The Earth’s climate changes over time. This record of he Earth’s global average temperature over the past 3 million years was reconstructed from proxy data (oxygen isotope ratios from deep-sea sediment cores). It shows oscillations with a period of approximately 40 thousand years until about 1 million years before the present and a period of approximately 100 thousand years during the last 1 million years. Explaining the mechanism behind this periodicity and the change in the periodicity are among the major topics of research in climate mathematics.