A paper led by the fosterlab’s Dr. Eleni Anagnostou and featuring Prof. Gavin Foster is published today in Nature (here). It represents the latest in our efforts to use the geological record to provide information about how the Earth’s climate system operates when significantly warmer than today (e.g. see here). It was work funded as part of the “descent into the icehouse” project that involved researchers from a number of UK universities, and is part of the NERC life and planet theme.
Here is the press release from the University of Southampton. And here is a short video that discusses the wider project:
Our principle finding in this new paper is that atmospheric CO2 was around 1400 ppm during the warmest part of the Eocene and approximately halved through the Eocene. We determined this by using boron isotopes.
Last year Dr. Gordon Inglis from Bristol who is also involved in this work published this paper that better demonstrated that: (a) the early Eocene (~53 million years ago) was warm (the best estimates, here, are at +14 oC compared to the pre-industrial); (b) there was around 8 oC of cooling for the high latitude surface ocean and around 2 oC of low latitude cooling through the Eocene (from ~53 to ~34 million years ago).
We know from earlier work that some of the warmth of the Early Eocene was caused by changes in the position of the continents, vegetation change, and the lack of any ice sheets on the South or North poles. This leaves around 9 oC of the early Eocene warmth to be explained. CO2 is a potent greenhouse gas (see here for example; ) and when its concentration is doubled in the atmosphere (www.ipcc.ch) the Earth should warm by 1.5 to 4.5 oC. Levels of CO2 of 1400 ppm is ~5 times the pre-industrial concentration of atmospheric CO2. This means, if the Earth’s climate system really behaves like we think, the early Eocene should be between 4 and 11 oC warmer than the pre-industrial (when CO2 was 280 ppm). The 9 oC warmth of the early Eocene is thus entirely explained by the enhanced greenhouse effect due to the higher CO2 at the time. This finding also supports our earlier work that suggests the sensitivity of the climate system to forcing from CO2 doesn’t depend hugely on climate state.
We also know from previous work that surface temperature in high latitude regions tend to change more than the global mean, and low latitude regions tend to exhibit a more muted change than the mean. Given the nature of this “polar amplification”, we further demonstrated in this new paper that the CO2 drop through the Eocene (from 53 to 34 million years ago) was sufficient to drive the observed (2 oC low latitude and 8 oC high latitude) cooling in sea surface temperature.
These two findings (that CO2 drove much of the warmth of the Eocene, and its decline drove much of the cooling through the Eocene) confirm that not only are the IPCC estimates of climate sensitivity consistent with the geological record, but that CO2 change was a major player in driving the switch between the Cretaceous/early Cenozoic greenhouse climate state to the late Cenozoic ice house climate we currently find ourselves. The next question of course, and one we are working on, is why did CO2 decline through the Eocene? A recent paper in Science by MacKenzie et al. suggest it’s all about volcanoes and their emissions of CO2. Whatever the cause, its only with CO2 records like we present here we will be able to provide a deeper understanding of the role of CO2 in natural and anthropogenic climate change.