Solar variation Guide, Meaning , Facts, Information and Description
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/m². 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] [1] or about 1 W/m² 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.
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/m² 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 [1] and show various results including Lean et al. (1995) [1]. More recently Lean et al (GRL 2002, [1]) say:
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).
Solar irradiance 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:
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.
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.
Some researchers have correlated solar variation with changes in the Earth's average temperature and climate - sometimes finding an effect, and sometimes not. When effects are found they have tended to be greater than can be explained by direct response to the change in radiative forcing from solar change, so some kind of feedback or amplification mechanism is required. See attribution of recent climate change for a discussion of attribution of causes of current global warming.
Research by Soon and Baliunas presents evidence that variations in solar radiation produced the warming that "put the green in Greenland" and led to a "Little Ice Age" from which the earth has been recovering since 1890.
Spencer Weart wrote:
This is an Article on Solar variation. Page Contains Information, Facts Details or Explanation Guide About Solar variation Sunspots and Solar luminosity
Solar cycles
Solar irradiance of Earth and its surface
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 earths 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.Other effects due to solar variation
Global warming
Historical studies of the importance of solar variations
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