PETM Weirdness

If (and this is a key assumption that we’ll get to later) this was the only forcing associated with the PETM event, how much warmer would we expect the planet to get? One might be tempted to use the standard ‘Charney’ climate sensitivity (2-4.5ºC per doubling of CO2) that is discussed so much in the IPCC reports. That would give you a mere 1.5-3ºC warming which appears inadequate. However, this is inappropriate for at least two reasons. First, the Charney sensitivity is a quite carefully defined metric that is used to compare a certain class of atmospheric models. It assumes that there are no other changes in atmospheric composition (aerosols, methane, ozone) and no changes in vegetation, ice sheets or ocean circulation. It is not the warming we expect if we just increase CO2 and let everything else adjust.

In fact, the concept we should be looking at is the Earth System Sensitivity (a usage I am trying to get more widely adopted) as we mentioned last year in our discussion of ‘Target CO2‘. The point is that all of those factors left out of the Charney sensitivity are going to change, and we are interested in the response of the whole Earth System – not just an idealised little piece of it that happens to fit with what was included in GCMs in 1979.

Now for the Paleocene, it is unlikely that changes in ice sheets were very relevant (there weren’t any to speak of). But changes in vegetation, ozone, methane and aerosols (of various sorts) would certainly be expected. Estimates of the ESS taken from the Pliocene, or from the changes over the whole Cenozoic imply that the ESS is likely to be larger than the Charney sensitivity since vegetation, ozone and methane feedbacks are all amplifying. I’m on an upcoming paper that suggests a value about 50% bigger, while Jim Hansen has suggested a value about twice as big as Charney. That would give you an expected range of temperature increases of 2-5ºC (our estimate) or 3-6ºC (Hansen) (note that uncertainty bands are increasing here but the ranges are starting to overlap with the observations). ALl of this assumes that there are no huge non-linearities in climate sensitivity in radically different climates – something we aren’t at all sure about either.

But let’s go back to the first key assumption – that CO2 forcing is the only direct impact of the PETM event. The source of all this carbon has to satisfy two key constraints – it must be from a very depleted biogenic source and it needs to be relatively accessible. The leading candidate for this is methane hydrate – a kind of methane ice that is found in cold conditions and under pressure on continental margins – often capping large deposits of methane gas itself. Our information about such deposits in the Paleocene is sketchy to say the least, but there are plenty of ideas as to why a large outgassing of these deposits might have occurred (tectonic uplift in the proto-Indian ocean, volcanic activity in the North Atlantic, switches in deep ocean temperature due to the closure of key gateways into the Arctic etc.).

Putting aside the issue of the trigger though, we have the fascinating question of what happens to the methane that would be released in such a scenario. The standard assumption (used in the Zeebe et al paper) is that the methane would oxidise (to CO2) relatively quickly and so you don’t need to worry about the details. But work that Drew Shindell and I did a few years ago suggested that this might not quite be true. We found that atmospheric chemistry feedbacks in such a circumstance could increase the impact of methane releases by a factor of 4 or so. While this isn’t enough to sustain a high methane concentration for tens of thousands of years following an initial pulse, it might be enough to enhance the peak radiative forcing if the methane was being released continuously over a few thousand years. The increase in the case of a 3000 GtC pulse would be on the order of a couple of W/m2 – for as long as the methane was being released. That would be a significant boost to the CO2-only forcing given above – and enough (at least for relatively short parts of the PETM) to bring the temperature and forcing estimates into line.

Of course, much of this is speculative given the difficulty in working out what actually happened 55 million years ago. The press response to the Zeebe et al paper was, however, very predictable.

Page 2 of 3 | Previous page | Next page