It has previously been demonstrated that the mean land air temperature of the Northern Hemisphere could adequately be associated with a long-term variation of solar activity as given by the length of the approximately 11-year solar cycle. In this paper it is shown that the right cause-and-effect ordering, in the sense of Granger causality, is present between the smoothed solar cycle length and the cycle mean of Northern Hemisphere land air temperature for the twentieth century, at the 99% significance level. This indicates the existence of a physical mechanism linking solar activity to climate variations. ¿ 2001 American Geophysical Union