At GEOMAR we’ve been running some experiments for different marine CDR techniques already in the past few years, partly also with mesocosm facilities. It’s a difficult question right? Because in the end I think most of the optimistic IPCC projections assume a certain amount of CDR. Without it we wouldn’t be able to reach 1.5 or even 2 degrees. So it’s really important and urgent to do this research to assess if the different CDR techniques have ecological side effects, or unforeseen biogeochemical feedbacks like the ones shown in my presentation.

I think it’s important to do this research and see which techniques are actually an option and then there’s also the engineering part to it, like how to implement these things on global scales, on gigoton scales. And I think in the public discussion, it often seems problematic because CDR is seen as an excuse to not reduce our CO₂ emissions, but I think we need to clarify that we need to reduce emissions, but probably we also need CDR to stay within a safe range of climate change.

In this study we didn’t specifically look at the knock on effects on the carbon cycle because it’s tricky with the model. But you would expect that if the most important primary producer decreases its biomass by a lot, then it will probably change the food web and biogeochemical cycling.

If one phytoplankton group decreases in biomass, other phytoplankton groups will jump in and increase their productivity. But because diatoms are also important for trophic transfer in the food web I think it will affect the whole cycling of organic matter. 

Moreover, when you think of processes like the sinking speed of particles in the biological carbon pump, this is also controlled by the density of particles. So when the presence of silica is higher, it is lost at a slower rate. This might actually increase the sinking speed of particles, and therefore it might make the carbon pump more effective or protect the particles more from degradation. 

There’s different processes that should be looked at, but to look at processes at a smaller scale you need to go back to the lab and run smaller scale experiments. Physiological experiments are important, we have to know how organisms respond, but it’s like when you have these small incubation bottles, there’s just certain processes that you cannot look at. 

In the processes I mention in my presentation you need an ecosystem and a water column in order to look at the sinking particles. So these things, you cannot do in small scale experiments, but I think the two approaches should complement each other. So when you see something in these larger scale experiments, then you go back to the lab and look specifically at this organism or at these processes. That scale interplay is important to consider.

I would also say that on the organism level or ecosystem level, warming is probably an even bigger issue because like the physiological responses and disruption might be even stronger than for acidification alone. But then, of course, like when these two interact the combined effect is even stronger than the individual effects. I guess it also depends as certain regions or ecosystems might be more sensitive to warming while other parts of the food web might be more sensitive to acidification and vice versa.