Dr Lennart Bach from IMAS (Institute for Marine and Antarctic Studies), University of Tasmania joined us to talk about marine carbon dioxide removal — the science of using the ocean to draw down CO2, the range of methods being proposed, and where his own research on ocean alkalinity enhancement is headed.

Lennart Bach: The research I'm doing is partly funded by the Australian Research Council and partly through philanthropic sources — in this case an initiative called CDRIC, which is dedicated to investigating whether any of this makes sense. That's also my own goal in all this: trying to figure out if it's useful or not.

Just to bring everyone onto the same page — we're obviously here to look into climate change, and it's generally accepted that global warming should be kept below two degrees Celsius to maintain the Holocene mode of operation of the Earth system. If we pass a threshold — which doesn't necessarily have to be exactly two degrees, but somewhere in that margin — we may propel the climate system into a state that may have existed in prehistoric times, but not recently, and certainly not in the time humans have evolved. So it could be bad for us.

For me, the motivation to look into marine CDR started with the Paris Agreement, now eleven years ago. I was in Paris at the time — I was still a scientist in Germany, with the German ocean acidification programme — and I was there and could observe what was happening. It was a very interesting meeting, because there was a real positive energy around the issue back then, and everyone was on the same page that this is a problem that needs to be tackled, and quite ambitious goals were set. So what motivated me was figuring out what actually needs to happen to keep global warming below two degrees.

The emissions gap

What everyone already knew for a long time is that carbon emissions must be phased out very rapidly and drastically, and there's a plot I like to show because it was an eye-opener to me. Time is on the x-axis, and on the left you see global carbon emissions in gigatons of CO2 per year — historic emissions going up, and unfortunately still going up. This plot is from 2020, so a bit old now, but it shows that to maintain global warming within the Paris goal, we'd basically have to phase out fossil fuels down to close to zero by 2030. That's four years away, so it's not going to happen.

Because everyone already knew that, people started talking about negative emissions or carbon removal — to ease off the decline rate and make it more digestible to the public, essentially postponing the problem to later in the century. Our children will have to remove all the mess we've made, towards the end of the twenty-first century, with insane amounts of carbon removal — on the order of three to five hundred gigatons of CO2 per year. That's pretty much every fourth CO2 molecule that comes out of our cars, planes and so on, and somehow it would need to be mopped up. No one knows how.

There's a related plot showing the negative emissions needed under different scenarios. The most optimistic scenario — rapid phase-out, renewables and so on — needs less; a more divided, uncooperative world needs more, on the order of 20 gigatons of CO2 per year by the end of the century, versus around 10 in the best case — which is still a quarter of what we emit today. This is all IPCC data — according to the best available science, there is no future below two degrees without some sort of climate engineering. So the choice becomes: do we let warming go above two degrees, which we could in theory address with emissions reductions alone, or do we keep it below two degrees, which requires the same emissions reductions plus some form of climate engineering. It's not a replacement for emissions reductions — it's additional to them.

Three pillars of climate policy

Just to get clarity on what CO2 removal, or negative emissions, actually means: there are three pillars to the climate policy toolbox. The most important, obviously, is that emissions must be phased out immediately and rapidly — that's priority number one. On top of that, there's the challenge that CO2 is a potent greenhouse gas but not very abundant in the gas mix — roughly 400 parts per million — and these need to be filtered out somehow, whether through terrestrial methods (forests, geological storage) or through the oceans. Since I'm a marine scientist, I focus on ocean pathways to get these legacy emissions out of the atmosphere.

Where marine CDR stands

A report published two years ago found that with regard to conventional CDR — mainly forestry — we globally already operate at a scale of about two gigatons of CO2 per year. So we're not starting at zero; a lot of people think we are, but we're already doing quite a lot of carbon removal through forestry, and the readiness level there is very high, because we've done it for many years. But with the novel carbon removal methods — among which marine CDR sits — our readiness level is very low, partly because no one wanted to fund that research. It was flagged as geoengineering, seen as dirty and unnecessary — but then the emissions reductions didn't happen, and now we're in a situation where we rely on it without having studied it properly. A comment published in Nature a while back basically asked, "should we fertilise the oceans, seed clouds, or do anything?" — and the answer was: no one knows. That was the state of the science in 2019. We've come a long way since, but there's still much to learn.

Why look into marine CDR at all? The opportunities are that the ocean is very large — it offers large scaling potential to make a meaningful contribution, low land requirements since there's no competition for land area, and co-benefits for some methods, like ecosystem restoration, increased biodiversity, or counteracting ocean acidification. The risks: governance is very complex, because it's an internationally regulated system with transboundary risks — the ocean is an interconnected fluid that moves matter between jurisdictions, so an action taken in your own waters can affect others quite dramatically. Then there's the massive problem of monitoring, reporting and verification — showing that the ocean has actually taken up more carbon because of a given action is a huge challenge, because the volume is so large and the carbon is hard to trace. And of course there's environmental risk — some methods may affect the ocean environment negatively.

Six approaches

Roughly six approaches are widely discussed — four biotic, meaning photosynthesis is involved, and two geochemical, meaning weathering processes are involved. I'll step through them quickly, then focus on the one I think is most worth discussing at length.

Ocean iron fertilisation is one of the oldest ideas, initiated by the oceanographer John Martin, who famously said "give me half a tanker of iron and I will give you an ice age." The idea was that the Southern Ocean — everything south of Tasmania — is limited by the nutrient iron, which is very insoluble in seawater, so the environment is essentially starved of it. Add iron, and things can grow — and Martin thought the growth potential was so large that a small amount of iron could sequester enough CO2 to tip us into an ice age. The iron-limited areas are large parts of the globe. The idea is that we'd fertilise the surface ocean with iron salts, stimulating phytoplankton growth that sequesters carbon, which then sinks into the deep ocean. Long story short, research has shown this may have a very limited climate effect — it may work a little, but won't be a major contributor to marine CDR.

Ocean afforestation expands seaweed aquaculture into the open ocean — replacing the phytoplankton that naturally live there and support the food chain with seaweed, which grows faster and sequesters carbon. I call this the "Icarus method," because it's had a very limited flight. The ocean CDR space has hugely overhyped it — beautiful pictures of seaweed forests, and everyone loves seaweed forests, they're very appealing, I love diving in them myself — but an open-ocean seaweed farm would look very different, more like a crop field than a forest. Once people realised it wasn't really working, and wasn't as beautiful as imagined, it fell out of favour pretty quickly. I don't think there's much potential in it, though it would have been nice to study a bit more.

Artificial upwelling involves building pipes that pump nutrient-rich deep water up to the surface, where the nutrients are meant to stimulate production and, via photosynthesis, sequester carbon. The problem is that deep water is rich in CO2 as well as nutrients, so you bring a lot of CO2 to the surface too — which could go back into the atmosphere, unless you can prove the nutrients sequester more carbon than the CO2 you brought up released. That's a very difficult thing to demonstrate in an open system like the ocean. My personal opinion is there's essentially zero chance this will make any meaningful contribution to climate mitigation — it's an interesting concept, people like talking about it, but I don't think it's a good one.

Blue carbon refers to coastal ecosystems that sequester CO2 — seagrass meadows, mangroves, salt marshes. Very nice ecosystems, worth restoring. These are considered conventional marine CO2 removal because they already feed into countries' nationally determined contributions under the Paris Agreement — Australia has quite significant blue carbon environments and is trying to get them into its national carbon budget. Side note: most of the progress in Australia's carbon budget so far comes from terrestrial sinks, and these are part of that. My view is that blue carbon is a "no regrets" method because of its co-benefits — even if it doesn't sequester much carbon, it's worthwhile for biodiversity, coastal protection and so on. I wouldn't bet heavily on the carbon side — it's extremely hard to quantify how much carbon these systems really sequester, and they have very low scaling potential; even though they can sequester a lot of carbon per unit area, the total area they occupy is so small it won't make a meaningful contribution to climate mitigation on its own. Still worth doing for the co-benefits.

Ocean alkalinity enhancement

The one I want to talk about at greater length is ocean alkalinity enhancement — the first method that isn't biotic. There's no photosynthesis involved at all, ideally no biology whatsoever — it's purely geochemical, based on marine carbonate chemistry.

I'm not sure how firm everyone's chemistry is, but let's step through it — it's roughly high-school level. When CO2 dissolves in seawater, it reacts with water to form carbonic acid — that's the soda water reaction, which is why soda water is sour. That carbonic acid partially dissociates into bicarbonate and a proton, and further dissociates into carbonate ion and another proton — so two protons in total. Those protons are basically the acidity of seawater, which is why we get ocean acidification.

If you capture those protons — through rock-weathering reactions — you shift the chemical equilibrium: instead of CO2, you get more bicarbonate and carbonate, which leaves more room for atmospheric CO2 to dissolve into the ocean afterwards. So by capturing the protons, you make more space for physically dissolved carbon in seawater — no biology involved at all. The weathering reaction does release other elements — nutrients, trace metals — which can potentially affect biology, but that's not the primary goal.

Here's how the idea came about. If you wonder what happens to the CO2 we currently emit over the very long run, the Earth system will eventually mop it up — but it takes a very long time. Much of it dissolves in the ocean and eventually reaches the deep ocean, where it reacts with seafloor sediments, which dissolve via that same reaction — consuming acidity and making more room for atmospheric CO2. On land, slightly acidic rain dissolves certain rocks, which also capture protons and convert CO2 into bicarbonate. Over roughly a hundred thousand years, all the CO2 we emit today ends up as bicarbonate and CO2 in the ocean. The ocean does this naturally — the idea behind alkalinity enhancement is simply to speed that process up, because naturally it's very slow.

Practically, this resonates with Australia — I suppose we're really a mining country. The idea is to accelerate mineral dissolution by extracting certain minerals, grinding them into a fine powder to increase their surface area so they dissolve faster, then transporting and distributing that material into the ocean environment.

The problem is that all the less-effective methods happen to be biological, and everyone likes biology, so there's an instinct that this problem should be solved by biology. Alkalinity enhancement isn't that — it doesn't really involve biology — and it can be perceived as "dirty," because when you explain to people that you have to dig up a lot of rock, they don't love it. There's a study on public perception showing the idea is quite unpopular — that's from the United States, so it may not translate directly to Australia, but at least there, the comfort level is low.

Ocean alkalinity enhancement is really an umbrella term for over fifty different methods of achieving the same chemistry — many different ways to grind up rocks or otherwise capture those protons. There was a field trial by a commercial enterprise wanting to do some research on this in St Ives, in Cornwall, UK, which drew a large public protest. I found it interesting that the aesthetics of that protest looked quite similar to the anti-salmon-farming protests here in Tasmania — the demographics and visual style of environmental opposition seem to travel.

So is ocean alkalinity enhancement — OAE, as we call it — destined to fail like a lot of the other marine CDR proposals? Not yet, at least: the good news is that there's relatively broad scientific consensus in the expert community that OAE is our sharpest blade — the method most likely to make a meaningful contribution.

What we're doing in Tasmania

I have a research team where we run lab experiments to see how different organisms respond — we've tested this with Ecklonia, a kelp species that's very important in Tasmania, and with plankton communities in seawater. After the lab experiments, we moved to small, contained field studies to test how seafloor sediments respond, and whether the process would work at all — dispersing ground-up rock powder onto the sea floor in a small, fairly rough first field test, just to see how it could practically be done.

That's the research we're doing, and there's one thing I wanted to raise and get your feedback on. We were pleasantly surprised to secure the Investigator research vessel for a reasonably large field study in 2027 and 2028 — Investigator is the Australian marine research flagship, the biggest proper, dedicated research vessel we have. The proposal is to add twenty tonnes of rock powder to the surface ocean, in Bass Strait, to see whether this works at all, and how the environment would be affected. The research team — which isn't just Tasmanian, but a pan-Australian effort — thinks this is important research to do, but we also wanted to get public feedback on whether people think it's worthwhile, and understand how people feel about the research. So that's perhaps my question back to you all.

Q&A

Host: Thank you — wow, that was fascinating. We've got about three minutes for questions — sorry we started a little late. Does anybody have a burning question, before I ask mine? I wonder if we could get Lennart back at some point to take more questions at a future meeting, if that's possible — we won't put pressure on you tonight. Anthony, go ahead.

Guest (Anthony): Lennart, have you spoken to Andrew Forrest about Fortescue's mining?

Lennart Bach: Fortescue — Andrew Forrest's mining company. Well, he's not exactly an ocean scientist, so —

Guest (Anthony): They're going 100% electric by 2030.

Guest: He's seen as a world leader on climate change in mining.

Lennart Bach: So, yeah — you've got a lot of rock there. I think I'm perhaps a little too unimportant to speak to him directly.

Guest: No, you're not.

Lennart Bach: No — well, if I did, I do know his opinion on carbon removal a little. I understand he's not particularly convinced by it — that's what I've heard, at least, secondhand. So no, I haven't spoken to him, but I'd like to.

Host: Don't wait — just do it, we need everything we can get. We'll see if we can help connect you with him, Lennart. Claire, your turn.

Guest (Claire): I just wanted to know about the implications of the sources of all that calcium carbonate as well, because you're going to need an awful lot of it — is that another problem?

Lennart Bach: Great question. Calcium carbonate itself is a very abundant resource — you could sequester all the carbon in the atmosphere something like three times over with the amount of calcium carbonate available. It's also used a lot in agricultural liming, so the supply chains are already in place — you can go to a landscape supplier and ask for twenty tonnes of calcium carbonate and get it. In our case, that would come from a limestone quarry at Mole Creek, which is easy to purchase from. It's abundant across Australia and the world, so this specific material isn't limiting — but you could do this with other materials, and those may well be limiting at some point. You're absolutely right that you need massive amounts.

Guest: Can you quickly answer how much CO2 removal we're actually talking about — is it physically possible, in terms of mass?

Lennart Bach: Good question — let's talk mass. You shouldn't think of carbon removal as one silver-bullet method doing the whole job — it'll be a portfolio of different approaches, fine-tuned to whichever environment suits them best. With fifty-plus alkalinity enhancement methods alone, some will make bigger contributions than others. But if we assumed everything was done with one method only, we'd be talking about an industry as large as the coal mining or concrete industries — a massive global scale, because the problem itself is massive: we emit forty gigatons of CO2 a year. So the answer can't be "we move around a couple of thousand tonnes" — it's massive by design. But it has to be.

Host: Excellent answer — and that's a good point for us all to remember when talking about this to others, that it's one tool in the toolbox while we rapidly stop using fossil fuels. Lennart, that's been fantastic, and I'm sure there are lots more questions, but we'll leave it there for tonight. We've got your contact details — are you open to us connecting you with others working on this on the mainland? I'm thinking of Jane Morton, for people involved with Extinction Rebellion, and David Spratt from Breakthrough.

Lennart Bach: Absolutely — I'm very happy to talk to everyone and share more about this. If anyone has questions, my email will be in the link, and I'm also happy to come back and go deeper on this specific field study at some point. Whatever's useful — just let me know.