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Croplands can suck lots of CO2 from air if treated with crushed rock

REUTERS/Ilya Naymushin
If two-thirds of the world’s most productive farmland were treated, it could absorb ongoing emissions at a rate equivalent to 10 percent of current CO2 output.

Most home gardeners know at a least a bit about soil amendments — how you can make plants grow better by working in compost to fertilize and loosen the loam, maybe some crushed limestone to lower acidity.

What if the same principle could be applied, at a massive scale, to take globe-warming carbon dioxide out of the atmosphere?

What if it could work relatively quickly, compared to other carbon-removal methods, and perhaps even cheaply? While simultaneously improving plant health and crop yields for a world with ever more hunger in its future? And reducing the need for fertilizers and pesticides?

This is the bold, if still somewhat blurry, vision outlined in a fascinating paper published last week in the respected journal Nature Plants.

Prepared by an international team of scientists, with America’s renowned James Hansen among them, the paper makes a strong conceptual case that significantly reversing CO2 buildup need not require “geoengineering” approaches as elaborate as covering the ocean with wiffle-ball blankets, as expensive as gigantic carbon-sucking sequestration plants, or as risky as creating an artificial cloud layer out of reflective aerosols.

There might just be a role, they suggest, for a solution as simple as dirt — in this case, soil with pulverized volcanic rock worked into it, in a way that mimics natural weathering, and accelerates certain soil-improving benefits that farmers have recognized since antiquity.

And the potential contribution to easing the climate problem is impressive. While calculations are still at back-of-the-envelope stage, the research suggests that if two-thirds of the world’s most productive farmland were treated, it could absorb ongoing emissions at a rate equivalent to 10 percent of current CO2 output.

The paper’s lead author is David Beerling of the UK’s University of Sheffield, where he directs the Leverhulme Centre for Climate Change Mitigation. In announcing the findings, he said:

Human societies have long known that volcanic plains are fertile, ideal places for growing crops without adverse human health effects, but until now there has been little consideration for how adding further rocks to soils might capture carbon.

This study could transform how we think about managing our croplands for climate, food and soil security. It helps move the debate forward for an under-researched strategy of CO2 removal from the atmosphere —  enhanced rock weathering —  and highlights supplementary benefits for food and soils.

Just add rainwater

Volcanic soils are highly silicate, basalt being a good example of one globally abundant type, and also tend to be rich in calcium and magnesium. Their mineral makeup reacts with the slight acidity in rainwater to draw carbon dioxide out of the air, forming carbonates and bicarbonates.

(Those are alkaline substances, so I guess you could think of the process as sort of the chemical reverse of the way sulfide mining mixes waste rock with water to produce polluting acid runoff.)

If these alkaline compounds stay in the soil, they become an immediate, long-term carbon sink. But what if they should flow to the ocean, an important though temporary carbon sink which is already experiencing a troubling degree of acidification because of atmospheric CO2?

No worries, says the paper. The carbonates and bicarbonates are sufficiently stable to remain undissolved, and keep the carbon locked up, for 100,000 to 1 million  years.

What would it take to crush and distribute all that rock in a global program of “enhanced weathering”? This is among the practical questions that the paper suggests are worthy of additional inquiry, but it offers some general thoughts and more back-of-the-envelope figuring.

Happily, basalt and other silicate rocks are plentiful throughout the world. Unhappily, crushing and transporting them for massive soil amendment would require a lot of energy, much of it from fossil fuels, reducing the efficiency of its carbon recapture by maybe 10 percent, possibly as much as 30 percent.

As to cost, the paper says the best estimate right now is between $52 and $480 per ton of recaptured CO2. That compares unfavorably with, say, a bioenergy power plant that captures its own CO2  at a cost of $39 to $100 per ton.

But remember, recapture from the atmosphere is way more expensive; 10 times more expensive seems to be a rule of thumb. One Swiss project that's considered promising sucks CO2 back out of the air at “industrial scale,” which means 900 tons per year, or the equivalent of emissions from 200 cars — at a cost of perhaps $1,000 per ton.

Keeping new carbon out of the atmosphere in the first place is always the best idea, the most efficient, the cheapest way to keep global warming from getting worse. Problem is, the evidence increasingly suggests it won’t be sufficient on its own.

As Hansen, formerly of NOAA and now at Columbia University’s Earth Institute, said of the new paper: "Strategies for taking CO2 out of the atmosphere are now on the research agenda and we need realistic assessment of these strategies, what they might be able to deliver, and what the challenges are."

The urgency of that assessment can hardly be understated, nor the pervasive glumness around our chances. Last week I stumbled on yet another discouraging climate headline, “Sighing, Resigned Climate Scientists Say To Just Enjoy Next 20 Years As Much As You Can,” and failed to recognize it as pitch-perfect satire from The Onion.

Agriculture's climate shadow

Agriculture is itself a large contributor of atmospheric CO2 and other greenhouse gases, by some measures accounting for nearly half of atmospheric carbon, and much attention already focuses on finding ways to reduce those contributions.

The Drawdown project organized by Paul Hawken, for example, favors expanded efforts in the practice of  “conservation agriculture” and “regenerative agriculture,” which abandon tillage in favor of expanded crop rotation and more use of cover crops. Also, greatly expanded use of compost to return valuable, natural nutrients to the soil while reducing manufacture and application of synthetic fertilizers.

And the reason these proposals hold such great potential is the same that supports, I think, a serious investigation of the “enhanced weathering” idea: There’s a whole lot of farming going on, and there always will be.

Making cropland better at retaining or recapturing carbon builds on existing efforts and practices already well understood, as the Leverhulme announcement points out:

Critically, enhanced rock weathering works together with existing managed croplands. Unlike other carbon removal strategies being considered, it doesn’t compete for land used to grow food or increase the demand for freshwater. Other benefits include reducing the usage of agricultural fertilizers and pesticides, lowering the cost of food production, increasing the profitability of farms and reducing the barriers to uptake by the agricultural sector.

Crushed silicate rocks could be applied to any soils, but arable land is the most obvious because it is worked and planted annually. It covers some 12 million square kilometres or 11 per cent of the global land area.

Arable farms already apply crushed rock in the form of limestone to reverse acidification of soils caused by farming practices, including the use of fertilizers. Managed croplands, therefore, have the logistical infrastructure, such as the road networks and machinery, needed to undertake this approach at scale.

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The full paper, “Farming with crops and rocks to address global climate, food and soil security,”  can be read here but access is not free for nonsubscribers.

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