Do abrupt climate shifts occur as part of a natural cycle? Despite growing evidence that humans affect climate via urbanization and greenhouse gas emissions, the natural climate cycle may have the final say. Research from Dr. Stefan Rahmstorf at Germany`s Potsdam Institute for Climate Impact Research suggests that the earth`s climate is characterized by an extraordinarily regular cycle of about 1470 years. He found that the five most recent cycles had a standard deviation of only 32 years. Rahmstorf examined ice cores from Greenland. Going back before the 20th century, when weather stations were nonexistent or widely dispersed, ice cores...