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 feedback or amplification mechanisms are required.[12] 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 [13]. More recently Lean et al 2002 say that total solar irradiance may also lack significant secular trends.
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.
In 2003, Shaviv and Veizer compared a temperature reconstruction of the last 500 million years to expected changes in cosmic ray flux as the solar system moves around the galaxy. They concluded that, at least over very long time scales, cosmic ray variations had a much larger impact on climate than other processes (such as greenhouse gas changes) [14]. Since, as described above, cosmic rays are also affected solar variations, their work may imply a larger role for solar variability in recent climate change than has previously been appreciated. Also, by looking at the temperature changes not ascribed to cosmic rays, they estimated that the climate response to doubling CO2 is only about 0.75 °C as compared to the 1.5-4.5 °C reported by the IPCC [15]. However, long-term processes occuring over millions of years may make it impossible to interpret of Shaviv & Veizer's results over the short time scales relevant to recent warming. [16].
Solar variation in climate models
Solar forcing 1850-2050 used in a NASA GISS climate model. Recent variation pattern used after 2000.
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 [17] and show various results including Lean et al. (1995) [18]. However the Lean 1995 value may well be too high: more recently Lean et al (GRL 2002, [19]) 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.
In 2003, Stott et al found that "current climate models underestimate the observed climate response to solar forcing over the twentieth century as a whole, indicating that the climate system has a greater sensitivity to solar forcing than do models." [20]
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.
) causing the difference.
