In the words of the four solar scientists, their findings indicate that "periods of higher solar activity and lower cosmic ray flux tend to be associated with warmer climate, and vice versa," and that "the major part of this correlation is due to similar long-term trends in the data sets." In fact, both the sunspot number and cosmic ray parameters exhibit low-frequency variations that closely match the declining temperatures of the Roman Warm Period-to-Dark Ages Cold Period transition that was in progress at the start of the 1800-year records, the subsequent warming that produced the Medieval Warm Period, the following cooling that led to the Little Ice Age, and the most recent warming that produced the Current Warm Period. Hence, it would appear that the millennial-scale oscillation of Earth's climate that has given us these distinctive multi-century warm and cold periods was likely driven by a similar-scale oscillation of the solar-modulated cosmic ray flux.
This does not rule out human activity as a contributing factor to global climate change, but it does rule it out as the sole factor.
Links to other related articles and extracts can be found here.