Although the burning of fossil fuels has been humanity's first attempt at changing the concentration of greenhouse gasses in the atmosphere, it's not the first time the planet has experienced something of the sort. About 50 million years ago, the Earth saw a dramatic change in the atmosphere's carbon isotopes, which implies a large influx of carbon dioxide and/or methane with biological origins. This event, the Paleocene-Eocene thermal maximum (PETM), was accompanied by a geologically sudden change in temperatures, major disruptions in ecosystems, and a significant extinction event.

The PETM could potentially inform us about some of the changes that may accompany the current rise in atmospheric carbon concentrations. Unfortunately, reconstructing an event that's 50 million years old has proven pretty challenging; we're still not entirely sure how much carbon was in the atmosphere before the sudden influx, or what the source of the additional carbon was. Nonetheless, a couple of papers published in recent weeks paint a potential picture of what might have taken place, although some significant uncertainties remain.

The first appeared last week in Nature, and it attempts to describe the precise timing of the temperature rise—or, as its authors argue, rises. The paper uses isotope ratio information obtained from tooth enamel preserved in fossils of the time. Since these sorts of data are very variable—they change based on diet and other factors—the authors chose a specific tooth from a single set of related species of mammals to do their sampling. Even so, the data is still very noisy and depends on some significant assumptions; the authors also caution that it may not fully represent global conditions.

Nevertheless, authors see a fairly consistent signal in the data that suggests there was actually a period of warming ahead of the change in carbon isotopes; a second warming followed the influx of carbon. Obviously, greenhouse forces can explain the second rise in temperatures, but the authors are at a bit of a loss to explain to the first. The only thing definitive they say is that, if the first was also caused by increased atmospheric carbon, its source must have matched the isotope ratios of what was already in the atmosphere.

The idea of two bursts of warming fits nicely with the second paper, which appeared in the latest issue of Geology. This paper attempted to model what was going on in the oceans during the PETM (the authors used the UK Met Office HadCM3L model) using a series of different levels of atmospheric carbon dioxide. These started with preindustrial levels (280 parts per million) and went up by multiples of two, topping out at 1680ppm. The model was fed these conditions and run to equilibration, after which the temperature and the ocean's circulation were examined.

Temperature displayed a fairly linear response to the doublings of carbon dioxide levels. In contrast, the ocean circulation showed a dramatic threshold effect. At the lower two levels of CO 2 —280 and 560ppm—nothing much changes with ocean circulation. But, at 1120ppm, a region of ice in the Southern Ocean no longer forms during the winter, which stops the turnover of the oceans in the area. This has some significant consequences, among them the warming of deeper waters in the Southern Atlantic. The authors suggest the warming could be strong enough to destabilize the methane clathrates present in many sediments, causing a massive release of methane into the atmosphere.

Again, there are some significant caveats to this work; other researchers have attempted to model ocean circulation during this period, and not all of them have seen the sort of changes the authors report. If these authors are right, however, then this could easily explain the carbon isotope changes associated with the second period of PETM warming, and nicely explains how the first warming could trigger it. Obviously, that still leaves the question of what caused the first period of warming, although some potential candidates have been proposed.

How much can we say about current conditions based on this? Potentially, very little. Scientists are still trying to pin down how much carbon dioxide was in the atmosphere before the PETM started, and many estimates place that value significantly higher than current levels. 50 million years ago, the continents weren't in the same locations, and no land bridge existed between North and South America, so ocean circulation was dramatically different. Nevertheless, the destabilization of methane clathrates may represent a major tipping point in the Earth's carbon cycle and climate, so the more we know about these events, the better.

Nature, 2010. DOI: 10.1038/nature09441 (About DOIs).

Geology, 2010. DOI: 10.1130/G31184.1 (About DOIs).