In the popular imagination, the ‘age of mammals’ began after the demise of the non-avian dinosaurs at the end of the Cretaceous Period, about 66 million years ago. That was the dawning of the Palaeogene Period (66.0-23.03 million years ago [mya]), which was followed by the Neogene Period (23.03-2.58 mya) then the Quaternary Period (2.58 mya to present day). The Palaeogene Period consisted of three parts or epochs: the Palaeocene (66.0-56.0 mya); the Eocene (56.0-33.9 mya); and the Oligocene (33.9-23.03 mya)1.
One of these epochs, the Eocene2, is the interval in time with which we are concerned here.
It lasted approximately 22 million years and its beginning is marked by a very sharp spike in global temperature, called the Palaeocene-Eocene Thermal Maximum (PETM). At its maximum the global temperature was up to 8-9°C warmer than the 1850-1900 standard2,3. This spike only lasted for about 200,000 years and is associated with a massive injection of carbon dioxide into the atmosphere which may have happened in two pulses, with the first pulse lasting less than 2,000 years. These injections are estimated to have been at a rate of about 0.9 Gigatonnes of carbon dioxide per annum4. For context, currently, humans are adding about 10 Gigatonnes of carbon dioxide per annum to the atmosphere5. If that doesn’t scare you, nothing will.
Some 200,000 years later, the climate cooled again in the early Eocene. Then, over the next few million years the global temperature rose again to a level similar to that during the PETM. This is termed the Eocene climatic optimum, but it was anything but optimum. While it may sound nice to have forests growing in Antarctica with palm trees and crocodiles, and high latitude (60°S) ocean surface temperatures up to 30°C, there were likely torrid dead zones in the tropics too hot for animal or plant life of any sort6. This is what you get when you have an atmosphere with around 1,000 parts per million (ppm) of carbon dioxide. Given that the level of carbon dioxide in the atmosphere has recently passed 400 ppm and we are on target to reach 1,000 ppm by the end of this century, this is very concerning.
Global temperature reconstructions in the Eocene have until now suffered from a mismatch between interpreted sea-surface temperature (SST) and terrestrial (i.e. land) temperature with the latter being up to 10°C less and more consistent with climate modelling simulations. Using a relatively new technique, termed (wait for it!) the branched glycerol dialkyl glycerol tetraether (brGDGT) palaeothermometer, has given an indication of terrestrial temperature 5-10°C higher than most previous estimates, and more in line with SST estimates. These estimates are 10-15°C warmer than those being experienced currently. This organic molecule (brGDGT) comes from bacteria, and its structure is temperature sensitive. The molecule is found in lignites (fossilised peat) and has allowed an estimate of terrestrial temperature to be made based on the sampling and analysis of such lignites7.
The fact that these new terrestrial temperature estimates are in line with the well established SST estimates indicates that they are likely more accurate than previous estimates. However, the fact that these temperature estimates exceed those obtained from climate modelling, indicates that climate modelling is missing something. This could be due to the omission of the generation of methane from climate models. Numerous studies have shown that emissions of methane from peatlands and elsewhere increase significantly with increasing temperature. As methane is a very potent greenhouse gas, this could indicate the presence of a significant positive feedback which will exacerbate the global temperature increase7. If that is so, we are in much deeper trouble than a simple linear relationship between carbon dioxide and temperature would indicate. This was recently suggested in a survey of possible positive feedbacks in the carbon cycle. In this, it was suggested that these positive feedbacks could lead to an extra half a degree of warming by 21008. This recent assessment of terrestrial temperatures suggest that this feedback effect may be much greater and that we are in much more trouble than climate modelling currently suggests.
- Naafs, B.D.A., Rohrssen, M., Inglis, G.N., Lähteenoja, O., Feakins, S.J., Collinson, M.E., Kennedy, E.M., Singh, P.K., Singh, M.P., Lunt, D.J. & Pancost, R.D., 2018. High temperatures in the terrestrial mid-latitudes during the early Palaeogene. Nature Geoscience,https://doi.org/10.1038/s41561-018-0199-0