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PETM: a sudden hothouse
56 million years ago· Extinction or crisis· Earth, climate & extinctions
Well establishedStrong fossil, genetic or geological evidence.
A sudden global warming spike 56 million years ago, driven by a massive release of carbon into the air and oceans.
See it on the Life map →What happened
The Paleocene–Eocene Thermal Maximum (PETM) warmed the planet by roughly 5–8 °C over perhaps 3,000–20,000 years (estimates vary). The carbon likely came from volcanic heat cooking buried organic-rich sediments in the North Atlantic. Oceans acidified, and 30–50% of deep-sea foraminifera (tiny shelled organisms on the seafloor) died out. On land, early primates and hoofed mammals spread across the northern continents, and some mammals shrank in size. The climate took roughly 150,000–200,000 years to recover.
Why it matters
It is the best ancient parallel for today's carbon emissions, showing how fast warming and acidification reshape life, and how long recovery takes.
The evidence (fossils, genes, rocks)
A sharp drop in heavy carbon in seafloor sediments and fossil soils, chemical temperature signals in plankton shells, dissolved-carbonate layers in ocean cores and mammal fossils from Wyoming's Bighorn Basin.
Still debated
The carbon source (volcanoes, methane ice, burning peat or several) and the release speed are debated; most estimates say it was slower than today's emissions.
Connections (2)
▸PETM: a sudden hothouse spurred the spread of Mammals radiate after the dinosaursStrong evidence
The connection
During the PETM's warmth, forests extended toward the poles, opening routes across Arctic land bridges between Asia, Europe and North America. Early primates, even-toed hoofed mammals (ancestors of deer and pigs) and odd-toed ones (ancestors of horses) appeared across the northern continents geologically almost at once. Some mammals also shrank as they adapted to the heat.
When and how
Northern continents, about 56 million years ago; Wyoming's Bighorn Basin holds the clearest record, studied by Philip Gingerich and others.
How sure are we?
In the Bighorn Basin these groups first appear exactly at the carbon-isotope shift marking the PETM; early horse Sifrhippus shrank by about 30%, then grew back.
Debates and open questions
Where these groups came from, probably Asia, and whether the heat itself drove their spread or simply opened migration routes is debated.
▸PETM: a sudden hothouse is a past model for Human-caused climate changeStrong evidence
The connection
About 56 million years ago, thousands of billions of tonnes of carbon entered air and oceans within a few thousand years, warming the planet roughly 5–8°C and acidifying seas. Scientists use it to gauge how Earth responds to a carbon pulse. Today's emissions are roughly ten times faster, though the total released so far is smaller.
When and how
Discovered in 1991 by James Kennett and Lowell Stott in deep-sea cores from off Antarctica; since studied in ocean cores and rocks such as Wyoming's Bighorn Basin.
How sure are we?
A sharp drop in carbon-13 in shells and soils marks the carbon release; oxygen isotopes record warming; dissolved seafloor carbonate shows acidification.
Debates and open questions
Where the carbon came from (volcanic heating of buried sediments, methane ice, thawing permafrost) and exactly how its pace compares with today's.