CLIMATE

The Hottest the World Has Been

~94 to 83 million years ago

Phanerozoic · Mesozoic · Cretaceous

For more than ten million years there was no permanent ice anywhere, the sea stood at its highest level of the last half billion years, and the gap in temperature between the tropics and the poles was about half what it is today. Relatives of crocodiles lived within reach of the Arctic.

Why it matters

This is the upper bound: the warmest sustained state a planet with modern geography and modern plankton is known to have held. Every projection of a high-carbon future is measured against it. The awkward part is that the measurements run hotter than the models will go, and the only way found so far to reconcile them is to assume very heavy rain at high latitudes.

Dating & uncertaintyDisputed

Peak warmth is placed in the Turonian by both organic and oxygen-isotope thermometers. The absolute numbers are contested: sub-Antarctic surface waters above 27 degrees, and tropical shells suggesting 41 to 45 degrees, are warmer than climate models will produce, and can be brought into line only by assuming very heavy rainfall at high latitudes.

Sources

1. O'Brien et al. (2017), Earth-Science Reviews · Cretaceous sea-surface temperature evolution: constraints from TEX86 and planktonic foraminiferal oxygen isotopes
2. Huber et al. (2018), Global and Planetary Change · The rise and fall of the Cretaceous hot greenhouse climate
3. Křížová et al. (2024), Palaeogeography, Palaeoclimatology, Palaeoecology · Late Cretaceous (Cenomanian-Turonian) temperature evolution and biotic response in the Adriatic Carbonate Platform region of Friuli, northeast Italy

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