Here is a very mixed bag of reporting from ScienceDaily, when you read this article it says it's the Sun and then says it's not the Sun. What it is trying to say is that they can follow a correlation in the solar activity in the past but are unable to apply the same logic in recent years as the CO2 theory would not work!
The reason they are unable to apply the solar activity in modern times is that it will put a spanner in the works. A higher amount of solar activity since the 1950's has warmed the oceans releasing more CO2 into the atmosphere. This article does not account for this, instead it insists the extra CO2 from man made emissions has driven world temperatures out of control, if this is the case then why are world temperatures falling and CO2 is still rising?
“While changes in the solar activity were a main driver of temperature variations in the pre-industrial period, the temperatures in the Altai have shown a much higher rate of increase than that of solar activity during the past 150 years. The strong increase in the industrial period, however, correlates with the increase in the concentration of the greenhouse gas CO2 over this time.
This article is full of contradictions, but still worth a read.