CLIMATE

The Cretaceous Thermal Maximum

~94 to 83 million years ago

Phanerozoic · Mesozoic · Cretaceous

The Cretaceous thermal maximum, between 94 and 83 million years ago, is the warmest interval of the last half billion years, when the Earth had no permanent ice and sea level was the highest of that half billion years.

The Cretaceous thermal maximum is an interval of more than ten million years during which the Earth was warmer than at any other time in the last half billion years. No permanent ice existed anywhere, and sea level was the highest of that half billion years. The latitudinal temperature gradient is the difference in temperature between the tropics and the higher latitudes, those nearer the poles. It was under fourteen degrees, smaller than today's gradient of about twenty degrees. This difference in temperature drives the winds, so a reduced gradient means weaker atmospheric circulation. Relatives of crocodiles lived close to the Arctic.

Why it matters

No later period has been as warm as the Cretaceous thermal maximum. It has become the reference case for any climate model, a computer simulation of climate, of a high-carbon future. The measurements, however, indicate temperatures above those that the models produce. The best-measured warm period available is therefore one that the models cannot yet reproduce.

Dating & uncertaintyDisputed

Two methods place the thermal maximum in the Cenomanian and the Turonian, two successive ages of the Cretaceous. One uses organic thermometers, molecules whose composition varies with temperature. The other uses oxygen isotopes, the variants of the element oxygen. Researchers contest the temperatures: sub-Antarctic surface waters above 27 degrees, and tropical shells that indicate 41 to 45 degrees, exceed what climate models produce. Models match the measurements only if very heavy rainfall is assumed at high latitudes.

Earth history as one day

23:30:1129 min 49 s before midnight

if Earth’s 4.54 billion years were compressed into a single day