Climate change scientists and activists have long harboured an uncomfortable secret: they don’t know how to achieve net zero emissions. That goal is imperative if the rise in global temperature is to be halted. It means human society can ultimately put greenhouse gases into the atmosphere only to the extent they are captured or removed and stored. But how the “removal and storage” part of that equation can be achieved is still something of a mystery.
In its 2018 report on limiting global warming to 1.5°C above preindustrial levels (the Paris climate agreement’s widely accepted goal), the Intergovernmental Panel on Climate Change was somewhat coy about this. It stated unambiguously that all its 1.5°C scenarios included significant use of carbon dioxide removal, or CDR, and it listed possible CDR methods. And then it described all of these as “subject to multiple feasibility and sustainability constraints.”
That’s a scientifically careful way of saying that none of the methods yet looks capable of safely storing the quantities of CO₂ — tens of gigatonnes per annum — required to achieve net zero.
But the good news is that the race is on to identify and develop processes that might do this. And in that race a possible winner seems to be emerging: carbon sequestration in the world’s oceans.
There are essentially three ways to remove carbon from the atmosphere. (Carbon removal is a separate field from “carbon capture and storage,” which uses industrial processes to capture emissions produced by chemical or power plants before they enter the atmosphere.)
The first is via vegetation. Photosynthesis is carbon removal: the more vegetation planted, the more carbon removed. Since trees and other forms of plant life can also support local livelihoods and biodiversity, considerable efforts are being made in many parts of the world to restore degraded and deforested land.
But this method has its limits. Using land for carbon storage means not using it for food production. And the increase in vegetation must be permanent, which is hard to guarantee. Only if the vegetation is burned to produce energy and the emissions directly captured and stored (a process called “bioenergy with carbon capture and storage”) is carbon definitely removed.
The second possible method of carbon removal is direct air capture, or DAC. CO₂ is retrieved from the atmosphere by solid or liquid absorbents operating either in a vacuum or at high temperature, then stored underground or in building materials. DAC technologies are in their infancy; since they use highly energy-intensive processes they are dependent on abundant and cheap renewable energy as well as accessible storage sites. Eighteen DAC facilities are currently operating in Europe, Canada and the United States, but even the largest of them, in Iceland, will only be able to capture 36,000 tonnes (0.000036 gigatonnes) of CO₂ a year.
This is why attention is increasingly turning to ocean-based methods. The world’s oceans already absorb more than a quarter of all carbon emissions; in principle they have the physical capacity to sequester the many additional gigatonnes required in the IPCC’s scenarios. This is the new scientific and commercial field of “mCDR” — marine carbon dioxide removal. The question is whether it can be done safely and cheaply enough to make net zero possible.
Some mCDR is well under way. Around the world, coastal ecosystems — mangroves, reefs, seagrass meadows and tidal salt marshes — are being conserved and restored to enhance “blue carbon” uptake. Such methods offer vital economic benefits to local communities, but their carbon removal potential is relatively small. The major gains can only be made by enhancing the oceans’ natural “carbon cycle” — the processes by which carbon is redistributed between the atmosphere, surface waters and deep waters.
One way of doing this is by adding alkalinity to the ocean. Human-made CO₂ has made the oceans more acidic, with adverse ecological effects including damage to coral reefs. Adding alkaline minerals to the ocean would convert dissolved inorganic CO₂ into carbonates and bicarbonates, which are stable forms of carbon with lifetimes of thousands of years. CO₂ equilibrium would then be restored in surface waters by natural carbon removal from the atmosphere. De-acidification would also have significant ecological benefits.
The ocean alkalinity produced naturally by the runoff from slowly weathering rocks could be greatly enhanced. Billions of tons of alkaline minerals — olivine, basalt, limestone — are readily available to be mined and could be applied to beaches or the open ocean. A number of commercial firms are developing “enhanced weathering” processes of these kinds. Others are looking at the electrochemical filtration of seawater to remove excess acid caused by human-made CO₂ and return alkaline seawater in its place.
A different element of the marine carbon cycle is powered by photosynthesis. Microscopic algae living in the ocean’s surface waters — known as phytoplankton — photosynthesise dissolved CO₂. This again pulls in more carbon from the atmosphere in a natural rebalancing, with a portion of the organic matter sequestered in the ocean depths. The more phytoplankton, the more carbon removed and stored.
Phytoplankton growth can be stimulated by adding mineral nutrients, particularly iron, to the ocean. This could be done either in engineered substrates — a controlled but expensive method — or in the open ocean. Artificial pumping could then push greater quantities of organic matter downward for sequestration. Perhaps surprisingly, large areas of ocean have almost no microalgae, and therefore little life of any kind. Fertilising these areas with iron to stimulate phytoplankton could therefore have wider ecological benefits as well as removing carbon from the atmosphere.
Equally, though, it could be environmentally damaging. Any interference with the sensitive ecology of the oceans risks unforeseen and adverse effects, and it makes environmentalists nervous.
In the United States, Friends of the Earth has little doubt: “These technologies would have to be implemented at truly massive scales to have any impact on the climate, imposing disastrous side-effects on ocean ecology, marine life [and] coastal communities.” Arguing that carbon removal is a distraction from the primary task of reducing emissions — and thereby lets polluting companies off the hook — they oppose all forms of what they call “marine geoengineering.”
But other NGOs are keener to engage. In the US the Environmental Defense Fund is taking part in scientific mCDR research. The EDF accepts that, even with rapid emissions reduction, carbon removal will be essential to achieve global warming goals, and believes that trials of the different removal methods are necessary to explore their technical and commercial feasibility and environmental impact.
Iron fertilisation is generating particular attention because it looks likely to be the cheapest method of large-scale carbon removal. Phytoplankton are very efficient at absorbing carbon dioxide and iron is cheap. It is estimated that the cost of carbon removal could be less than $50 a tonne, against a benchmark of $100/tonne generally used as the threshold for commercial viability.
In the last few months the scientific prospectus for mCDR has seen some important advances. In November the European Marine Board, an independent advisory body, recommended a set of standardised protocols for research and feasibility studies. In January a network of scientific bodies, NGOs and philanthropies, Ocean Visions, published a detailed research framework for investigating phytoplankton-based carbon removal, identifying how remaining uncertainties and knowledge gaps can be reduced.
The next stage is major field trials. More than twenty small-scale experiments in ocean iron fertilisation have already been conducted, but larger and longer studies are needed to understand sequestration potential and environmental impacts. The Woods Hole Oceanographic Institution in the US has set out a program for such trials, with the Gulf of Alaska in the Northeast Pacific a particularly promising location.
But the obstacles to large-scale deployment of mCDR remain formidable. For a start, the international ocean waste governance regime will need to be amended; the 1972 London Convention and its 1996 London Protocol forbid “dumping” of materials in the ocean. And mechanisms for financing mCDR deployment still need to be devised: if this were through “offset” markets allowing polluting companies to substitute cheap removal for more expensive emissions reduction it would almost certainly generate furious opposition from environmental NGOs.
Yet the attraction of marine carbon dioxide removal is also evident. Ocean-based solutions wouldn’t compete with other uses of scarce land, such as food production. Because they use naturally occurring biological and chemical processes of carbon absorption they will almost certainly prove cheaper than direct air capture methods. Most of all, the sheer abundance of the world’s oceans — if the ecological impacts can be proven positive — makes it possible to imagine global carbon removal on the scale that will be needed to keep 1.5°C of warming within reach. As that goal becomes more pressing over the next two decades, the world’s major emitting countries may be willing to pay for such solutions.
Last week Fiji and Tuvalu were announced as hosts of the “pre-COP” meetings in October that will prepare the next UN climate conference, COP31. It might not have escaped the attention of the Pacific islands that they have some of the largest territorial ocean waters in the world. •
