Climate Has Varied Significantly in the Past 70 Million Years

The relative amount of two oxygen (O) isotopes, ¹⁸O relative to the more common ¹⁶O, depends on temperature. This ratio can therefore be used as an indicator of average ocean temperature.

A graph measuring the ratio of oxygen isotopes from 5 to 0 and temperature in degrees Celsius from 0 to 12 overlayed on the x-axis. Age in millions of years ago from 0 to 70 on the y-axis. The data show negative temperatures at 0 m-y-a, 1 degree at 10 m-y-a, 5 degrees at 15 m-y-a, 4 degrees at 20 m-y-a, 5 degrees at 25 m-y-a, 1 degree at 30 m-y-a, 6 degrees at 40 m-y-a, 12 degrees at 50 m-y-a, 8 degrees at 60 m-y-a, 9 degrees at 65 m-y-a, 8 degrees at 70 m-y-a. — The graph shows the ratio of oxygen isotopes (expressed as δ) in fossils from ocean sediment cores representing a span of 70 million years. Time is on the vertical axis, which is common in geoscience because younger deposits are formed at the top.

These large swings in ocean temperature, ranging across 12°C, are driven by changes in atmospheric CO₂ concentration.

You may wonder why the present temperature reads below 0 degrees. This is because the conversion of isotope ratio to temperature is based on a model for an ice-free planet and does not describe the past 2 million years with polar ice caps and ice ages.