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Solar variations are changes in solar activity - or sunspots.
Solar variations are changes in the amount of radiant energy emitted by our Sun. Solar variations are periodic components, the principal one being described as the 11 year solar cycle or Sunspot Cycle, and Aperiodic fluctuations. Small solar variations have been measured from satellites during recent decades. Climate scientists are interested in understanding what sort of effect these solar variations have on the temperature of Earth's atmosphere. As such, solar variations are referred to "solar forcing".
The amount of solar radiation received at the outer surface of Earth's atmosphere varies little from an average value of 1366 watts per square meter (W/m2). The variations in total solar 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 approximately 0.1% or about 1.3 W/m2 peak-to-trough during the 11 year sunspot cycle. 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 2000 years. Solar variation has probably 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. It should be stressed, 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. However, these influences cannot be confirmed, they add, because physical models for such effects are still too poorly developed."
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 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, known as the Maunder Minimum. By the 1800s, 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 millenial-scale chronologies.
Statistical studies that correlate weather and climate with solar activity remain popular. 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: Sunspots.
Sunspots are relatively dark areas on the surface of the Sun where intense magnetic activity inhibits convection and so cools the surface. The number of sunspots correlates with the intensity of solar radiation. The variation is small (of the order of 1 W/m² or 0.1% of the total) and was only established once satellite measurements of solar variation became available in the 1980s. Based on work by Abbot, Foukal et al. (1977) realised that higher values of radiation are associated with more sunspots. Nimbus 7 (launched October 25, 1978) and the Solar Maximum Mission (launched February 14, 1980) detected that because the areas surrounding sunspots are brighter, the overall effect is that more sunspots means a brighter sun.
There had been some suggestion that variations in the solar diameter might cause variations in output. But recent work, mostly from the Michelson Doppler Imager instrument on SOHO, shows these changes to be small, about 0.001% (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.
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 over 8,000 years ago. 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.
Solar variations: Solar cycles.
Solar cycles are cyclic changes in behavior of the Sun. Many possible patterns have been suggested; only the 11 and 22 year cycles are clear in the observations.
The sensitivity of climate to cyclical variations in solar forcing will be higher for longer cycles due to the thermal inertia of the ocean, which acts to damp high frequencies. Scafetta and West (2005) found that the climate was 1.5 times as sensitive to 22 year cyclical forcing relative to 11 year cyclical forcing, and that the thermal inertial induced a lag of approximately 2.2 years in cyclic climate response in the temperature data.
Predictions based on patterns of solar variations.
Solar irradiance of Earth and its surface.
Solar irradiance, or insolation, is the amount of sunlight which reaches the Earth. The equipment used might measure optical brightness, total radiation, or radiation in various frequencies. Historical estimates use various measurements and proxies.
There are two common meanings:
Various gases within the atmosphere absorb some solar radiation at different wavelengths, and clouds and dust also affect it. Hence measurements above the atmosphere are needed to observe variations in solar output, within the confounding effects of changes to the atmosphere. Indeed, 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 relative amount of radiation reaching regions of the Earth. 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 24º during boreal summer at near the time of the Holocene Climatic Optimum.
Milankovitch cycles: Solar interactions with Earth.
There are several ways that 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 solar irradiance.
Changes in ultraviolet irradiance.
Changes in the solar wind and the Sun's magnetic flux.
Solar variation and the effects on clouds.
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. It has been speculated that a change in cosmic rays could cause an increase in certain types of clouds, affecting Earth's Albedo.
Solar variation: 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.
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.
Solar variation: 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 solar magnetic field deflects 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.
Solar variation on cloud effects.
Changes in ionization affect the abundance of aerosols that serve as the nuclei of condensation for cloud formation. As a result, ionization levels potentially affect levels of condensation, low clouds, relative humidity, and Albedo due to clouds. Clouds formed from greater amounts of condensation nuclei are brighter, longer lived, and likely to produce less precipitation. 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 50º 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 such correlations include that many aspects of solar variability change at similar times, and some climate systems have delayed responses.
The production of radiocarbon (carbon-14:14C) also is related to solar activity. Radiocarbon is produced in the upper atmosphere by cosmic ray bombardment of atmospheric nitrogen (14N) with neutrons, which may have come from the atmosphere. Increased solar activity reduces cosmic rays and reduces14C production. Therefore, the14C concentration of the atmosphere is lower during sunspot maxima and higher during sunspot minima. By measuring the captured14C 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 the14C 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: Global warming.
Researchers have correlated solar variation with changes in the Earth's average temperature and climate - sometimes finding an effect, and sometimes not. Researchers who have found an effect include Willie Soon and Sallie Baliunas or Douglass and Clader, Geophysical Research Letters, 2002.
The IPCC questions the magnitude of long-term (last hundred or more years) solar variation in section 6.11 of the TAR and show various results including Lean et al. (1995). However the Lean 1995 value may well be too high: more recently Lean et al (GRL 2002,) say:
More recently, a study and review of existing literature published in Nature in Sept. 2006 suggests that the evidence is solidly on the side of solar brightness having relatively little effect on global climate, and downplays the likelihood of significant shifts in solar output over long periods of time.
Solar variation theory.
Solar variation theory is one attempt to account for global warming. Various hypotheses have been proposed to link terrestrial temperature variations to solar variations. The meteorological community has responded with skepticism, in part because theories of this nature have come and gone over the course of the 20th century.
Sami Solanki, the director of the Max Planck Institute for Solar System Research in Göttingen, Germany said:
Over the last twenty years, however, Solanki agrees with the nearly unanimous scientific consensus that the marked upswing in temperatures is indeed to be attributed to human activity.
Willie Soon and Sallie Baliunas of the Harvard Observatory correlated historical Sunspot counts with temperature proxies. They report that when there are fewer sunspots, the earth cooled (see Maunder Minimum, Little Ice Age) - and that when there are more sunspots the earth warmed (see Medieval Warm Period, though since sunspot numbers were only counted from 1700 the link to the MWP warmth is speculative).
The theories have usually represented one of three types:
Although correlations often can be found, the mechanism behind these correlations is a matter of speculation. Many of these speculative accounts have fared badly over time, and in a paper "Solar activity and terrestrial climate: an analysis of some purported correlations" (J. Atmos. and Solar-Terr. Phy., 2003 p801-812) Peter Laut demonstrates problems with some of the most popular, notably those by Svensmark and by Lassen (below). 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.
In 1991, Knud Lassen of the Danish Meteorological Institute in Copenhagen and his colleague Eigil Friis-Christensen found a strong correlation between the length of the solar cycle and 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. This relationship appeared to account for nearly 80 per cent of the measured temperature changes over this period. Damon and Laut, however, show that when the graphs are corrected for filtering errors, the sensational agreement with the recent global warming, which drew worldwide attention, has totally disappeared. Nevertheless, the authors and other researchers keep presenting the old misleading graph. Note that the prior link to "graph" is one such example of this.
Sallie Baliunas, an astronomer at the Harvard-Smithsonian Center for Astrophysics, has been among the supporters of the theory that changes in the sun "can account for major climate changes on Earth for the past 300 years, including part of the recent surge of global warming."
On May 6, 2000, however, New Scientist magazine reported that Lassen and astrophysicist Peter Thejll had updated Lassen's 1991 research 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 ºC 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."
Later that same year, 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" and not the indirect effects mediated through cosmic rays for which there is still no accepted mechanism - these ideas are still being fleshed out. cosmic rays, Clouds, and Climate, K. S. Carslaw, R. G. Harrison, and J. Kirkby Science Nov 29 2002: 1732-1737 and those articles citing it. 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.
References to Solar variation.
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