The climate shock of 56 million years ago bears a striking resemblance to the present: a dramatic increase in temperature in a very short time. Experts have long puzzled over the cause. Global warming, acidifying seas and collapsing ecosystems: What sounds like the near future most likely happened 56 million years ago. In just a few thousand years, a huge amount of greenhouse gases entered the atmosphere. The average global temperature rose by about five degrees. It took more than 100,000 years for the climate to return to normal. The event, called the Paleocene-Eocene Thermal Maximum (PETM), shows striking parallels with human-induced climate change that has fascinated experts for years. But the cause of the extreme geological event is controversial. Carbon isotope data from marine sediments, which show a clear enrichment of lighter carbon ¹² C compared to ¹³ C at the start of the PETM, evidenced by strong greenhouse gas emissions over a short period of time. But which climate-influencing gases were responsible, and where did they come from? (Lars Fischer)