Here on Bitsofscience.org we’ve discussed the Paleocene-Eocene Thermal Maximum (PETM) – that sudden CO2 and methane-induced peak climate warming (or ‘hyperthermal’) 55.8 million years ago of around 6 degrees over 20,000 years – on several occasions, because it offers an important warning with respect to the escalating warming potential of positive climate feedbacks.

Another interesting period in the Earth’s history is the Early Eocene Climatic Optimum, following the PETM event, when both CO2 and temperature remained high for millions of years – with a few lesser hyperthermals superimposed. Altogether Earth´s temperatures during the early Eocene were at their highest in at least 65 million years till now, possibly some 400 million years.

Just like the PETM, the Early Eocene too offers important insights into the Earth’s climate system, as it shows the correlation between elevated levels of CO2 and temperature over a much longer period – an indication of ancient climate sensitivity.

The Eocene climate

It remains important to note the Earth of around 50 million years ago, is not the same as today’s Earth, although continents were already closing in on their current positions. Plate tectonics could have played an important role in the Eocene Climatic Optimum, as around 50 million years ago the Indian subcontinent hit Eurasia and began sliding underneath it, starting the Himalayan orogenesis and perhaps provoking increased volcanic activity – thus releasing yet more [the planet also had not recovered from the PETM] CO2 into the atmosphere.

Another important difference was the relative position of Australia and Antarctica – with the latter not yet isolated by the Antarctic Circumpolar Current, the important isolating factor that allows glaciations. From these tectonic deviations stem other thermohaline differences [compared to the modern Earth], for instance warm deep ocean currents and a smaller temperature gradient between the equator and both poles, as characteristic to the Eocene’s climate.

The Eocene exaggeration?

It has even been suggested Arctic and Antarctic regions may have been hotter than 30 degrees Celsius, without much indication of just how much warmer the tropics and subtropics may have been.

Now a new study, conducted by researchers of Syracuse and Yale Universities, and published in Geology, suggests the Eocene may not have been that hot.

The fossil record around the US Gulf coast shows [through clumped-isotope and tetraether-lipid analysis on fossil shells and sediments – this is considered state of the art methodology] the average Gulf water temperature was around 27 degrees Celsius, which is less than 3 degrees warmer than today. More remarkable is that studied fossils showed the temperature hardly changed over the seasons, just 3-5 degrees – whereas the Gulf’s water temperature fluctuates by 12 degrees presently.

The team is currently using the same methods to determine average temperatures and seasonal variability in the polar regions. Including these measurements could give us a better picture of the entire Earth’s climate during the hot part of the Eocene – and better understand the CO2 climate sensitivity of that time.

Climate sensitivity – and regionality

The researchers expect to find previous estimates have overestimated early Eocene temperatures around the poles. The world was definitely hotter though – and there was definitely more CO2 around. “Our results support predictions that increasing levels of atmospheric CO2 will result in a warmer climate with less seasonality across the globe,” says lead author Caitlin Keating-Bitonti of Syracuse University.

This work touches on the all-important issue of climate sensitivity, or ‘what happens’ to the temperature when you double atmospheric CO2 concentrations [at least several degrees warming – but you need to switch from the perspective of a physicist to the perspective of a climatologist to see that]. It is reason number one* why we want to keep track of this research group – and any new publications. But in the above quote is a second important reason:

Decreasing seasonality is what we are witnessing under the present climate warming too. And that may have large consequences for today’s other bit of climate news.

[*) According to a 2006 Science publication atmospheric CO2 concentrations during the Early Eocene may have been as high as 1125 ppm, which would translate to ‘climate sensitivity times two’ as this is roughly four times as much CO2 as the preindustrial CO2 level of modern Earth. Translating this to IPCC AR4’s  climate sensitivity range – and fully ignoring all complexities – the Early Eocene should have been 4-9 degrees hotter than our own Earth around 1850.]

© Rolf Schuttenhelm | www.bitsofscience.org