One of the most common skeptical arguments against global warming is that the Sun is to blame for our current warming. This argument is presented in various forms and involves varying methods. For many skeptics, it is the primary reason for their disbelief in the idea that humans could be causing global warming. According to them, there's an enormous ball of fire in the solar system, which puts out massive amounts of energy, so no human activity could possibly compete with that.

The trick here is that these skeptics are trying to separate the two competing theories as if they had nothing to do with each other. The problem with this is that the theory of a greenhouse gas induced global warming does not discount the Sun as the primary source of heat for our planet. It is simply recognized that while this may be the case, the Sun's total energy output remains largely constant at 1366 watts per meter squared (W/m^2). There is some slight variation in 11 years solar cycles of sunspot activity (see chart at left), but the energy level varies only 1 W/m^2 during these cycles.

Therefore, any changes in length or strength of these cycles could only result in an increase of a fraction of that amount. The IPCC has assigned a radiative forcing value of only 0.12 W/m^2 to solar irradiance (See "Radiative Forcing Components" chart at bottom). This is not a large enough change in energy to be able to cause a significant increase in global temperatures.

What I think is commonly misunderstood by many skeptics is that the competing theory of greenhouse gases causing a warming effect actually builds upon this same 1366 W/m^2 from the Sun, rather than dismisses it. This energy reaches our planet in the form of high frequency infrared radiation, which is absorbed by the Earth. The planet then attempts to cool off by emitting this energy as low frequency infrared. Some of it escapes into outer space, but a certain portion is absorbed by greenhouse gases in our atmosphere, which release the energy as heat. Therefore, greenhouse gases are only amplifying the constant energy of the Sun, so greenhouse gases can be the cause of an increase in temperature without completely discounting the Sun's role in the process.

Still, many insist on solar activity as the reason for global warming. The most recent, and maybe even most popular, argument in favor of this theory appeared in the controversial documentary, "The Great Global Warming Swindle", which aired a few months ago on Channel 4 in the UK. This film makes the case that solar activity is responsible for global warming, both through direct radiation and the indirect effect of cosmic ray flux:

New evidence shows that as the radiation coming from the sun varies (and sun-spot activity is one way of monitoring this) the earth seems to heat up or cool down. Solar activity very precisely matches the plot of temperature change over the last 100 years. It correlates well with the anomalous post-war temperature dip, when global carbon dioxide levels were rising. In fact, what is known of solar activity over the last several hundred years correlates very well with temperature. This is what some scientists are beginning to believe causes climate change. Others feel that solar activity only explains the fine details of temperature change. So how does the sun affect the earth's temperature? The process scientists suggest is that as earth moves through space, the atmosphere is constantly bombarded by ever-present cosmic rays. As these particles hit water vapour evaporating from the oceans, clouds form in the atmosphere. Clouds shield Earth from some of the sun's radiation and have a cooling effect. When solar activity is high, there is an increase in solar wind and this has the effect of reducing the amount of cosmic radiation which reaches Earth. When less cosmic radiation reaches Earth, fewer clouds form and the full effects of the sun's radiation heats the planet. - Channel 4

Since that documentary aired, many climate scientists complained that it was very misleading. There were even accusations that graphs were manipulated in order to make the arguments featured in the film:

Despite the blatant manipulations of science in the film, many of its arguments have continued to circulate throughout the world of global warming skepticism. The claims in the documentary have actually become prevalent enough that it prompted a new study to analyze these assertions. This new study, published in the Royal Society's journal Proceedings A, "concludes that changes in the Sun's output cannot be causing modern-day climate change":

"This should settle the debate," said Mike Lockwood, from the UK's Rutherford-Appleton Laboratory, who carried out the new analysis together with Claus Froehlich from the World Radiation Center in Switzerland.... "All the graphs they showed stopped in about 1980, and I knew why, because things diverged after that," he told the BBC News website. "You can't just ignore bits of data that you don't like," he said. - BBC News

The recent study disputes both prominent theories that recent warming could be caused by the Sun:

[The scientists] compared temperature and solar data for the past 100 years. The records show that solar activity peaked between 1985 and 1987. Since then, trends in sunshine, sunspot number and cosmic rays have all been in the opposite direction to that required to explain global warming - while temperatures at the Earth's surface rose steadily by more than 0.3C. The two scientists conclude: "Our results show that the observed rapid rise in global mean temperatures seen after 1985 cannot be ascribed to solar variability, whichever mechanism is invoked." In pre-industrial times, Prof Lockwood said, there was considerable evidence that the sun played a significant role in driving global climate, but he was concerned this genuine area of study had been done a "great disservice" by climate sceptics who were trying to confuse people about recent global warming. He said: "I know we're attacking a bit of a strawman here because there is no serious scientific debate about recent warming, but those who disagree are very vocal. We wrote this up specifically to show they are wrong, and wrong in a dangerous way." - Guardian

The bottom line is that solar activity is not a primary driver of our recent warming. As the IPCC shows in it's latest report, greenhouse gases, namely carbon dioxide, are the dominant forcings that cause climate change:

 

This is why it is imperative that we take action to reduce our carbon emissions. This is also why the solar forcing argument is so popular with skeptics - it turns global warming into a natural event that we can't do anything about. However, the opposite is true, and we are getting to the point now where we need to stop fooling with these nonsense arguments and, instead, start making a difference.