“Solar Variation” describes the change in the Sun’s radiation output. The Sun is fundamentally the source of all energy on the Earth and so regardless of other limiting and even amplifying factors, if the amount of radiation from the Sun is changed then it has a knock on effect on insolation and therefore temperature.
The radiation can vary in both quantity and quality. Quantity simply describing the amount of radiation, but quality also referring to the type within the spectrum ie, the type of ultraviolet light emitted. (Goudie, 1992) Quantity of radiation often varies through the amount of activity seen upon the Sun’s surface, which can manifests as a ‘sunspot’, an isolated area on the Sun’s surface that is cooler and visibly darker. Sunspots are generally twice the diameter of the Earth, they generally form in pairs and are at a significantly cooler temperature of around 4240 kelvins (3,966.85 °C) compared to the regular temperature of the Suns surface at approximately 6000 kelvins (5,726.85°C). Sunspots can form in groups of up to 100 and can last up to 2 months. (Science Week, 2004). The occurrence and prevalence of sunspots take on regular cycles, known as Hale cycles. (Goudie) These Hale cycles have a periodicity of 22 years (Usoskin, 2009). The cycles describe the magnetic intensity upon the Sun, which itself correlates with the Sunspots. The magnetism switches polarity within this 22 year period. The polarity itself does not influence sunspot number and so we can observe them upon an 11 year cycle.
The total energy emitted from the Sun is highest at a sunspot maximum, when the greatest amount of sunspots are present, meaning that similarly the Earth receives slightly more, however between a maximum and a minimum energy output only varies 0.1%, calculated from the 20 years of records that exist on the matter. (Soon & Baliunas, 2003) There have been claims that this minute variation is so small that it is negligible, such as in this documentary by the BBC, especially given the 11 years it takes to make this change. However, there is a more drastic change of the Sun over further past centuries, if we take “proxy data” from Beryllium 10 (which is created when the Sun is in a minimum as its magnetic field is week and so allows more cosmic rays, which are energetic particles, to enter the earth’s atmosphere which creates more Beryllium). This identified a correlation with a time called the Maunder Minimum (1620-1720), else known as the “little ice age”, where the River Thames froze over and temperatures were at a significant low.
The activity of the Sun and the terrestrial temperature do, in fact, correlate until the 1980s. A study by Lev Pustilnik and Gregory Yom Din found that there is significant evidence to suggest that wheat prices in the 17th century correlated to the amount of sunspots, this was repeated in the 20th century and the same result was found. (New Scientist, 2004) It is this break in the trend that the aforementioned BBC documentary commented on. Sunspots have actually fallen in number after 1987 (Lockwood & Frohlich, 2008), yet the temperature since has continued to rise and thus is the area where the argument for anthropogenic global warming can be applied.
The “continued warming” statement is disputed as it fundamentally depends on the scale of time in which you reference. For example, 2007-2008 have been significantly cooler, but yet the consensus is that this is not long enough to identify it as a trend. (Christy, 2008) Furthermore, the current solar minimum was predicted to end in 2007, but has still not until this day. This extensive delay replicates events of the Maunder Minimum. It is the only period which replicates the low levels of sunspots such as we are currently experiencing. Some predict that we could therefore be entering a similar period of sparse sunspot prevalence such as we did from 1880 and 1915 which correlated with a global -0.4°C decrease in temperature, or further a more drastic cooling The Maunder Minimum if the period were to be further elongated. (mi2g, 2010) We can only wait to see who is proven correct with regards to this.
A further argument given in relation to the indirect effect of solar variation. For instance, changing the solar energy might change the oceanic surface temperature, which could increase evaporation and in turn contribute itself to the greenhouse effect. (Soon & Balliunas) Water vapour itself is the most prevalent greenhouse gas in the atmosphere at about 0.3% of the mass of the atmosphere, but only 0.06% for CO2. The radiative forcing for CO2 and water is not equal, however models have shown that water vapour contributes to around 60% of the greenhouse effect. (RealClimate, 2005) Furthermore, we can extend this reasoning to realize that it is not just solar variation that can influence water vapour, but also things such as land use or any other factor that could effect evaporation. However, its important that we understand that it is a feedback system, water vapour will adjust alongside humidity and so if the earth warms for any given reason, water vapour will rise and amplify the effect of warming. It can therefore be deemed a contributing amplifier. Evidence for this effect can be seen from the eruption of Mount Pinatubo, on the island of Luzon. Cooling occured for 3 years from the albedo effect of aerosols (discussed on this site), alongside this was a decrease in the proportion of water vapour. This has correlated with models based upon the feedback effect of water vapour. (RealClimate)
I previously mentioned the change in the quality of the Sun’s radation, which is the type of radiation that it emits. I also stated that the changes in the energy within a sunspot cycle only varies by 0.1%. However, with regard to quality of sunspot, specifically within the UV range of the spectrum this impact can increase from between 1% and 10% (BBC, 2007). The knock-on effect of this is that UV radiation is absorbedf by the Earth’s stratosphere and also produces ozone, which is a greenhouse gas and can therefore be deemed an amplifier of radiation. So without a significant change in solar radiation, changes in the quality of radiation can amplify itself further down the line.
Whilst not dependant on the suns energy emissions itself, it has been suggested that the solar variation that we receive is due to “clouds of interstellar matter”, which is effectively dust. The Earth sometimes passes through these, or even through the spiral dust lanes of the Milky Way, and by being positioned between the Earth and the Sun, this would also effect the warming we receive (Goudie). For instance, if we were to exit a dust lane, the planet would warm. There is one final interstellar factor which I am yet to discuss and that is Milankovitch Cycles, which I examine here.