Most people know that we have tried to judge what may happen during global warming by creating gigantic models of the Earth system, and see how it responds to forcing from different factors.

Another way that we approach the problem, is, (in my opinion) a much more interesting line of research, and that is looking in the geological record to see how the Earth responded in the past to global warming events, and to use these to inform us about what may happen now.

One of the most famous global warming events in the geological record is the PETM, which took place at 55.9 Ma. This caused the Earth's temperature to rise by about 6 degrees, which we know from oxygen isotope ratios in foram shells, causing a mass extinction of benthic forams, and, weirdly, a diversification of mammals. As for what caused it, all that we know is that some process dumped a 5000 or so Pg of light carbon in the atmosphere.

The Paleocene (via BBC)

Many studies such as this one out this week hail the PETM as the best analogy for modern day global warming that we have. But is this reasonable? And, can we assume that the Earth will react in a similar way?

We have to remember that, at the moment, geologically speaking, we are still living in an ice age, by which we mean that there is permanent ice cover on the Earth at the poles. This is not a trivial distinction; the Earth behaves very differently when there is ice to reflect light back to space. At the time of the PETM, there is no such cover, and not only that, the Earth started much warmer as a whole before entering into the global warming event. Like or not, the Earth is actually almost as cold as it gets at this present moment in time; significantly colder than at the end of the Paleocene. Moreover, levels of CO2 were significantly higher before even entering into the PETM; before all that excess carbon was dumped.

Its also important to note the geography at the time. Atmospheric circulation plays a key role in regulating our climate, and in particular, ocean currents, which suck up a lot of CO2 and by downwelling can sequester it away very deep.

Whilst it may not look that different to you, there is no bridge between North and South America and many of the other continents were still stuck together. Consequently, there was warm circulation round the world, which moderates the climate. Crucially, the Drake passage; a strait that separates Antarctica from Patagonia, was closed. Antarctica is cold today purely because it is almost entirely isolated from the global circulation system because of the circumpolar current, which goes round and round antarctica. This has a fundamental role in regulating climate due to the amount of CO2 that it up wells - which doesn't happen without the circulation.

So, if the PETM isn't an analogy, what is? Well, we could try the Middle Miocene Climate Optimum (MMCO) of 15Ma, when the atmospheric circulation was similar to today and CO2 levels similar to today, but notably, at Mount Boreas in Antarctica it was warm enough for a lake and trees of Notophagos

However, the temperature was still 3°C higher than present, so not perfect. Another candidate might be the Pliocene warming event, about 4.5 Ma? CO2 levels were similar to todays. Unfortunately, there was no sea ice, and sea levels were already higher, and we don't know the effects of the widespread boreal forests at the time.

So, I would say that there were no ancient analogues for modern day global warming. But that doesn't mean that such events aren't useful. What they are, are incredibly important proxies for how the climate can behave in response to forcing. But to simply say that the Earth will react in the same
way - or even similar way - to fossil fuel burning, is a tad short sighted.


Thanks to Mark Williams for raising some of these points.