There are others who think the solar contribution is much larger. Observe the plot from a 1991 paper in the journal Science Correlation with solar cycle that correlates global temperature with the solar cycle. This explains the ups and owns of the global temperature record without having to rely on CO2 concentrations.
Variations in the length of the solar cycle affect the amount of energy reaching earth from the sun, the primary driver of global temperature. This appears to explain the major features of the temperature record, not just 1/3 of the warming. This correlation has been extended back centuries by others and it provides a pretty good match with the reconstructed global temperature record.
This is my first attempt at a link. The web address is http://solar-center.stanford.edu/sun-on-earth/glob-warm.html in case the link doesn't work.
Solar blow to low cloud could be warming planet
And below is how the IPCC evaluated it. One thing that I think is of interest when you read this (if you choose to) is that it shows how much research is considered by the scientific arm of the IPCC. Many of the criticisms of the IPCC process are of the "politicization" of the science findings when they attempt to translate them into policy recommendations. I agree with much of that criticism. But I also think that the IPCC does a pretty good job of evaluating the state of climate science. Where they've gone overboard is in not stating maximum likelihood parameters for their predictive models, and that allows the media to run wild with the worst-case (and usually quite implausible) scenarios.