As shown by Figure 2 below, there was essentially no trend in cosmic rays during the 19th century, but a significant decrease during the 20th century, and a slight uptick at the end of the 20th century. Per Svensmark's theory, solar activity controls cosmic rays which in turn affect cloud formation, with cosmic rays inversely related to global temperature. Thus, figure 2 is highly supportive of the Svensmark theory and cause of 20th century warming.
Climate alarmists such as Stefan Rahmstorf of Michael Mann's site RealClimate claim Svensmark's theory is debunked by only cherry-picking the tiny uptick at the end of the 20th century in cosmic ray counts, absurd in comparison to the long-term view.
The post also reviews recent additional papers demonstrating the Sun controls climate, not man-made CO2.