Climate-Biosphere Feedbacks & Tipping Points

The Earth’s climate has changed ever since the Earth formed, but the drivers of these changes are not clear. If solar luminosity drove climate change alone then the Earth would have started out much chillier than evidence suggests and gradually heated throughout its history – a pattern at odds with the geological record, indicating stabilising feedbacks providing Earth System resilience. However, palaeorecords also show occasional abrupt climate shifts (such as the Palaeocene-Eocene Thermal Maximum or Eocene-Oligocene Transition) on an otherwise smooth history, which also suggests the presence of destabilising feedbacks and tipping points in the Earth System. The biosphere plays a major role in these feedbacks, with the risk of key biomes transitioning from net negative to net positive feedbacks on climate change. These feedbacks could result in a higher than expected climate sensitivity, while tipping points have been hypothesised of being capable of “cascading”, both of which could increase the tail-risk of catastrophic warming in the emergent Anthropocene. To fully understand these risks requires mapping and modelling of the key interactions and feedbacks between Earth System elements, investigating how changing ecosystem dynamics modulate the strength and form of these interactions, and assessing the potential for vulnerable elements (‘systempunkte’) capable of triggering tipping cascades.

In my PhD and a follow-up Post-Doc project I studied the potential drivers and dynamics of some of the major perturbations to the carbon-climate system during the Cenozoic, while in my current postdoc I’m focusing on mapping and modelling nonlinear biosphere-climate interactions, feedbacks, and tipping points in the modern Earth system.

PhD: ‘What were the drivers of carbon-climate system perturbations during the Cenozoic?’ (2011-2015)

Over the course of the Cenozoic (the last 66 million years) the Earth system has shifted from a CO2-rich ‘Greenhouse’ climate state to a CO2-poor ‘Icehouse’ climate state. This trend is punctuated by numerous perturbations to the carbon-climate system, but the extent of the coupling between the carbon cycle and climate system, the drivers of these perturbations, and their relationship to the longer-term Cenozoic trend is still debated.

In my PhD thesis I used biogeochemical modelling and numerical analysis to explore the key research question: What were the drivers of carbon-climate system perturbations during the Cenozoic?, with a focus on perturbations during the Eocene-Oligocene Transition and the mid-Miocene, the role of tipping points during these periods, and the long-term evolution of the ocean carbonate system.

So far I have published 3 papers based on my PhD (see my writing page for details and links) on the impact of the Columbia River Basalt large igneous province eruptions on the carbon-climate system in the mid-Miocene (around 16 million years ago) and the drivers of the carbon cycle perturbation during the glaciation of Antarctica in the Eocene-Oligocene Transition (~34 million years ago). I also co-authored a paper on the likely extent of ocean acidification as a result of the dinosaur-killing asteroid impact at the Creataceous-Palaeogene boundary (~66 Ma) and the role of Methane Hydrate dissociation during the infamous Palaeocene-Eocene Thermal Maximum (PETM, ~55 Ma).

Post-Doc 1.2: Can early warning signals be reliably detected in the Cenozoic palaeoclimate record? & Outreach: The point of no return? An Interactive Stall and Website Starting Conversations on Climate Tipping Points (Uni. Southampton 2016)
In the summer of 2016 I received early career researcher funding from the EPSRC/ReCoVER network for a 3 month Research Fellow position at NOCS, University of Southampton, which follows on from some preliminary tipping point / early warning research in my PhD. There are many points in Earth’s history where the Earth System is hypothesised to pass a ‘tipping point’ beyond which a rapid transition to a new and very different state occurs. These critical transitions are common in other complex dynamical systems and are often preceded in datasets by ‘early warning signals’ (EWS) such as critical slowing down (i.e. the system’s recovery time in response to perturbations slows down) and increasing variability (as the data gradually contains more extreme values). Dakos et al. [2008] and subsequent studies found that EWS can be detected prior to several past climate shifts, suggesting that critical transitions can successfully be detected in the palaeorecord and that palaeo tipping points can be identified. However, doubts have been raised about the reliability of EWS analysis on palaeoclimate records, the degree to which parameter selection can affect the results, and the risk of committing the ‘prosecutor’s fallacy’ when analysing suspected critical transitions. In my PhD I did a pilot study in which I analysed the highest-resolution palaeorecords currently available across a number of perturbations to the Cenozoic carbon-climate system, and found some promising results even when using a cautious approach to counter potential problems. In this Post-Doc I extended these analyses, and found evidence of significant declines in carbon cycle resilience prior to several Cenozoic climate events such as the Palaeocene-Eocene Thermal Maximum.

As part of the above Post-Doc project, I also received funding from the ReCoVER Network to run a public outreach project about Climate Tipping Points, for which I set up the website, commissioned a youtube animation, and gave a series of public talks. See the website for more information!

Post-Doc 2.1: Earth Resilience in the Anthropocene (Stockholm Resilience Centre, 2018-2021)
In my second postdoc stint at Stockholm Resilience Centre (SRC) I was on the Earth Resilience in the Anthropocene (ERA) project led by PI Prof. Johan Rockström, working on modelling nonlinear biosphere-climate feedbacks in the Earth system. A key task of the ERA project is the analysis of the nonlinear biosphere dynamics governing Earth resilience in the Anthropocene, in order to improve our understanding of tipping points in biophysical systems. This requires modelling of the key interactions and feedbacks between Earth system elements, investigating how changing ecosystem dynamics modulate the strength and form of these interactions, and assessing the potential for cascading tipping points across them. Key interactions include between climate change, the marine biological pump, and acidification in the ocean, and between climate change, biodiversity, and ecosystem resilience on land (in e.g. tropical and boreal biomes). I used a selection of Earth system models (e.g. ecoGEnIE), ecosystem models (e.g. LPJmL and Madingley), and more stylised models to analyse these interactions and their potential for nonlinear dynamics within a socio-ecological framework. I finished this postdoc in December 2020, but will continue as a visiting researcher at SRC for a while longer and continue to be involved with the ERA project and associated research networks (such as LOOPs).