Ancient global warming shows the limits of our knowledge

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The scenario should sound familiar. The amount of carbon in the oceans and atmosphere changes suddenly and dramatically. The oceans are acidified and significant extinctions result. On land, global temperatures increase anywhere from five to nine degrees Celsius, causing widespread habitat disruptions. Despite the sudden onset of the event, its impact lingers for 100,000 years.

This might sound like a worst-case situation for the current anthropogenic influences on climate, but it's actually an historic event that the public is generally unaware of: the Paleocene-Eocene Thermal Maximum, or PETM, which occurred about 55 million years ago.

If there's a gap between the scientific community and the public when it comes to climate change, the PETM is pretty illustrative. While portions of the public are still arguing over whether changes in atmospheric carbon dioxide are likely to do anything bad to us, the scientific community has concluded that, based on everything we know, the sudden change in carbon was a significant contributor to the rise in global temperatures and drop in oceanic pH that occurred during the PETM.

That said, there are some significant uncertainties regarding the precise events that triggered the PETM. The change in the carbon cycle is indicated by a sudden change in isotope ratios; lighter-weight 12C became much more common, which suggests a biological source, but that's consistent with both deep ocean sediments and methane clathrates. More significantly, although we do know there was a sudden influx of carbon, we don't know how much in total wound up in the oceans and atmosphere.

That latter issue is where a new publication in Nature Geoscience comes in, as the authors were able to make some estimates of the amount of carbon involved in the PETM.

The authors relied on the fact that deep ocean waters, being colder, can dissolve more CO2 and get more acidic. As a result, below a certain depth, calcium carbonate from the shells of dead organisms dissolves back out of sediments. By using sediment cores from several sites, they were able track how that depth changed in response to the ocean acidification that occurred during the PETM. Since the additional carbon induced this acidification, the results put some constraints on how much carbon was involved.

In the end, the numbers they came up with indicated that the amount of carbon in circulation went up to about 1.7 times the level just prior to the PETM. Using the best estimates of the atmospheric carbon levels at that time, they calculate that 3,000 Petagrams of carbon were involved. That took atmospheric CO2 levels from 1,000 parts-per-million up to 1,700 ppm (it was much warmer during the Paleocene than it is now, so atmospheric CO2 was likely to be higher). In contrast, human activity has taken current levels from 280ppm to 390ppm.

Estimates of human-driven climate change rely on a climate sensitivity where doubling the atmospheric CO2 will drive somewhere between 1.5°C and 4.5°C of warming. Based on those numbers, the influx of carbon during the PETM would cause, at most, about 3.5°C of the 5-9°C change that occurred during the PETM.

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