New research on the Paleocene - Eocene Thermal Maximum (PETM), which was a short lived extreme global warming event 56 million years ago, is providing clues to future ocean acidification due to man-made climate change. Although not a direct analogue for future change (the continents and mountains were different 56 million years ago, as well as many other things!) it does provide some interesting insight into the how the Earth system responds to a sudden increase in atmospheric carbon dioxide. Probably most worrying is that following the rapid increase in CO2, the oceans remained acidic for 70 000 years. If this isn't worrying enough, the speed of climate change 56 million years ago that led to such devastation was slower than our current human induced climate change.... Not good for marine ecosystems or the planet in general!