Records of significant changes in temperature in the past provide the context for understanding the potential magnitude and importance of future changes. The figure on the right shows results from an Antarctic ice core analysis of climate changes over the past 160 thousand years. During interglacial periods (about 130 thousand years ago, and beginning 15 thousand years ago), carbon dioxide concentrations increased, possibly due to the effects of increased temperature on the carbon cycle. Increases in carbon dioxide concentrations over the last century are primarily a result of human activities. The curve below shows the observed changes in the mean global temperature over the last hundred years. Credit: J. Hansen, NASA.