A reconstruction of the Earth’s climatic history during a key hot period 55 million years ago has highlighted a yawning gap in our understanding: this period’s rise in carbon dioxide accounts for just half of its warming. Some as-yet-unidentified climate feedbacks could be at work, the scientists behind the research conclude.
The era under scrutiny is the Palaeocene-Eocene Thermal Maximum (PETM). Paleoclimatologists believe that the PETM could mimic our own future climate, because it’s thought to have kicked off with a pulse of carbon dioxide roughly equivalent to what humans are currently pumping out by burning fossil fuels. In a study published in Nature Geoscience (subscription), Richard Zeeb of the Universtiy of Hawaii and colleagues make a new, more precise estimate of the PETM’s carbon dioxide release based on ocean sediment records.
The increasing carbon levels caused ocean acidification that dissolved deep-sea carbonate compounds. By using measurements of this process along with a carbon-cycle model, the team inferred that during the period’s initial CO2 spike, no more than 3 billion tonnes of the gas was released over 5,000 years. Even before then, the planet looked like a greenhouse – it had a much warmer climate than today and about 1,000 parts per million CO2 in the atmosphere. Over the main phase of the PETM, the group estimates the CO2 level rose to 1,700 parts per million.
But according to the IPCC’s best guess at climate sensitivity, that 70% rise should have pushed up global temperatures 3.5 degrees Celsius at most. Other proxy records indicate, though, that temperatures soared by 5 to 9 degrees. In other words, the consensus climate sensitivity – the value, devilishly hard to pin down, for how much warming will result from a given greenhouse gas increase – doesn’t seem to be holding.
The authors suggest the extra warming came from mechanisms yet to be taken into account, such as amplification by methane leaking out of swamps. Finding other paleoclimate records that verify the group’s estimates would solidify their argument for mystery feedbacks, writes David Beerling of the University of Sheffield, UK, in an accompanying News and Views article.
Meanwhile, a crucial question is whether these feedbacks lie in wait for us. Beerling says,
The upshot of the study by Zeebe and colleagues is that forecasts of future warming could be severely underestimating the extent of the problem that lies in store for humanity as greenhouse gases accumulate in the atmosphere.