New Paper Out: Reduced Carbon Cycle Resilience across the PETM

Much like buses, after a waiting a while for new papers to be published two have come along in short succession. This time though we’re back in the palaeoclimate domain, with a paper based on my work on a ReCoVER-funded Early Career Research award hosted at Ocean & Earth Science at Southampton which applied ‘early warning signal’ methodology to Cenozoic palaeoclimate records. It’s now available open access from Climates of the Past, and as with other papers I’ll summarise it here on my blog as well.

CP cover enlarged
My first EGU journals paper , and a pleasant public review process!

The setting of this paper is the Palaeocene-Eocene Thermal Maximum (i.e. the PETM), which is a natural case of carbon cycle disruption and linked rapid global warming that happened about 56 million years (My) ago. The triggers of this event are still being investigated, but palaeorecords point to the release of several thousand gigatonnes of carbon being released over a few thousand years driving ~5oC of global warming. As a comparison to today, this is a similar amount of carbon as humans are likely to emit from fossil fuel burning but over ~10 times the time, making it a partial but limited analogue to current climate change. The PETM was then followed by several smaller ‘hyperthermal’ events on a regular timescale into the warm Eocene.

As with many other big ancient climate shifts, the PETM was preceded by more gradual changes before a rapid shift, which has led many to hypothesise that it involved some sort of ‘tipping point’ (i.e. when gradual changes can eventually lead to a sudden shift in a system after reaching a critical threshold – see for more info!) that led to lots of carbon from parts of the Earth system like methane hydrates or peat being suddenly released. Alternatively, the PETM also coincided with a time of mass volcanism associated with the opening of the North Atlantic (of which Iceland is now the distant hangover of), and so could have been directly triggered by volcanic eruptions without any sort of tipping point involved.

Theory suggests that tipping points are often preceded by small but detectable ‘early warning signals’ (EWS), which can be found using statistical analysis of data. After an early proliferation of EWS techniques a few years ago though researchers have found them to have important limitations, with data quality being a big constraint and a propensity for false or missed alarms. Despite this, using multiple EWS indicators of different types along with strong statistical significance testing can still give us a pretty good idea of changes in a system’s overall resilience, with increasing variability and system ‘memory’ indicating the weakening of the system’s stabilising negative feedbacks and therefore a greater risk of being disturbed.

In our study we put this to the test by analysing some good quality long palaeorecords covering 5 My before the PETM and ~2 My after in order to look for any significant changes in carbon-climate system resilience that might help explain the origins of the PETM. We found consistent evidence from several different methods of a gradual destabilisation of the geological carbon cycle in the ~2 My before the PETM, and long-lasting carbon-climate system instability in the aftermath. This period coincides with the North Atlantic volcanism, leading us to suggest that these eruptions helped to gradually destabilise the carbon cycle by suppressing organic carbon burial (in particular either the marine biological pump or peat on land) as the result of volcanism-driven warming .

However, although this could mean the PETM itself was a tipping point resulting from this destabilisation it cannot solidly prove it, and we find no evidence of a tipping point in just the climate system either. Despite this, a decline in carbon cycle resilience would’ve still made it easier for the PETM to occur and last longer than it would’ve been otherwise, as weaker negative feedbacks would slow down the carbon cycle’s recovery to pre-PETM conditions. We also find evidence that the subsequent hyperthermal was preceded by slightly different dynamics than the PETM itself, which fits with the hypothesis that the PETM required an extra “push” from say volcanism but that the later events were more traditional tipping points.

To find out more, the full article is open access and free to read for all, and direct questions are welcome. Future follow-up work include a similar analysis of the Cretaceous/Palaeogene boundary and the Deccan Traps (paper TBC), and other Cenozoic climate shifts as more long and high-resolution records become available. Thanks also go to EPSRC/ReCoVER for funding the initial project, OES at Uni. Southampton for hosting the project back in Summer 2016, SRC where I did the final revisions/reanalyses, and Stockholm University for funding the open access publication.