They are called hyperthermals - periods of intense and sudden rises in temperature, lasting tens of thousands of years. And until now they were thought to be rare geological events, dating back to a time when CO2 levels were several times higher than they are now. But in a paper published in Nature, two geologists from Scripps Institution of Oceanography have shown that hyperthermals were a regular occurrence some 50 million years ago.
Richard Norris and Phil Sexton made the discovery by looking sediment cores drilled off of the South American coast. Within the thick layers of green mud, they found slices of gray clay-rich sediment. Analysis showed that these striking clay bands formed because the usual microscopic calcium-rich shells were absent - dissolved away by acidic sea waters.
Image: Sediment samples: Image Credit: Scripps Institution of Oceanography, UC San Diego
It is the acidic nature of the oceans which is the tell-tale signal for a hyperthermal event. When CO2 levels rise in the atmosphere, the oceans soak up atmospheric CO2, forming weak acids. This CO2-driven acidification of the oceans is already under way in our own epoch of global warming - and that same oceanic response in the past coincides with massive rises in temperature - the hyperthermal.
Starting 50 million years ago, these hyperthermal events seem to have been triggered every 400,000 years, and involved temperature rises of 3 F to 5 F (2 C to 3 C) that lasted up to 40,000 years. This cycle appears to have been repeated until 40 million years ago, spanning the transition from the Paleocene to the Eocene epochs. Global temperatures were at their peak (7 F to 12 F higher), during the Paleocene-Eocene Thermal Maximum. At that point hyperthermal events were much stronger, and needed 200,00 years to come down from their high.
The authors believe their research helps pin the cause of hyperthermals on long-term rhythms in the CO2 cycle for that 10 million year period. Other proposed culprits, such as the release of methane hydrates or peat burning, seem unlikely in a time of warm oceanic temperatures. And their regularity rules out another candidate, cometary impacts. Instead, they see a periodic slowdown in the vast turnover of oceanic currents as locking in CO2 into the deep oceans. There is some evidence that oceanic circulation systems can back up and reverse - and this may then have released the stored CO2.
Image of Richard Norris: Credit: Scripps Institution of Oceanography, UC San Diego
Whatever their cause, such hyperthermals have not been seen for 40 million years - until now. The rate of current temperatures rises, caused by mass CO2 released from fossil-fuel burning, may be kicking off the first hyperthermal event in tens of millions of years. This research helps to broaden our understanding of where we may be heading.
Norris said ''These hyperthermals seem not to have been rare events, hence there are lots of ancient examples of global warming on a scale broadly like the expected future warming. In 100 to 300 years, we could produce a signal on Earth that takes tens of thousands of years to equilibrate, judging from the geologic record.''