot) cycle and luminosity with a solar cycle peak-to-peak amplitude of about 0.1%. Luminosity has also been found to decrease by as much as 0.3% on a 10-day timescale when large groups of sunspots rotate across the Earth's view and increase by as much as 0.05% for up to 6 months due to faculae associated with the large sunspot groups.
- 22 years: Hale cycle, named after George Ellery Hale. The magnetic field of the Sun reverses during each Schwabe cycle, so the magnetic poles return to the same state after two reversals.
Hypothesized cycles 
Periodicity of solar activity with periods longer than the sunspot cycle has been proposed. Some of these proposed longer cycles include:
- 87 years (70–100 years): Gleissberg cycle, named after Wolfgang Gleißberg, is thought to be an amplitude modulation of the 11-year Schwabe Cycle (Sonnett and Finney, 1990), Braun, et al., (2005).
- 210 years: Suess cycle (a.k.a. "de Vries cycle"). Braun, et al., (2005).
- 2,300 years: Hallstatt cycle
- 6000 years (Xapsos and Burke, 2009).
Other patterns have been detected:
- In carbon-14: 105, 131, 232, 385, 504, 805, 2,241 years (Damon and Sonnett, 1991).
- During the Upper Permian 240 million years ago, mineral layers created in the Castile Formation show cycles of 2,500 years.
The sensitivity of climate to cyclical variations in solar forcing will be higher for longer cycles due to the thermal inertia of the oceans, which acts to damp high frequencies. Using a phenomenological approach, Scafetta and West (2005) found that the climate is 1.5 times as sensitive to 22-year cyclical forcing relative to 11-year cyclical forcing, and that the thermal inertia of the oceans induces a lag of approximately 2.2 (± 2) years in cyclic climate response in the temperature data.
Predictions based on patterns 
- Perry and Hsu (2000) proposed a simple model based on emulating harmonics by multiplying the basic 11-year cycle by powers of 2, which produced results similar to Holocene behavior. Extrapolation suggests a gradual cooling during the next few centuries with intermittent minor warmups and a return to near Little Ice Age conditions within the next 500 years. This cool period then may be followed approximately 1,500 years from now by a return to altithermal conditions similar to the previous Holocene Maximum.
- There is weak evidence for a quasi-periodic variation in the sunspot cycle amplitudes with a period of about 90 years ("Gleisberg cycle"). These characteristics indicate that the next solar cycle should have a maximum smoothed sunspot number of about 145±30 in 2010 while the following cycle should have a maximum of about 70±30 in 2023.
- Because carbon-14 cycles are quasi periodic, Damon and Sonett (1989) predict future climate:
Solar irradiance and insolation are measures of the amount of sunlight that reaches the Earth. The equipment used might measure optical brightness, total radiation, or radiation in various frequencies. Historical estimates use various measurements and proxies.
|Cycle length||Cycle name||Last positive
|232||--?--||AD 1922 (cool)||AD 2038|
|208||Suess||AD 1898 (cool)||AD 2210|
|88||Gleisberg||AD 1986 (cool)||AD 2030|
Solar irradiance of Earth and its surface 
There are two common meanings:
- the radiation reaching the upper atmosphere
- the radiation reaching some point within the atmosphere, including the surface.
Various gases within the atmosphere absorb some solar radiation at different wavelengths, and clouds and dust also affect it. Measurements above the atmosphere are needed to determine variations in solar output, to avoid the confounding effects of changes within the atmosphere. There is some evidence that sunshine at the Earth's surface has been decreasing in the last 50 years (see global dimming) possibly caused by increased atmospheric pollution, whilst over roughly the same timespan solar output has been nearly constant.
Milankovitch cycle variations 
Some variations in insolation are not due to solar changes but rather due to the Earth moving closer or further from the Sun, or changes in the latitudinal distribution of radiation. These have caused variations of as much as 25% (locally; global average changes are much smaller) in solar insolation over long periods. The most recent significant event was an axial tilt of 24° during boreal summer at near the time of the Holocene climatic optimum.
Solar interactions with Earth 
There are several hypotheses for how solar variations may affect Earth. Some variations, such as changes in the size of the Sun, are presently only of interest in the field of astronomy.
Changes in total irradiance 
- Total solar irradiance changes slowly on decadal and longer timescales.
- The variation during recent solar magnetic activity cycles has been about 0.1% (peak-to-peak).
- Variations corresponding to solar changes with periods of 9–13, 18–25, and >100 years have been detected in sea-surface temperatures.
- In contrast to older reconstructions, most recent reconstructions of total solar irradiance point to an only small increase of only about 0.05% to 0.1% between Maunder Minimum and the present.
- Different composite reconstructions of total solar irradiance observations by satellites show different trends since 1980; see the global warming section below.
Changes in ultraviolet irradiance 
- Ultraviolet irradiance (EUV) varies by approximately 1.5 percent from solar maxima to minima, for 200 to 300 nm UV.
- Energy changes in the UV wavelengths involved in production and loss of ozone have atmospheric effects.
- The 30 hPa atmospheric pressure level has changed height in phase with solar activity during the last 4 solar cycles.
- UV irradiance increase causes higher ozone production, leading to stratospheric heating and to poleward displacements in the stratospheric and tropospheric wind systems.
- A proxy study estimates that UV has increased by 3.0% since the Maunder Minimum.
Changes in the solar wind and the Sun's magnetic flux 
|This section does not cite any references or sources. (August 2012)|
- A more active solar wind and stronger magnetic field reduces the cosmic rays striking the Earth's atmosphere.
- Variations in the solar wind affect the size and intensity of the heliosphere, the volume larger than the Solar System filled with solar wind particles.
- Cosmogenic production of 14C, 10Be and 36Cl show changes tied to solar activity.
- Cosmic ray ionization in the upper atmosphere does change, but significant effects are not obvious.
- As the solar coronal-source magnetic flux doubled during the past century, the cosmic-ray flux has decreased by about 15%.
- The Sun's total magnetic flux rose by a factor of 1.41 from 1964–1996 and by a factor of 2.3 since 1901.
Effects on clouds 
Changes in ionization affect the abundance of aerosols that serve as the nuclei of condensation for cloud formation. During periods of low solar activity (during solar minima), more cosmic rays reach Earth, potentially creating ultra-small aerosol particles which are precursors to cloud condensation nuclei. Clouds formed from greater amounts of condensation nuclei are brighter, longer lived, and likely to produce less precipitation. It has been speculated that a change in cosmic rays could cause an increase in certain types of clouds, affecting Earth's albedo.
- Galactic cosmic rays have been hypothesized to affect formation of clouds through possible effects on production of cloud condensation nuclei.
- Several percent variation in cosmic rays and in tropospheric ionization occurs when the interplanetary magnetic field changes over the solar cycle, greater than the typically 0.1% variation in total solar irradiance meanwhile.
- Particularly at high latitudes where the shielding effect of Earth's magnetic field is less, some studies suggest cosmic ray variation may impact terrestrial low altitude cloud cover (unlike a lack of correlation with high altitude clouds), making such partially influenced by the solar-driven interplanetary magnetic field (as well as passage through the galactic arms over longer timeframes).
Other effects due to solar variation 
Interaction of solar particles, the solar magnetic field, and the Earth's magnetic field, cause variations in the particle and electromagnetic fields at the surface of the planet. Extreme solar events can affect electrical devices. Weakening of the Sun's magnetic field is believed to increase the number of interstellar cosmic rays which reach Earth's atmosphere, altering the types of particles reaching the surface.
Geomagnetic effects 
The Earth's polar aurorae are visual displays created by interactions between the solar wind, the solar magnetosphere, the Earth's magnetic field, and the Earth's atmosphere. Variations in any of these affect aurora displays. Solar coronal mass ejections, associated with high solar activity, will produce enhanced auroral activity, and visible aurorae at lower latitudes than usual.
Sudden changes can cause the intense disturbances in the Earth's magnetic fields which are called geomagnetic storms.
Solar proton events 
Energetic protons can reach Earth within 30 minutes of a major flare's peak. During such a solar proton event, Earth is showered in energetic solar particles (primarily protons) released from the flare site. Some of these particles spiral down Earth's magnetic field lines, penetrating the upper layers of our atmosphere where they produce additional ionization and may produce a significant increase in the radiation environment.
Galactic cosmic rays 
An increase in solar activity (more sunspots) is accompanied by an increase in the "solar wind," which is an outflow of ionized particles, mostly protons and electrons, from the sun. The Earth's geomagnetic field, the solar wind, and the solar magnetic field deflect galactic cosmic rays (GCR). A decrease in solar activity increases the GCR penetration of the troposphere and stratosphere. GCR particles are the primary source of ionization in the troposphere above 1 km (below 1 km, radon is a dominant source of ionization in many areas).
Levels of GCRs have been indirectly recorded by their influence on the production of carbon-14 and beryllium-10. The Hallstatt solar cycle length of approximately 2300 years is reflected by climatic Dansgaard-Oeschger events. The 80–90-year solar Gleissberg cycles appear to vary in length depending upon the lengths of the concurrent 11-year solar cycles, and there also appear to be similar climate patterns occurring on this time scale.
Carbon-14 production 
The production of carbon-14 (radiocarbon: 14C) also is related to solar activity. Carbon-14 is produced in the upper atmosphere when cosmic ray bombardment of atmospheric nitrogen (14N) induces the Nitrogen to undergo β+ decay, thus transforming into an unusual isotope of carbon with an atomic weight of 14 rather than the more common 12. Because cosmic rays are partially excluded from the Solar System by the outward sweep of magnetic fields in the solar wind, increased solar activity results in a reduction of cosmic rays reaching the Earth's atmosphere and thus reduces 14C production. Thus the cosmic ray intensity and carbon-14 production vary inversely to the general level of solar activity.
Therefore, the atmospheric 14C concentration is lower during sunspot maxima and higher during sunspot minima. By measuring the captured 14C in wood and counting tree rings, production of radiocarbon relative to recent wood can be measured and dated. A reconstruction of the past 10,000 years shows that the 14C production was much higher during the mid-Holocene 7,000 years ago and decreased until 1,000 years ago. In addition to variations in solar activity, the long term trends in carbon-14 production are influenced by changes in the Earth's geomagnetic field and by changes in carbon cycling within the biosphere (particularly those associated with changes in the extent of vegetation since the last ice age).
Solar variation and climate 
Both long-term and short-term variations in solar activity are hypothesized to affect global climate, but it has proven extremely challenging to directly quantify the link between solar variation and the earth's climate. The topic continues to be a subject of active study.
As discussed above, there are three suggested mechanisms by which solar variations may have an effect on climate:
- Solar irradiance changes directly affecting the climate ("Radiative forcing"). This is generally considered to be a minor effect, as the amplitudes of the variations in solar irradiance are much too small to have significant effect absent some amplification process.
- Variations in the ultraviolet component. The UV component varies by more than the total, so if UV were for some (as yet unknown) reason having a disproportionate effect, this might explain a larger solar signal in climate.
- Effects mediated by changes in cosmic rays (which are affected by the solar wind) such as changes in cloud cover.
Early research attempted to find a correlation between weather and sunspot activity, mostly without notable success. Later research has concentrated more on correlating solar activity with global temperature.
Crucial to the understanding of possible solar impact on terrestrial climate is accurate measurement of solar forcing. Unfortunately accurate measurement of incident solar radiation is only available since the satellite era, and even that is open to dispute: different groups find different values, due to different methods of cross-calibrating measurements taken by instruments with different spectral sensitivity. Scafetta and Willson found significant variations of solar luminosity between 1980 and 2000. But Lockwood and Frohlich find that solar forcing has declined since 1987.
Effect on global warming 
The Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR) concluded that the measured magnitude of recent solar variation is much smaller than the amplification effect due to greenhouse gases but acknowledges in the same report that there is a low level of scientific understanding with respect to solar variation.
Estimates of long-term solar irradiance changes have decreased since the TAR. However, empirical results of detectable tropospheric changes have strengthened the evidence for solar forcing of climate change. The most likely mechanism is considered to be some combination of direct forcing by changes in total solar irradiance, and indirect effects of ultraviolet (UV) radiation on the stratosphere. Least certain are indirect effects induced by galactic cosmic rays.
In 2002, Lean et al. stated that while "There is ... growing empirical evidence for the Sun's role in climate change on multiple time scales including the 11-year cycle", "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 ... because the stochastic response increases with the cycle amplitude, not because there is an actual secular irradiance change." They conclude that because of this, "long-term climate change may appear to track the amplitude of the solar activity cycles," but that "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 suggests that general circulation model (GCM) simulations of twentieth century warming may overestimate the role of solar irradiance variability." More recently, a study and review of existing literature published in Nature in September 2006 suggests that the evidence is solidly on the side of solar brightness having relatively little effect on global climate, with little likelihood of significant shifts in solar output over long periods of time. Lockwood and Fröhlich, 2007, find that there "is considerable evidence for solar influence on the Earth's pre-industrial climate and the Sun may well have been a factor in post-industrial climate change in the first half of the last century," but that "over the past 20 years, all the trends in the Sun that could have had an influence on the Earth's climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures."
A paper by Benestad and Schmidt concludes that "the most likely contribution from solar forcing a global warming is 7 ± 1% for the 20th century and is negligible for warming since 1980." This paper disagrees with the conclusions of a Scafetta and West study, who claim that solar variability has a significant effect on climate forcing. Based on correlations between specific climate and solar forcing reconstructions, they argue that a "realistic climate scenario is the one described by a large preindustrial secular variability (e.g., the paleoclimate temperature reconstruction by Moberg et al.) with the total solar irradiance experiencing low secular variability (as the one shown by Wang et al.). Under this scenario, according to Scafetta and West, the Sun might have contributed 50% of the observed global warming since 1900. Stott et al. estimate that the residual effects of the prolonged high solar activity during the last 30 years account for between 16% and 36% of warming from 1950 to 1999.
Solar variation theory 
A 1997 U.S. National Academy of Sciences study concluded that variations in total solar irradiance (TSI) were the most likely cause of significant climate change in the pre-industrial era, before significant human-generated carbon dioxide was put into the atmosphere.
A 2007 paper by Scafetta and West correlating solar proxy data and lower tropospheric temperature for the preindustrial era, before significant anthropogenic greenhouse forcing, suggested that TSI variations may have contributed to 50% of the global warming observed between 1900 and 2000 (although they conclude "our estimates about the solar effect on climate might be overestimated and should be considered as an upper limit.") This contrasts with the results from global circulation models that predict solar forcing of climate through direct radiative forcing is too small to explain a significant contribution. The relative significance of solar variability and other forcings of climate change during the industrial era is an area of ongoing research.
In 2000, Peter Stott and other researchers at the Hadley Centre in the United Kingdom published a paper in which they reported on the most comprehensive model simulations to date of the climate of the 20th century. Their study looked at both "natural forcing agents" (solar variations and volcanic emissions) as well as "anthropogenic forcing" (greenhouse gases and sulphate aerosols). They found that "solar effects may have contributed significantly to the warming in the first half of the century although this result is dependent on the reconstruction of total solar irradiance that is used. In the latter half of the century, we find that anthropogenic increases in greenhouses gases are largely responsible for the observed warming, balanced by some cooling due to anthropogenic sulphate aerosols, with no evidence for significant solar effects." Stott's team found that combining all of these factors enabled them to closely simulate global temperature changes throughout the 20th century. They predicted that continued greenhouse gas emissions would cause additional future temperature increases "at a rate similar to that observed in recent decades". It should be noted that their solar forcing included "spectrally resolved changes in solar irradiance" but not indirect effects mediated through cosmic rays (discussed above and in the following section); these ideas are still being fleshed out. In addition, the study notes "uncertainties in historical forcing" — in other words, past natural forcing may still be having a delayed warming effect, most likely due to the oceans. A graphical representation of the relationship between natural and anthropogenic factors contributing to climate change appears in "Climate Change 2001: The Scientific Basis", a report by the Intergovernmental Panel on Climate Change (IPCC).
Stott's 2003 work mentioned in the model section above largely revised his assessment, and found a significant solar contribution to recent warming, although still smaller (between 16 and 36%) than that of the greenhouse gases.
The sun has been at its strongest over the past 60 years and may now be affecting global temperatures... the brighter sun and higher levels of so-called "greenhouse gases" both contributed to the change in the Earth's temperature, but it was impossible to say which had the greater impact.
Nevertheless, Solanki agrees with the scientific consensus that the marked upswing in temperatures since about 1980 is attributable to human activity.
"Just how large this role [of solar variation] is, must still be investigated, since, according to our latest knowledge on the variations of the solar magnetic field, the significant increase in the Earth's temperature since 1980 is indeed to be ascribed to the greenhouse effect caused by carbon dioxide."
Maunder Minimum 
One historical long-term correlation between solar activity and climate change is the 1645–1715 Maunder minimum, a period of little or no sunspot activity which partially overlapped the "Little Ice Age" during which cold weather prevailed in Europe. The Little Ice Age encompassed roughly the 16th to the 19th centuries It is debated whether the low solar activity caused the cooling, or whether the cooling was caused by other factors.
The Spörer Minimum has also been identified with a significant cooling period between 1460 and 1550. Other indicators of low solar activity during this period are levels of the isotopes carbon-14 and beryllium-10.
On the other hand, in a 2012 paper, Miller et al. link the Little Ice Age to an "unusual 50-year-long episode with four large sulfur-rich explosive eruptions," and notes "large changes in solar irradiance are not required."
Recent research had suggested that a new 90-year Maunder minimum would result in a reduction of global average temperatures of about 0.3 °C, which would not be enough to offset the ongoing and forecasted average global temperature increase due to increased forcing from rising levels of carbon dioxide (generally referred to as global warming).
Correlations to solar cycle length 
In 1991, Eigil Friis-Christensen and Knud Lassen of the Danish Meteorological Institute in Copenhagen claimed to see a strong correlation of the length of the solar cycle with temperature changes throughout the northern hemisphere. Initially, they used sunspot and temperature measurements from 1861 to 1989, but later found that climate records dating back four centuries supported their findings. They reported that the relationship appeared to account for nearly 80 per cent of the measured temperature changes over this period.
Although correlations often can be found, the mechanism behind these correlations is a matter of speculation. In a 2003 paper "Solar activity and terrestrial climate: an analysis of some purported correlations" Peter Laut demonstrates problems with some of these correlation analyses. Damon and Laut report in Eos that
the apparent strong correlations displayed on these graphs have been obtained by incorrect handling of the physical data. The graphs are still widely referred to in the literature, and their misleading character has not yet been generally recognized.
Damon and Laut stated that when the graphs are corrected for filtering errors, the sensational agreement with the recent global warming, which drew worldwide attention, has totally disappeared.
On 6 May 2000, New Scientist magazine reported that Lassen and astrophysicist Peter Thejll had updated Friis-Christensen and Lassen's 1991 research (which originally only went to 1989) and found that while the solar cycle still accounts for about half the temperature rise since 1900, it fails to explain a rise of 0.4 °C since 1980. "The curves diverge after 1980," Thejll said, "and it's a startlingly large deviation. Something else is acting on the climate.... It has the fingerprints of the greenhouse effect." Likewise a 2005 review by Benestad found that the solar cycle length does not follow Earth's global mean surface temperature.
Solar variation and weather 
There are some suggestions that there may also be regional climate impacts due to the solar activity. Measurements from NASA's Solar Radiation and Climate Experiment show that solar UV output is more variable than the total solar irradiance. Climate modelling suggests that low solar activity may result in, for example, colder winters in the US and southern Europe and warmer winters in Canada and northern Europe, with little change in globally averaged temperature. More broadly, links have been suggested between solar cycles, global climate and events like El Nino. In other research, Daniel J. Hancock and Douglas N. Yarger found "statistically significant relationships between the double [~21-year] sunspot cycle and the 'January thaw' phenomenon along the East Coast and between the double sunspot cycle and 'drought' (June temperature and precipitation) in the Midwest."
Recent research at CERN's CLOUD facility examined links between cosmic rays and cloud condensation nuclei, demonstrating the effect of high-energy particulate radiation in nucleating aerosol particles which are precursors to cloud condensation nuclei. Dr. Jasper Kirby, a team leader at CLOUD, said, "At the moment, it [the experiment] actually says nothing about a possible cosmic-ray effect on clouds and climate, but it's a very important first step."
1983–1994 data from the International Satellite Cloud Climatology Project (ISCCP) showed that global low cloud formation was highly correlated with galactic cosmic ray (GCR) flux; subsequent to this period, the correlation breaks down. Changes of 3–4% in cloudiness and concurrent changes in cloud top temperatures have been correlated to the 11 and 22-year solar (sunspot) cycles, with increased GCR levels during "antiparallel" cycles. Global average cloud cover change has been found to be 1.5–2%. Several studies of GCR and cloud cover variations have found positive correlation at latitudes greater than 50° and negative correlation at lower latitudes. However, not all scientists accept this correlation as statistically significant, and some that do attribute it to other solar variability (e.g. UV or total irradiance variations) rather than directly to GCR changes. Difficulties in interpreting such correlations include the fact that many aspects of solar variability change at similar times, and some climate systems have delayed responses.
Historical perspective 
Physicist and historian Spencer R. Weart in The Discovery of Global Warming (2003) writes:
The study of [sun spot] cycles was generally popular through the first half of the century. Governments had collected a lot of weather data to play with and inevitably people found correlations between sun spot cycles and select weather patterns. If rainfall in England didn't fit the cycle, maybe storminess in New England would. Respected scientists and enthusiastic amateurs insisted they had found patterns reliable enough to make predictions. Sooner or later though every prediction failed. An example was a highly credible forecast of a dry spell in Africa during the sunspot minimum of the early 1930s. When the period turned out to be wet, a meteorologist later recalled "the subject of sunspots and weather relationships fell into dispute, especially among British meteorologists who witnessed the discomfiture of some of their most respected superiors." Even in the 1960s he said, "For a young [climate] researcher to entertain any statement of sun-weather relationships was to brand oneself a crank."
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- Solanki, S.K.; Fligge, M. (2000). "Reconstruction of past solar irradiance". Space Science Review 94 (1/2): 127–38. doi:10.1023/A:1026754803423.
- Reid, George C. (1995). "The sun-climate question: Is there a real connection?". Rev. Geophys. 33 (Suppl).[dead link] Aeronomy Laboratory, NOAA/ERL, Boulder, Colorado. U.S. National Report to IUGG, 1991–1994
- Recent Total Solar Irradiance data updated every Monday