The following graph shows the variation in the output of solar radiant energy over the last 400 years.

http://www.junkscience.com/Greenhouse/irradiance.gif

One important thing to note about this is that the variation in solar output tracks very closely with sunspot activity, temperature changes on Earth and atmospheric carbon dioxide levels. Man-made global warming proponents have argued correctly that the increase in solar irradiance (the amount of light given off by the sun) is not enough to account for temperature changes on Earth. However, this ignores the fact that the solar wind fluctuates in sync with solar irradiance. This is important because the solar wind shields the Earth from high energy radiation from space (coming primarily from within the Milky Way galaxy), which heavily influences cloud formation, which in turn heavily influences global temperatures.

A close relationship has been shown to exist between solar activity and global temperatures over the last thousand years, and GW advocates have been trying to explain it away. Most notably, a 2007 study by Lockwood and Frolich claims to have debunked the idea that solar variations are responsible for significant temperature changes, however Steve McIntyre at Climate Audit points out

Of particular note, the part where they use CLIMAX data for the cosmic ray flux is a red herring, as CLIMAX measures mainly lower energy cosmic ray flux (>3 GV) while the higher energy cosmic rays that correlate with low level clouds are best detected by Huancayo and Haleakala (>13 GV).

The CLIMAX data is swamped by the lower energy cosmic rays which effectively masks the higher energy cosmic ray signal of interest - as an analogy, would anyone here try to pass off a study using IR data to try to discredit what is observed to be happening in the UV band?

You should also note that higher energy cosmic rays are claimed to effect lower level clouds, which is not even examined in the paper.

What ever way you look at it, they are comparing apples with oranges.

The last peak of the sunspot cycle was in 2000. Global temperatures peaked in 1998 and have been on a steady decline ever since.

http://solarscience.msfc.nasa.gov/SunspotCycle.shtml

http://www.junkscience.com/MSU_Temps/UAHMSUglobe.html

Lastly, Professor Richard Lindzen of MIT notes:

http://www.heartland.org/events/newyork09/pdfs/lindzen.pdf

Unlike greenhouse forcing, solar forcing is so vague that one can’t reject it. However, acting as though this is the alternative to greenhouse forcing is asking for trouble simply because it endorses the incorrect paradigm underlying AGW. Remember, we are dealing with a small amount of warming (concentrated in two relatively brief episodes) in an inadequately observed system. The proper null hypothesis is that there was no need whatsoever for external forcing in order to produce such behavior. The unsteady and even turbulent motions of the ocean and atmosphere are forever moving heat from one place to another on time scales from days to centuries, and, in doing so, they leave the system out of equilibrium with the sun leading to fluctuations in temperature. The thought that these turbulent fluctuations demand specific causes is absurd – almost as absurd as calling for specific causes for each whirl in a bubbling brook.