The 20th Century has seen a notable temperature rise, generally attributed to the greenhouse effect of anthropogenic gases, and a future "business as usual" policy is generally believed to be catastrophic. However, significant evidence indicates that the sun plays a major role in climate change. We will review the evidence which proves the existence and quantifies the physical mechanism linking between solar activity and climate—galactic cosmic ray ionization of the atmosphere and its effect on cloud cover. In particular, we will show that the link operates on geological time scales, linking our galactic motion to long term climate variations. We will see that once the link is taken into account, a much more consistent picture for the 20th century global warming is obtained. In it, climate sensitivity is low and future climate change is benign.