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).
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.
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.
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 %).
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!