The Paris Agreement includes in Article 2.1.a an alternative 1.5C target for global warming, as an aspiration. The IPCC has been tasked with preparing a special report on what this target implies. Somebody with political guile leaked an early working draft of the summary for policymakers of the report. This has interesting things to say on sequestration, page 18.
3.5 All mitigation pathways compatible with limiting global warming to 1.5°C by 2100 involve removal of CO2 from the atmosphere. […] The total amount of CO2 removal projected in 1.5°C pathways in the literature is of the order of 380-1130 Gt CO2 over the 21st century.
Converting from CO2 to carbon (the ratio is 3.67, from the atomic weights in the molecule – no guesswork here), we get a wide range from 104 to 308 Gt carbon. Annual emissions today are about 9 Gt.
Suppose we allow ourselves 50 years for the task. Assuming that 1.5 degrees becomes the policy goal, we will have to bury 2 to 6 Gt a year. Ouch.
BTW, we should convert to carbon. Nobody can imagine a tonne of a gas, but carbon is a solid. A tonne of carbon in coal is for instance typically 1.2 m3. Using my proposed journalistic unit of the Cheops (the volume of the Great Pyramid at Giza), the sequestration effort will be from 1,000 to 3,000 pyramids’ worth. Every year.
There is a one-stop miracle fix. From 1 January 2020, just say to all fossil fuel companies, in the spirit of the Spanish proverb: “Take what you like, said God. Take it, and pay for it”: anyone can emit as much CO2 as they like, but must put it back within ten years. No pesky negotiations over the carbon tax are needed. Simple!
Somehow I don’t feel this is likely to gain acceptance, so we need to work on the costs, technologies, and incentives that could conceivably inform policy in the real world. My initial thoughts below the jump.
The technology list
Start by leaving out atmospheric geoengineering (too dangerous), extra-atmospheric (pies in the sky), and power station CCS (tried at huge expense and found wanting). Here are some candidates still standing. They rely on souping up existing natural processes, so the risks are limited.
Biology group
- Reafforestation
This technology is obvious and shovel-ready. All you need to do is reverse net deforestation and switch to net reafforestation. The problems are political, arising ultimately from the clash over land use with livestock rearing and mining (Brazil) and palm oil plantations (Indonesia). Fixing these is likely to require compromises over the type of replanting. For maximum sequestration, you probably want a recreated natural forest, with very limited human use. A mixed-use European-style managed forest with selective harvesting of mature trees for construction timber trades off some sequestration for higher economic value: worth doing IMHO to secure acceptance. (I’m not sure of my facts here. A stable virgin forest is in carbon equilibrium. A sustainably managed one can in theory allow net extraction of timber, sequestering carbon in construction for another century). The fastest biomass growth is from monocultures of fast-growing species like eucalyptus and bamboo, but their economic use at harvest is for paper and cardboard, for which world demand is stagnant. We can forget about this for practical purposes.
- BECCS (Bio-energy with carbon capture and storage)
Not a pet
This relies on burning biomass, recovering the CO2 from the flue, liquefying it, and burying it. The scheme has eminent support – the IPCC, Hadley Centre, etc. But almost all of this is just the same as the technology used in the expensively failed power station CCS plants. This smells to me like the same snake oil, sold by engineers in love with Big Tech. Swapping a black mamba for a green mamba doesn’t make you safer.
For power generation, there are real if overstated technical advantages to big reliable plants run by qualified people that can be ramped up and down at a moment’s notice. For sequestration, this does not apply at all. Precise timing does not matter. The simpler and more distributed the technology, the better.
A simple idea: grow biomass (of any type), pyrolyse it to charcoal, and add the charcoal to farmland. It improves soil quality, and the carbon is not released. Pretty well researched, but not tested at scale.
This is a classically “green” movement that reduces inputs of fertilisers and pesticides, as well as soil disturbance through ploughing. Since the practice increases organic matter in soil, it fixes carbon, at least in the transition, swinging from the net carbon release of intensive agriculture. The potential contribution to sequestration looks limited, but it does not look expensive either.
- Biomass burial at sea
Without going through the fuss of pyrolysis, you can grow trees or sugarcane or bamboo or whatever grows fastest, and dump it in landfills. You soon run out of landfill sites, but there’s an enormous storage capacity on the deep abyssal plains of the ocean floor. (Tie the biomass in bales and weight with a few rocks.) This could run into problems with the 1972 London Convention against ocean dumping, but that could be changed. The slow decomposition process on the ocean floor would have to be researched. A priori, you are only supplying extra food to seabed fauna to supplement their limited current diet of fish shit, whale carcasses, and each other.
A twist on this is to cut out the land plants stage and go straight to seaweed farms. Gigatonnes of kelp are already washed down every year from seaweed forests off California and the Bahamas, as a fully natural process. Grow the seaweed from giant submerged rafts, in situ on the high seas. When it’s grown, cut, weight and sink. You avoid land use conflicts entirely. China already has 500 km2 of seaweed farms in the shallow Yellow Sea, so large scale is demonstrated.
Mineral carbonation group
- Olivine weathering
Olaf Schuiling proposed in 2006 a scheme to grind up olivine, a very common magnesium silicate mineral, and exposing it it in large windrows.
Weathering is the neutralization of an acid (usually carbonic acid) by rocks, turning CO2 into the innocuous bicarbonate ion in solution. For the abundantly available magnesium-silicate olivine, the reaction is as follows:
Mg2.Si.O4 + 4 CO2 + 4 H2.O → 2 Mg2+ + 4 H.C.O3- + H4.Si.O4
These bicarbonate solutions are carried by rivers to the sea, where they are ultimately deposited as limestones and dolomites.
The final stage involves conversion of the bicarbonate by corals and diatoms into their exoskeletons as calcium carbonate, and their deposition on the ocean floor. This process is slow (over 100,000 years). However, the intermediate storage in the ocean is stable over long periods. The carbon in solution there is ca. 40,000 Gt, 100 times the high estimate for required total sequestration. There is little risk of destabilising the system. The required scale of mining is less than that of the current mining industry.
Schuiling sparked a lot of work on mineral carbonation, including the next two. One snag others identified with olivine is that it tends to be associated with nickel and chrome. Weathering releases these, and they risk getting into food chains, where they are damaging. It’s not a killer objection, but a serious difficulty.
- Basalt on farmland
David Beerling et al (an impressive list) have just published an important paper proposing a variant carbonation scheme. Start with basalt, also available by the trillion tonnes from past supervolcanoes (Deccan and Siberian Traps, Yellowstone, etc). The magnesium and calcium silicates weather slower than olivine, but the rock doesn’t have the pesky metals, and adds in useful amounts of phosphorus. The scheme is to grind up the rock and spread it on farmland. The weathering reaction and ocean storage is the same as for olivine. The process provides useful benefits, from soil texture to the replacement of silicon lost in biomass. This has been tested in small trials. Basalt is already spread commercially on sugarcane fields in Brazil, whose enterprising, export-oriented farming sector contrasts with its timid and protected industry.
- Injection into basalt rocks
In Iceland, a medium-scale pilot, funded by the EU, is underway to inject liquefied CO2 directly into deep basalt beds, which the country has in plenty. The CO2 reacts directly with silicates to form carbonates, as in the final stage of the previous two approaches. This works. The problem is the high-tech first stage of concentration and liquefaction of CO2, which has hamstrung coal plant CCS.
- Technologies X and Y
In such a new field, it would be a mistake to rule out some other new idea. Consider. We are heading for a world with abundant and very cheap electricity from wind and solar, on an intermittent basis. The latest German auction came out at €4c/kwh for both wind and solar, and they have rather poor resources. Good sites elsewhere price at half that, and prices will continue to drop. Costs rise if you add in secure supply, but that isn’t necessary for long-term sequestration projects. How can you use cheap electricity to capture CO2 from the air and turn it into bicarbonate in solution? Imagine a fleet of automated floating wind powered factories in the Roaring Forties.
This is not a comprehensive list. It is enough to show that there is already a portfolio of seriously thought-out and in some cases tested technologies that, separately or together, offer a very good hope of solving the sequestration problem.
What are the costs?
From the Beerling paper, we have guesstimates for a couple of technologies:
- Weathering of crushed basalt on farmland US$14–130 per tonne C (ignoring cobenefits to farmers)
- Bioenergy with carbon capture and storage US$11–27 per tonne C
At such an early stage in the development of technologies destined for large-scale use (the small scale is a waste of time except for research), little reliance can be placed on such estimates. Still, let’s run with them to see where we go.
Pick midrange illustrative numbers of $25 per tonne and 4 Gt a year of sequestration. That would cost $100bn a year. World GDP (GWP) was $107 trn in 2014, and it grows by about 3.5% a year, or $3.7 trn. $100bn would be 0.09% of GWP, or 2.7% of annual growth.
This order of magnitude is clearly affordable, especially when you think of the alternative. The general rule is that industrial processes become cheaper with scale and experience, and we have a lot of candidates in competition. (One example: several companies are experimenting with automated tree planting from drones.) It is worth remembering that the market cost of aggressive mitigation - not emitting CO2 in the first place - is effectively nil for electricity today and will be nil for vehicles within a few years (I’m guessing five). Both turn into massive free lunches if you add in avoided health damage and costs, as you jolly well should.
Balanced incentives
The GOP tax cut bonanza included, to general surprise, a $50 per tonne carbon tax credit for coal and gas CCS. This was a conceptual breakthrough, as it conceded the reality of the problem, and the magnitude is realistic. It is however hopelessly biased towards the worst technology on offer, apart from thorium reactors.
Le us dream of a market for rational policy and think ahead a little.
1. Sequestration requires subsidies. It is extremely unlikely that any large-scale sequestration can become profitable without them.
2. At this early stage, the playing field should be kept as level as possible. A wide variety of methods should be investigated and their early deployment supported. The subsidies should however be differentiated according to the maturity of the technology, as in the great German EEG of 2000. This is the opposite of the picking-winners and big tech style followed so far with CCS and basalt injection. Consider a big prize fund, rewarding significant partial advances in exchange for open IP.
3. Politically, massive sequestration revives the free rider problem that bedevilled early climate mitigation diplomacy, until falling costs for renewables and rising estimates for local health costs made it go away. International cooperation will be essential. Burden-sharing will be tricky. Developing countries will reasonably insist on a “polluter pays” principle, sharing costs according to past cumulative emissions. This won’t be easy. The hope is that the problem will be addressed on the back of a massive joint success on mitigation, which is now reasonably likely.
4. Since sequestration costs money and mitigation is free, it pays to minimise the former through ambition on the latter.
5. We aren’t going to stay within the 1.5 degree carbon budget. So let’s get started on sequestration options, now, with real money.
Afterthought
6. There is no reason to stop at 1.5 degrees of warming on the way down. The 1 degree warming we have now, at 400 pm of atmospheric CO2, is already having alarming effects on droughts, floods, hurricanes, Arctic sea ice, and the melting of the great ice sheets. The ultimate goal should be to get back to the equilibrium preindustrial level, say 350 ppm, and let the temperature stabilise then. Sequestration technology will get cheaper as time goes on, and we will have already set up the systems and paid for the equipment. So our grandchildren should just keep it running until they have white Christmases again.
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