We had a new paper out a month ago in Science, in which we reassess climate tipping points based on the past ~15 years of climate science. We conclude that five climate tipping points could already be possible at current warming levels, four of which becoming likely beyond 1.5°C. You can find a free referral link to the final published version over on my publications page (plus links to the accepted version and preprint), and a blogpost explaining the paper over at climatetippingpoints.info.
Summary figure for the paper, showing the climate tipping elements we identify and what global warming level they might tip at
This paper first started out in late 2019 during my last year as a postdoc at Stockholm Resilience Centre on the ERA project, with preliminary results first presented at EGU2020, so I’m really pleased to finally have this out (it’s effectively been my covid project!). It’s also in a way the academic manifestation of my climatetippingpoints.infoscience outreach site, writing for which led me to build up a big database of papers on various climate tipping points which I felt would be useful to bring together in a scientific paper. Along the way I teamed up with Prof. Tim Lenton, who had been planning on such an update of his 2008 paper that kicked off a lot of climate tipping points research, along with various colleagues from Stockholm Resilience Centre and the Earth Commission (who helped fund the latter stages).
Talking about the paper at the Exeter Tipping Points conference
The paper also tied in with the “Tipping Points: from climate crisis to positive transformation” conference in Exeter in mid-September that I was on the programme committee for. We had over 200 delegates attend in-person from across academia, business, policymaking, and social movements (with more online and at the public debate), and had a lot of great insights on topics ranging from the risks from climate tipping points and socio-ecological cascades through to the possibility of triggering ‘positive’ socio-economic tipping points to accelerate decarbonisation. Outputs will be online soon and form the basis of a follow-up working paper and a new annual pre-COP ‘State of Tipping Points’ report.
So it’s been a busy few weeks on climate tipping points, capping off a few years of science synthesis and outreach. Hopefully this paper will serve as a useful reference point, and help show where some of the gaps are that bigger follow-up projects and assessments can tackle.
I’ve got a new paper out at EGU’s Earth System Dynamics this week, looking at the impact of climate change on the biological pump and ocean carbon sink, and in particular the role of ecological complexity in Earth system models in resolving non-linear climate-biosphere feedbacks. This is the first paper out of my recently-completed postdoc at Stockholm Resilience Centre on climate-biosphere feedbacks and tipping points, with a couple more to be submitted soon.
Following up on yesterday’s twitter thread explainer, here’s a blog version explaining what we did and why:
Pump down the Carbon
The oceans act as a massive carbon sink, taking up around a quarter of human CO₂ emissions so far. This CO₂ dissolves in to the surface ocean (making it more acidic in the process) before being transported to the deep ocean where it stays for hundreds of years (the “solubility pump”). Some of this dissolved CO₂ is used by photosynthesising plankton in the surface to make organic matter, which when they poo or die (or are eaten by zooplankton who then do likewise) produces “particulate organic carbon” (POC) which sinks through the ocean as “marine snow”.
As POC sinks it’s mostly consumed by microbes, who respire the organic matter and re-release the carbon & nutrients in dissolved form (known as “remineralisation”). The overall effect is transporting carbon & nutrients from surface to deep waters, i.e. the “biological pump”. Together the solubility and biological pumps transport carbon from the surface to deep ocean, allowing more CO₂ to dissolve in the surface and storing exported carbon in deep waters for hundreds of years before it is eventually mixed back to the surface and re-released.
Rising atmospheric CO₂ means more carbon is now dissolving in surface waters and being exported to deep waters, and so the ocean is acting as a net carbon sink (at least for a few hundred years before that deep water mixes back up).
Schematic illustrating the impact of warming on the soft tissue biological pump. On the left-side, under cooler preindustrial conditions the surface layer remains fairly well mixed with the deep ocean (large green arrow from deep to surface layers), returning dissolved nutrients and carbon (DNut & DIC) from the remineralisation of exported POC (red arrow from POC to DIC & DNut), while some POC is remineralised partly within the surface layer. On the right-side, warming leads to a shift to dominance by smaller plankton as well as stratification leading to less mixing between the shallow and deep ocean, while the average remineralisation depth getting shallower leads to greater recycling of nutrients and carbon close to the surface layer, combining to result in an overall reduction in POC export.
But the ocean is also warming up, and this changes the strength of the two pumps. Warmer water holds less dissolved CO₂, limiting the solubility pump. Warming also makes it harder for surface waters to mix with colder deep waters, causing the ocean to stratify and less nutrients to be returned to the surface – conditions that favour smaller phytoplankton. Warming speeds up metabolic rates too, and as respiration increases faster than photosynthesis this means that sinking POC is remineralised – and the carbon and nutrients it contains released – closer to the surface, increasing nutrient recycling but also CO₂ in the surface ocean.
Overall this means we expect both the solubility and biological pumps to weaken with climate change, gradually reducing the capacity of the current ocean carbon sink and the negative climate feedback it provides. However, due to computational limits most Earth system models used to project the future ocean carbon sink don’t resolve key relevant ecological processes such as the effect of warming on remineralisation, plankton size shifts, or plankton adapting to lower nutrient availability.
Ask the eco-Genie
In this study we use ecoGEnIE, a recently developed version of a simpler Earth system model featuring both remineralisation that increases with temperature (“temperature-dependent remineralisation”) and multiple sizes of plankton that can use nutrients flexibly depending on availability (“trait-based ecology”). This allows the effects of ecological dynamics on the biological pump and ocean carbon sink in response to climate change to emerge.
We separate out these effects by turning on temperature-dependent remineralisation (TDR) and trait-based ecology (ECO) (instead of the default simpler FPR & BIO settings respectively) both separately and together, and running ecoGEnIE with future emission scenarios (based on the IPCC’s RCP scenarios, from low [RCP2.6], moderate [RCP4.5], high [RCP6.0], to very high [RCP8.5] emissions) until the year 2500.
Graph showing ecoGEnIE simulation results for global POC export flux under different configurations and forcing scenarios. As time goes from left to right, going above the zero line means more POC is sinking from the surface ocean around the world, while going below the zero line means less POC is sinking from the surface ocean. Adding TDR (blue) leads to more sinking POC with warming than default (black), while adding ECO (yellow) leads to less sinking POC with warming (and adding both [pink] gives a smaller decline in sinking POC than default).
We find that turning on just temperature-dependent remineralisation (TDR) increases cumulative POC export relative to default runs (+∼1.3 %) as a result of increased nutrient recycling from remineralisation occurring closer to the surface with warming, whereas turning on just trait-based ecology (ECO) decreases cumulative POC export (−∼0.9 %) by enabling a shift to smaller plankton which produce less sinking POC.
EcoGEnIE POC export maps for default calibration model runs, showing baseline export patterns (left) and the change in POC export by 2100 relative to the 1765 pre-industrial baseline as a result of RCP4.5 (right). Darker colours on the left indicate areas where more POC sinks from the surface ocean (i.e. a stronger biological pump). On the right, blue areas show where sinking POC decreases with warming, while red areas show where it increases. In general, adding TDR means a smaller decline in sinking POC in non-polar oceans, while adding ECO means a greater decline.
In contrast, interactions with complex surface carbonate chemistry and ocean acidification cause opposite responses for the ocean carbon sink in both cases: activating temperature-dependent remineralisation (TDR) leads to a smaller sink relative to default runs (−∼1.0 %), whereas activating trait-based ecology (ECO) leads to a larger relative sink (+∼0.2 %).
Graphs showing ecoGEnIE simulation results for the absolute cumulative ocean carbon sink and the cumulative ocean carbon sink relative to BIO+FPR under different configurations and forcing scenarios.The bottom plot better shows the differences between the configurations – from the left-to-right, going above the zero-line (which represents the default model without the new features) means the ocean carbon sink is bigger in the new model configuration than the default configuration, while going below the zero-line means it’s smaller than the default configuration. In general adding ECO (yellow) leads to a bigger ocean carbon sink with warming, while adding TDR (blue) or combining ECO & TDR (pink) leads to a smaller sink.with warming
Down the sink
Combining both temperature-dependent remineralisation (TDR) and trait-based ecology (ECO) results in an overall strengthening of POC export (+∼0.1 %) and an overall reduction in the ocean carbon sink (−∼0.7 %) relative to default runs. Around 6 gigatonnes less carbon is taken up by the ocean in the 21st century as a result – a bit under 1 year of current human emissions.
This isn’t a huge difference, but is still more than current Earth system models project. There are also other important ecological processes not yet in the model (e.g. separated plankton shell types, ballasting, low resolution) that future work will need to look at to refine these estimates.
These results illustrate though the degree to which ecological dynamics & biodiversity modulate biological pump strength, and indicate that incorporating ecological complexity in Earth system models allows them to more fully resolve non-linear climate–biosphere feedbacks.
With thanks to co-authors Sarah Cornell, Katherine Richardson, and Johan Rockström, and the ERC-funded Earth Resilience in the Anthropocene (ERA) project for supporting this work during my postdoc at SRC!
I’m pleased to announce that I’ve been giving funding by the ReCoVER network to do some Outreach linked to the end of my current Research Fellowship, and so we’re now launching the project: “The Point of No Return? An Interactive Stall and Website Starting Conversations on Climate Tipping Points”.
In this project we’ll be hosting conversations about climate tipping points at a series of stalls, public discussions, and online during October and November 2016, focusing on how they happen, why they’re important to our lives, and how researchers are trying to understand and predict them.
We’ll be running stalls at Southampton Sustainability Week (8th-16th October) – including at the Family Festival of Science at Thomas Hardye School on October 8th (tomorrow!) and at Researchers Café at Mettricks on October 14th – and at TEDx Southampton (5th November). We’ll upload materials from the stall on the website along with additional articles, blog posts, interactive discussions, integrated social media feeds, a podcast, and videos (in collaboration with local film-maker global documentary and a local animator) about climate tipping points.
Dr. David Armstrong MᶜKay 