If we want to know what to expect from our climate as it continues to warm over the next few centuries, looking at similar examples of climate change in Earth’s past would be helpful. But there certainly haven’t been any similar temperature excursions in the instrumental record. Using indirect measures, we can tell that there probably haven’t been any since the last ice age. Even the exit from that ice age isn’t especially relevant; while the planet warmed considerably, it was driven by a complicated mixture of orbital changes, greenhouse gases, and melting ice.
To find a sudden warming that’s driven entirely by greenhouse gases, you have to go back 56 million years to the Paleocene-Eocene Thermal Maximum (PETM). At the start of the PETM, a geologically sudden surge of carbon dioxide into the atmosphere caused warming and a large change in the ocean’s pH. It took well over 100,000 years for conditions to return to anything normal. During that time, the extinction rate rose, and many ecosystems were disrupted or shifted by thousands of miles.
But understanding the PETM has proven a challenge, as it’s not clear how much carbon entered the atmosphere or where it came from. A new paper in today’s issue of Nature takes existing information about carbon dioxide levels and isotope ratios and combines them with data on the amount of carbon that dissolved into the oceans. The results provide a new indication of how much carbon entered the atmosphere—10,000 gigatonnes—and suggests volcanoes put it there.