No Leader to Follow Changes in the concentration of atmospheric CO 2 and surface air temperature are closely related. However, temperature can influence atmospheric CO 2 as well as be influenced by it. Studies of polar ice cores have concluded that temperature increases during periods of rapid warming have preceded increases in CO 2 by hundreds of years. Parrenin et al. (p. 1060 ; see the Perspective by Brook ) present a revised age scale for the atmospheric component of Antarctic ice cores, based on the isotopic composition of the N 2 that they contain, and suggest that temperature and CO 2 changed synchronously over four intervals of rapid warming during the last deglaciation.