Warm past climates: is our future in the past?
by Nathaelle Bouttes
When people talk about global warming, they sometimes wonder whether climate has already been warmer than today in the past. If so, then why would a warm future be so bad, and isn’t what’s going on now the same as before? When I think of a warmer climate, I tend to imagine the time of dinosaurs. Do we have to go as far back as the Cretaceous (more than 65 million years ago) to find a warmer climate?
A few months ago I went to a conference about past warm climates that was very informative. It turns out that, if we go back in time, although the climate cools back as far as the Last Glacial Maximum around 21,000 years ago (Figure1), it was warm (and indeed seemed to be warmer than today) during the last interglacial period (Eemian, around 125,000 years ago).
During the Eemian the climate was warmer with summer temperatures in the Arctic region about 2-4°C higher than today. The Greenland ice sheet was also smaller, bringing up interesting questions such as "will our warmer future also have a smaller Greenland ice sheet and higher sea level?" But it appears that most of the change of ice volume was due to the different orbital parameters (position of the Earth relative to the Sun) leading to a warmer Northern Hemisphere during the summer (Van de Berg, 2011). Additionally, the CO2 concentration then was similar to the one during the Preindustrial (around 280 ppm, EPICA, 2004) which is lower than the present one (around 380 ppm) and certainly lower than the one predicted for the next century. So in the end, the Eemian is not such a good analogue for our future (Ganopolski and Robinson, 2011).
After some more glacial-interglacial cycles during the Pleistocene (the last 2.6 million years), we find another period which was warmer than today: the Pliocene (between 5.3 million years and 2.6 million years). The CO2 concentration was also relatively high (Tripati, 2009). It is a very interesting period because it shows a general cooling associated to a decline of atmospheric CO2 and corresponds to the transition towards greater glaciations in the Northern Hemisphere (the increase of δ18O in figure 1 is due to the Northern ice sheets becoming bigger and the climate getting colder). That makes it a good period to study to better understand the links between CO2, climate and the ice sheets. It is unclear, however, as to why CO2 declined in this way.
The PETM (Paleocene-Eocene Thermal Maximum)
If we continue back in time (Figure 2), the climate progressively gets warmer during the rest of the Cenozoic (last 65 million years). A few periods are locally climate optimums such as the Mid-Miocene or the Mid-Eocene, but the probably most famous warm period is the Paleocene-Eocene Thermal Maximum (PETM, around 55.8 million years ago). It is a period much studied because it seems a good analogue to what we might experience in the future in terms of temperature and CO2 concentrations. It was around 5°C warmer with CO2 concentrations of around 1000 ppm (the uncertainty is very large). However, the causes of such high CO2 values were different from what we experience now. Current hypotheses involve the release of significant quantities of carbon, probably partly coming from methane hydrates (methane trapped in the ocean, Lunt, 2011). Additionally, the world itself was different from the one we know, with continents distributed differently. And of course there were no modern humans.
Was it such a bad climate to live in? There was of course life on earth, although it was different from today, so it wasn’t completely uninhabitable. However the climate was very different from ours today. Not only the global mean temperature was higher, but for example the distribution of precipitation was also changed. Nonetheless, because the distribution of continents was not the same as today, it is difficult to compare. It was just a different world. One very different variable was the timescale involved: the changes of atmospheric CO2 happened on much longer timescales (100 000 years) in the Cenozoic, which means climate change took more time and life had some time to adjust and adapt.
In conclusion, past climates are very informative and have been crucial for the discovery of new climate mechanisms, some of them having the potential to take place in the future. But there is no perfect analogue to what our future could be, because the world was different in the past (different distribution of continents for example) and the reasons for the CO2 change are different now. The latter means mostly that changes are happening much more rapidly compared to past events.
So the past presents no real analogues for our future, which is also why modelling is so important. The data are crucial to better understand the earth system and discover new processes, but they won’t directly tell us what lies in the future. Nor will the models, which will never be a perfect reproduction of our earth, but they can help us unravel part of our possible futures.
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