Fifty-five million years ago Earth got dramatically hotter. Temperatures rose 5-8ºC (9-14ºF) in – by geologic standards – a very short period in what is known as the Paleocene-Eocene Thermal Maximum (PETM). Dramatic as it was, a new study shows it was still a slow-motion version of what is happening today, with carbon released into the atmosphere at less than a tenth of modern rates. The research indicates most of the ancient emissions came from volcanoes, possibly associated with Iceland’s formation.
Any widely read article about climate change attracts numerous comments noting global temperatures have changed naturally in the past, by those apparently convinced this discredits either human involvement or any dangers. The first is an epic failure of logic, but the second might make sense if the previous temperature spikes occurred at similar rates.
Although scientists have long suspected the forces that drove the PETM took place over 1,000 years this has been difficult to confirm, in part because of uncertainty about what those forces actually were. In the course of her PhD at Columbia University, Laura Haynes may have gone a long way to resolving the doubts.
In Proceedings of the National Academy of Sciences, Haynes and Professor Bärbel Hönisch report the ocean added around 15 quadrillion metric tons of carbon 55.6 million years ago in a series of 4,000-5,000 year pulses.
Since oceanic and atmospheric carbon are in constant interchange, coming into balance relatively quickly, the resulting two-thirds rise in carbon concentrations would have been reflected in heat-trapping gasses.
Although the temperature increase is greater than all but the worst case human-induced scenarios, the timescale is much, much longer, so the rate of warming would have been far slower. “The PETM is not the perfect analog [to today], but it's the closest thing we have,” Haynes said in a statement.
Haynes reached her conclusions using an old tool in a new way. Paleoclimatologists rely heavily on foraminifera. These tiny marine organisms come in species adapted to different temperatures, and their shells settle in vast quantities on the sea floor. Changes in the abundance of warm and cold-adapted varieties provide a crucial record of conditions at the time.
Haynes tested living varieties of foraminifera in seawater with different concentrations of carbon dioxide. She found the greater acidity of high-carbon water reduced boron concentrations in foraminifera shells.
By measuring changing boron abundances Haynes could track carbon dioxide levels in the Paleocene oceans more precisely than previous work has estimated temperature.
Having established the timing of the carbon burst, the authors fingerprinted the carbon of the era. Volcanic CO2 has a different isotopic carbon ratio from the methane frozen on the floor of the ocean, the release of which has been a popular explanation for the PETM.
The foraminifera’s carbon pulse was primarily volcanic, the paper concludes, although rising temperatures probably melted enough methane to represent around 8 percent of the total carbon, creating a modest feedback loop.
Although land creatures had time to adapt, avoiding a mass extinction, many marine species could not cope with the increased acidity, with evidence inhabitants of the deep sea were particularly badly affected.
"If you add carbon slowly, living things can adapt. If you do it very fast, that's a really big problem," said Hönisch, "The past saw some really dire consequences, and that does not bode well for the future.”