evidence: In our study recently published in Nature, we identified the distinctive chemical fingerprint of this carbon – it pointed not to methane, but to emissions from intense and prolonged volcanic activity. We also show that atmospheric CO₂ levels more than doubled in less than 25,000 years. [...] We found more than 10,000 Gigatonnes of carbon must have been released into the atmosphere by volcanic activity during the PETM, which is an order of magnitude higher than all currently-accessible fossil fuel reserves taken together. [...] We found the PETM was caused by massive carbon emissions from Earth’s interior. It thus has many parallels to today, where we are ratcheting up CO₂ levels in our atmosphere and oceans by burning fossil fuels that have been buried for millions of years. This extra carbon is, in effect, permanent on human timescales. The PETM is giving us an increasingly clearer picture of what Earth will be like if we carry on, and take our planet to places it has not been in at least 56m years.