The great warming
5–8 degrees in 20,000 years — the closest parallel to today
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The Palaeocene–Eocene Thermal Maximum is the most studied rapid climate change in geological history, precisely because it is used as an analogue for today. An estimated 3,000–7,000 gigatonnes of carbon were released, possibly from methane hydrates or from volcanism linked to the opening of the North Atlantic. The decisive difference is rate: the PETM released its carbon over at least 3,000–20,000 years, while humanity is releasing comparable quantities over a couple of centuries — roughly ten times faster. Ocean acidification left a distinct dissolution horizon in deep-sea sediments. Remarkably, it caused no mass extinction on land; primates diversified in the heat.
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An enormous quantity of carbon was released into the air in a short time, and Earth warmed by five to eight degrees. Crocodiles lived at the North Pole, and the ocean turned acidic. It took 150,000 years before the climate returned to where it started.
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Part of Terra Saga — the story of Earth on a timeline. terrasaga.org