Ice Ages Have Waxed and Waned for the Past Million Years

This graph of oxygen isotope ratios compiled from 57 sites around the world shows a clear cyclical pattern of change over tens of thousands of years. This pattern is caused by variations in solar energy.

Graph of benthic oxygen isotope ratios ranging from 3.0 to 5.0 over the last million years. A double-sided arrow on the right indicates that lower numbers are warmer and higher numbers are colder. The data show a cyclical pattern over tens of thousands of years from troughs indicating ice ages to peaks indicating interglacials. — Graph of oxygen isotope ratios over the last million years. Troughs are ice ages, peaks are called "interglacials."