Findings of a new study published in the Nature journal prove that global warming some 66 million years ago – called the Palaeocene-Eocene Thermal Maximum (PETM) – was caused by the release of carbon from sedimentary reservoirs such as frozen methane.
During the PETM, atmospheric carbon dioxide more than doubled and global temperatures rose by 5 degrees Celsius, an increase that is comparable to the change that might happen by later next century on today’s Earth.
While there was a large amount of ecological disruption during the PETM, most species were able to avoid extinction through adaptation or migration. However, the rate of carbon addition during the PETM lasted for several thousand years, as outlined in a relation Nature Communications paper by Sandra Kirtland Turner, an assistant professor of earth sciences at the University of California.
Researchers focused on the ratios of different forms – or isotopes – of oxygen, carbon and boron within the shells, the latter of which offers tell-tale clues about the ocean’s acidity, a measure affected by levels of carbon dioxide that dissolved from the atmosphere into the ocean.
The carbon dioxide, researchers note, could either have been pumped directly into the atmosphere through volcanic events or have formed from other carbon sources, such as underwater methane deposits or organic-rich sediments.
However, carbon dioxide from the different sources would have a very different impact on carbon isotope ratios. What’s more, while volcanic eruptions gradually release carbon dioxide over time, gases from methane deposits or sediments are released rapidly.
Drawing these factors together with the level and duration of the increased ocean acidity, the team found that the carbon dioxide was probably released through volcanic eruptions, with such events accounting for up to 90% of the emissions.