Carbon-dioxide removal needs more attention

The Earth is isolated, ancient and the site of ceaseless change. This means that everything that happens on and below its surface is part of a cycle. For every change which uses things up, there must be another which supplies those things anew. The basic elements of life, such as carbon and nitrogen, cycle endlessly in and out of living things, the seas, the land and the atmosphere. Even the planet’s crust is recycled.

New crust is created where tectonic plates move away from each other, normally in the middle of an ocean, and molten rocks rise from the mantle below to fill the gap. Old crust is destroyed where two plates are pushed together and one is overridden by the other, sinking back into the mantle.The wheels of this great cycle have been turning slow and steady for billions of years. But there is the occasional glitch. The rocky peaks of the eastern Arabian peninsula bear witness to one.

Most geologists looking at the Arabian peninsula concentrate on the deep and extensive sedimentary basin that underlies the Persian Gulf and the lands around it. Organic matter in this basin’s depths has been cooked by heat and pressure into vast amounts of oil and gas which, having percolated upwards, now sit in rocks near the surface. They are very profitable rocks to study. Unfortunately their exploitation, along with that of carbon-rich rocks in other places, has destabilised the climate. On November 30th the governments of the world will meet in Dubai, a city built with the wealth from those rocks, to further negotiate their response to that destabilisation at the 28th Conference of the Parties (COP28) to the UN Framework Convention on Climate Change.

For those interested in tectonic glitches, though, it is the Hajar mountains to the east that matter. In an early stage of the collision between the Arabian plate and the Eurasian plate to its north a slab of ocean floor was caught between the two encroaching land masses in the sort of circumstances which would normally see it pushed down into the mantle. On this occasion, though, the rocks went not down, but up, lifted like a curl of wood by a carpenter’s plane. The ancient sea floor, mostly basalt, and some of the mantle it had rested on, a related rock called peridotite, ended up exposed to the open sky. In time, the sea floor formed the mountains.

Like all mountains the Hajar range is being eaten away by erosion; it is another part of the great recycling. The erosion endlessly exposes fresh rock, and that rock takes up carbon dioxide from the atmosphere through what is called “chemical weathering”. The alkaline minerals in the rocks react with rain and groundwater made slightly acidic by dissolved carbon dioxide to produce carbonate minerals of the sort that make up limestones. The peridotites in the Hajar are particularly susceptible to this weathering. Their dark stone is shot through with white veins of carbonate.

Chemical weathering is not the fastest of the natural processes which draw down carbon dioxide. Photosynthesis, carried out on land by plants and in the sea by algae and bacteria, operates on a far larger scale, removing more than 300 times as much carbon dioxide from the atmosphere every year. But it does not stay removed for long. The carbon is recycled to the sky by the plants themselves, the creatures that eat them and the soil into which they decay over timescales from days to centuries. The geological carbon cycle is far slower. Carbonates in places like the Hajar will be stable for hundreds of millions of years.

Until recently, adnoc, the national oil company of the United Arab Emirates (uae), concentrated its geological thinking on bringing carbon-rich oil and gas up from the bountiful sediments of the Gulf. Now, though, it has turned its eyes to the peridotites of the Hajar, and to pumping carbon dioxide down. In the hills above Fujairah, a city on the Gulf of Oman, adnoc and 44.01, an Omani startup, are working on a pilot plant at which 44.01 will inject carbon dioxide deep into the rock in a way that encourages its mineralisation into inert carbonate. Musabbeh Al Kaabi, head of “low-carbon solutions” for adnoc, sees his firm’s investments in this rapid mineralisation as part of a comprehensive decarbonisation strategy for the oil industry, one that aims to deliver its “very vital commodity in the most sustainable way”.

The Fujairah experiments are part of a nascent planet-wide effort to undo another glitch in the world’s great cycles: humankind’s transfer of fossil-fuel carbon from its quiet rest in the solid Earth to the hurly-burly of the atmosphere. Roughly 1trn tonnes of carbon dioxide have accumulated there thanks to human activity. The total is growing by a bit less than 20bn tonnes a year.

For a sense of scale, compare that with other planetary flows. It is about 60 times faster than carbon dioxide is removed by the weathering of the Earth’s rocks. It is around a tenth of the rate at which photosynthesis makes new biomass. That an accidental by-product of industry should be remotely comparable in its carbon flux to the process which powers all life on Earth is extraordinary.

It might also seem comforting; large though the human flow is, the biological one is comfortably larger. Can it not simply increase to accommodate humankind’s imposition? Alas, no. The biological carbon cycle is big, but it is also balanced; the rate at which the world’s biosphere photosynthesises is almost exactly the rate at which life’s other processes return carbon dioxide to the atmosphere. With carbon dioxide from fossil fuels added to the natural emissions, photosynthesis has valiantly tried to keep up, sucking back down as much as it can. But it cannot do enough. It only absorbs about a third of the emissions from human industry and agriculture (see chart).

The build-up of carbon dioxide in the atmosphere has raised the planet’s temperature by about 1.2°C (2.2°F). The temperature will go on rising until the accumulation stops, which is to say until annual additions are reduced to more or less zero. That is why the governments of the world agreed to work towards that end at the Paris climate conference of 2015.

For the most part, that means cutting emissions of carbon dioxide and other greenhouse gases. But some emissions—those from transport across oceans, from some types of farming, from a variety of industrial processes and more—seem highly unlikely to be entirely eliminated any time soon. So the Paris agreement specified that stabilisation need not be a matter of no emissions at all; instead it could be achieved by means of “a balance between anthropogenic emissions…and removals”. Residual, “hard to abate” emissions of greenhouse gases were to be balanced by the withdrawal of carbon dioxide already in the atmosphere. The project at Fujairah aims to show one of the ways in which what went up can come down, and the way of the world be righted.

This is the logic of “net zero”. Back in 2015 only one country had enunciated a net-zero target for its economy: Bhutan. Now the number is 101, and between them they account for just over 80% of global greenhouse-gas emissions. The increasingly vocal opponents of these net-zero targets on the political right say many of the domestic policies associated with cutting emissions are too expensive, or irksome, or both. Those focused on keeping global warming since the Industrial Revolution well below 2°C, as per the Paris agreement, know those steps being taken to reach net zero are also not yet ambitious enough. As the “emissions gap” report issued by the un Environment Programme in the run up to the Dubai COP points out, none of the G20 countries is reducing emissions at a pace consistent with its net-zero target.

Business not-as-usual

There is a lot less concern about the burgeoning removals gap. Few of those who have mouthed commitments to net zero appreciate how central greenhouse-gas removal is to the notion; of those who do, few recognise quite how vast the challenge is. Emission cuts of 90% would still see enough gas entering the atmosphere for a balancing level of removals to be a huge undertaking.

Studies by the Intergovernmental Panel on Climate Change suggest that if the planet is to stand a decent chance of staying below the 2°C limit on warming it would be wise to plan on removing an additional 5bn tonnes of carbon dioxide from the atmosphere every year. According to a report published in 2023 by an international team of academics, if you do not count managed forests, which have only limited room for expansion, the amount of carbon dioxide squirrelled away in durable storage in 2020 was 2.3m tonnes, or around a two-thousandth of that 2050 target. The Fujairah plant’s pilot phase runs at just 1,000 tonnes a year.

New forms of durable removal need to be scaled up far more quickly than is happening. And they need to earn trust. At the moment, many who realise that removals are needed remain sceptical of the technology, not least because it is championed by the oil industry. Mr Al Kaabi’s vision of a world free to produce and use oil “in the most sustainable way” does not sit well with those who think it necessary to stop the burning of all fossil fuels. The location of cop28 will bring such questions to the fore.

One reason oil companies are making the running is that they have expertise moving fluids in and out of the Earth’s crust. They also have lots of money, and carbon-dioxide removal currently looks very pricey. The obvious way to fund it efficiently is through markets. But none of the carbon markets around today is up to the job. This means that the net-zero strategies most of the world has embraced depend not just on inchoate technologies which can pull carbon dioxide out of the atmosphere and store it away, but on the creation of a carbon economy which makes doing so worthwhile. Climate policy insists that humans, their governments and their economy can and must be integrated into the planet’s great cycles of renewal. But how is that to be done? ■

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