20 years of solar irradiance data from satellites

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] (http://www.grida.no/climate/ipcc_tar/wg1/244.htm) [2] (http://www.aip.org/history/climate/solar.htm) 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 and Solar luminosity

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.

Graph showing sunspot numbers over the last several hundred years

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 [3] (http://www.grida.no/climate/ipcc_tar/wg1/244.htm) and show various results including Lean et al. (1995) [4] (http://www.grida.no/climate/ipcc_tar/wg1/fig6-5.htm). More recently Lean et al (GRL 2002, [5] (http://www.agu.org/journals/gl/gl0224/2002GL015880/index.html)) 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.

Solar cycles

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 of Earth and its surface

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:

• 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. 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.

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.

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.

Global warming

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 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 [6] (http://www.grida.no/climate/ipcc_tar/wg1/fig6-6.htm).

Historical studies of the importance of solar variations

Spencer Weart wrote:

"Adding solar variability to the sporadic cooling caused by dust from volcanic eruptions did seem to give a better match to temperature trends over the entire last millennium.... All this proved nothing..." [7] (http://www.jamstec.go.jp/jamstec-e/iugg/htm/abstract/abst/mc07_p/021986) and sourced this to Schneider, Stephen H., and Clifford Mass (1975). "Volcanic Dust, Sunspots, and Temperature Trends." Science 190: 741-46.

See Also

• Global dimming

External links

• The Variable Sun - The American Institute of Physics. http://www.aip.org/history/climate/solar.htm
• Gerrit Lohmann, Norel Rimbu, Mihai Dima (2004). Climate signature of solar irradiance variations: analysis of long-term instrumental, historical, and proxy data (http://www3.interscience.wiley.com/cgi-bin/abstract/109062443/ABSTRACT). 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
• S.K Solanski, M. Fligge (2001) Long-term changes in solar irradiance (http://www.astro.phys.ethz.ch/papers/fligge/solfli_rev.pdf) 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 (http://www.astro.phys.ethz.ch/papers/fligge/solfli_rev.pdf) 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? (http://www.agu.org/revgeophys/reid00/reid00.html) 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