Compared with trying to counteract climate change resulting from greenhouse gas pollution through solar radiation management (SRM) — essentially reflecting sunlight away, as with stratospheric sulfate injection — actually removing CO2 from the atmosphere by weathering rocks which form carbonates seems more attractive in many ways. The SRM approach may cause major side effects in terms of changes in precipitation, and any cessation in the injection of reflective aerosols in the upper atmosphere would lead to very abrupt climate change.
I asked David Keith about the idea when he was in Toronto talking about SRM-based geoengineering and he said that the problem is simply one of reaction rates. Even if we used zero-carbon energy to grind up vast amounts of ultramafic rock to absorb CO2, that process of absorbtion would happen so slowly that it would not counteract human-induced climate change on reasonable timescales.
I learned about the idea from Wallace Broecker and Robert Kunzig’s book Fixing Climate. Another oft-touted means of removing atmospheric CO2 is biochar. More recently, I read about the idea of speeding up natural rock weathering by biological means. I don’t know if this could somehow overcome Keith’s objection about reaction rates.
A paper just published in Science offers a possible solution. By burying CO2 in the right sort of rock, a team of alchemists led by Juerg Matter, a geologist at Southampton University, in Britain, was able to transmute it into stone. Specifically, the researchers turned it into carbonate minerals such as calcite and magnesite. Since these minerals are stable, the carbon they contain should stay locked away indefinitely.
Dr Matter’s project, called CarbFix, is based in Iceland, a country well-endowed with both environmentalism and basalt. That last, a volcanic rock, is vital to the process, for it is full of elements which will readily react with carbon dioxide. Indeed, this is just what happens in nature. Over geological timescales (ie, millions of years) carbon dioxide is removed from the air by exactly this sort of weathering. Dr Matter’s scheme, which has been running since 2009, simply speeds things up.
Between January and March 2012 he and his team worked at the Hellisheidi geothermal power station, near Reykjavik. Despite its green reputation, geothermal energy—which uses hot groundwater to drive steam turbines—is not entirely emissions-free. Underground gases, especially CO2 and hydrogen sulphide (H2S), often hitch a ride to the surface, too. The H2S, a noxious pollutant, must be scrubbed from the power-station exhaust before it is released, and the researchers worked with remainder, almost pure carbon dioxide.
The Paris agreement of 2015 calls for increases to the atmosphere’s carbon-dioxide level caused by fossil fuels to end by the second half of this century. Even if that deadline is not met, some mixture of policy, catastrophe and/or resource depletion will eventually bring the rise to an end. The flows of carbon between the atmosphere, oceans and biosphere will then come back into balance.
But the equilibrium thus restored will not be the pre-industrial one. The carbon-dioxide level will settle down not far short of whatever the 21st century’s peak level turns out to be. Which means that temperatures will stay high, too—with all that entails for crops, ice caps and the like.
This plateau will eventually subside. The erosion of the Earth’s crust exposes silicate minerals that react with carbon dioxide, eventually producing solid carbonate minerals from which the carbon cannot readily escape. But this “chemical weathering” works on a much longer timescale than the sinks. Geochemists think it would take 1,000 years for a post-fossil-fuel carbon-dioxide level of around 550 parts per million to be brought back below today’s 415ppm towards a mid-20th century level of 315ppm.
https://www.economist.com/schools-brief/2020/05/09/humanitys-immense-impact-on-earths-climate-and-carbon-cycle
Spreading rock dust on fields could remove vast amounts of CO2 from air
It may be best near-term way to remove CO2, say scientists, but cutting fossil fuel use remains critical
“The analysis, published in the journal Nature, estimates that treating about half of farmland could capture 2bn tonnes of CO2 each year, equivalent to the combined emissions of Germany and Japan. The cost depends on local labour rates and varies from $80 per tonne in India to $160 in the US, and is in line with the $100-150 carbon price forecast by the World Bank for 2050, the date by which emissions must reach net zero to avoid catastrophic climate breakdown.”
Potential for large-scale CO2 removal via enhanced rock weathering with croplands
Enhanced silicate rock weathering (ERW), deployable with croplands, has potential use for atmospheric carbon dioxide (CO2) removal (CDR), which is now necessary to mitigate anthropogenic climate change. ERW also has possible co-benefits for improved food and soil security, and reduced ocean acidification. Here we use an integrated performance modelling approach to make an initial techno-economic assessment for 2050, quantifying how CDR potential and costs vary among nations in relation to business-as-usual energy policies and policies consistent with limiting future warming to 2 degrees Celsius. China, India, the USA and Brazil have great potential to help achieve average global CDR goals of 0.5 to 2 gigatonnes of carbon dioxide (CO2) per year with extraction costs of approximately US$80–180 per tonne of CO2. These goals and costs are robust, regardless of future energy policies. Deployment within existing croplands offers opportunities to align agriculture and climate policy. However, success will depend upon overcoming political and social inertia to develop regulatory and incentive frameworks. We discuss the challenges and opportunities of ERW deployment, including the potential for excess industrial silicate materials (basalt mine overburden, concrete, and iron and steel slag) to obviate the need for new mining, as well as uncertainties in soil weathering rates and land–ocean transfer of weathered products.
U of A researcher pitches mineral carbonation as solution to climate change to U.S. policy makers | CBC News
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Wilson says there is potential to scale up mineral carbonation to a billion tonnes a year by the end of the century, but it would require all of the waste streams of carbon dioxide and mineral wastes that Canada produces.
https://www.cbc.ca/news/canada/edmonton/u-of-a-researcher-pitches-mineral-carbonation-as-solution-to-climate-change-to-u-s-policy-makers-1.5942956
U of A researcher pitches mineral carbonation as solution to climate change to U.S. policy makers | CBC News
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Wilson says there is potential to scale up mineral carbonation to a billion tonnes a year by the end of the century, but it would require all of the waste streams of carbon dioxide and mineral wastes that Canada produces.
Rare Mantle Rocks in Oman Could Sequester Massive Amounts of CO2 – Scientific American
https://www.scientificamerican.com/article/rare-mantle-rocks-in-oman-could-sequester-massive-amounts-of-co2/
The problem is cost. Climeworks says it costs between $600 and $800 to separate a tonne of CO2 from the Icelandic air and store it away, though it may do better in larger plants. It sells customers the assurance that a tonne of CO2 has been turned to stone at their behest for over $1,100. Because Orca is exciting and its capacity small, these offsets have more or less sold out. But when non-novelty offsets sell for a hundredth of the price it doesn’t look like a very scalable business. One serious rival, a Canadian firm called Carbon Engineering, says it can offer offsets at $300 a tonne when it gets its 1m-tonne-a-year plant operating in Texas by 2025. That fits with an analysis in an academic journal by the company’s founder, David Keith, that puts the costs of the technology it is using in the $90-240/tonne range.
https://www.economist.com/special-report/2021/10/27/why-the-world-needs-negative-emissions