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The soil carbon opportunity

The soil carbon opportunity

Let the world’s farmlands breathe and come alive, and they might just steer the planet to net zero

Two years ago a heavily promoted piece of research on trees suggested they alone could solve the planet’s carbon problem. The truth is not so simple. A new consensus is emerging that soil itself – and everything it nurtures – could be the biggest natural carbon sink available. The question is how to maximise its potential, and one answer comes from northern China.

In the 1990s the Loess Plateau, a France-sized area in China’s Yellow River Basin, was a vast tract of dust. The land was more or less barren – centuries of intensive grazing and agricultural production had turned its soils to mere dirt and left the area vulnerable to the effects of landslides, floods and droughts. A massive effort to restore the region’s ecosystems started in 1994. Over the course of a decade, the World Bank and the Chinese government invested $500 million in the “Grain to Green Project” (GTGP), aimed at reducing soil erosion and restoring sustainable agricultural production in a 35,000km2 area of the plateau. 

Twenty five years later and the project area is virtually unrecognisable. There are vastly more trees and grasslands and green terraces of fruit trees cascade down the hillsides towards flat grain fields at the bottom of gullies. 

Climate mitigation wasn’t the goal at the outset – the project was more concerned with land restoration, soil conservation, water management and rural poverty reduction – but studies tracking the programme found it also sequestered carbon, and therein lies an urgent lesson for the world.

How – and how much?
A 2019 paper showed that the average carbon sequestration during 2000 and 2015 was 415.34 gC per m2 per year. The carbon sequestration was measured as Net Primary Productivity (NPP), a sum of how much carbon plants take in during photosynthesis and how much they release during respiration. There is a read-across to soil carbon: the higher the NPP the higher the soil carbon sequestration over time. 

Land use change: Much of the soil carbon sequestration came from land use change, i.e. converting unproductive agricultural land into forests, shrublands and grasslands. A 2014 study in Nature’s Scientific Reports journal estimated that land use change from cropland to forest and grassland could give the Loess Plateau an annual average soil carbon sequestration rate of 0.29 tC per hectare per year. 

Better farming: The project re-engineered the farmlands at the bottom of gullies, converted hillside farms into managed terracing and encouraged sustainable practices such as agroforestry. These interventions improved soil health and soil carbon sequestration increased as a result. 

As an example, the reengineering of eroded gully farms into more productive managed gullies across 377.8km2 of farmland is estimated to have increased soil organic carbon sequestered from the atmosphere at a rate of 0.15GgC per year

What else happened?
Although the total cropland area decreased, farmland productivity increased over the course of the project and, by 2015, the Loess Plateau had reached 100 per cent self-sufficiency in terms of grain yields (although this self-sufficiency was likely only made possible by the out-migration of local residents to cities during this period).

Not everything has worked out: the project has been criticised for planting too much of the same kind of crops, and for planting short-lived, non-native trees in unsuitable desert areas, with tree die-off in drought years. Bans on grazing and logging in the conservation areas heavily impacted these parts of the local economy. 

What can we learn from it?
Soil carbon sequestration will be a hot topic at Cop – and to achieve it means investing in soil health and sustainable agricultural practices. A study by the Nature Conservancy found that protecting and restoring soils worldwide could mitigate 5.5GtC per year. 

The Loess Plateau project provides an example of what can be achieved when soils are managed and ecosystems are restored at scale. 

But the question of what to do with agricultural soils is complicated by issues like food security. Converting less productive agricultural land to forests and grasslands will certainly help sequester carbon, but will we still be able to feed the world? And will we be able to produce enough food if we adopt soil carbon friendly farming practices? “We don’t know,” was one expert’s answer at a recent ThinkIn at which we asked “Can soil save the planet?”

David Cleary of the Nature Conservancy also attended that ThinkIn and offered some data afterwards: 

Cropland: 0.87-2.68 GtC per year
Grazing land: 1.28-2.22 GtC per year

That’s how much carbon could be sequestered “if a range of generally accepted soil health management practices scaled to the places appropriate for them,” he said. For context, a much more substantial 5-15 GtC per year could be saved if we stop converting land from forests and grasslands to land for dwellings and agriculture; and the total remaining carbon budget to stay within 1.5C of warming is 230-440 GtC.

Increasing soil carbon stocks in agricultural soils will be “useful but not huge” as a carbon mitigation strategy, Cleary says. Obviously cutting emissions is the top priority, but in the fight against climate change we should do everything we can to reduce atmospheric carbon. “Useful but not huge” is still worth pursuing. 

At Cop26, soil carbon and sustainable agriculture will be firmly on the agenda. Soil carbon expert professor Joanna Clark hopes Cop will see soil in a holistic way: “Climate change is linked to other problems – air quality, flood and drought risk, pollution. They are all part of the same problem of unsustainable economies. These issues are connected and we are all connected to the problems and, importantly, the solutions,” she said.  

Soil carbon sequestration is one of these solutions: “Building soil carbon is good for soil health, to bring life back into our soil food webs, to support biodiversity, promote good soil structure which helps to reduce flood and drought risk, improve nutrient use and produce nutritious food,” Clark says. “It’s all connected and we can take action now.”


eco-nomics

Pricey oil
Eighteen months ago West Texas Intermediate crude fetched minus $37.63 per barrel. It’s now at nearly (plus) $80 a barrel – a seven-year high (£) – and Goldman Sachs reckons it could go higher. There are two main reasons: that surge in demand we can all hear as economies emerge from Covid and power producers switch turbines from gas to oil because of an even more severe gas price spike; and a decision by Opec+ to hold production pretty much steady rather than bow to American pleas to open the taps. And there are two main results: the big oil producers are now earning enough again to think about tapping expensive, hard-to-reach reserves, and the US is in the weird position of begging for cheap fossil fuel while trying to persuade the world it’s intent on giving it up altogether. This is an important reality check as Cop draws near. The world is still utterly dependent on oil.


Science and tech

Ultra cold
The past Antarctic winter has been the coldest on record. You read that right. The average temperature at the Scott-Amundsen research station at the South Pole between April and September was minus 61 Celsius, which is lower than in any year since 1957. One effect was to freeze up the waters around the continent. In August, Antarctic sea ice was at its fifth-most extensive for the time of year since records began. And now the caveats: Antarctic temperatures are notoriously variable and the trend is up. As for the ice, extent is not the same as volume and much of it had melted by the end of September. Even so, it’s been a remarkable few months for scientists over-wintering at the South Pole and those at Nasa’s Global Modelling and Assimilation Office who check their numbers. For jargon junkies, the intense cold is a function of an unusually intense polar vortex caused by the Southern Annular Mode (SAM) in positive phase. Translation: strong winds kept the cold stuck over the pole.


Engagement and activism

A sit-down too far?
Climate protesters blocking traffic at key points round London are driving motorists half-mad and helping to answer an important question: how far can you take direct action before it backfires completely? Looking only at coverage in the Daily Mail and a woman desperate to get to her mother in hospital, you’d think the Insulate Britain group, which wants to insulate Britain, had already passed that point. But that wouldn’t be quite fair. The group always lets ambulances through and the woman’s mother was indeed already in hospital, not prevented from getting there. And the justification for the protest? “We have tried lobbying,” a spokesman said. “We have tried targeting political leaders, government departments. People have been doing this for two, three, four, five decades, without any success at all.” It’s true. Britain’s homes need better insulation – and we’ll host a ThinkIn on the Renovation Revolution at Cop.


policy

Gas-free UK
Boris Johnson tells Tories gathering in Manchester this week that the UK’s energy mix must be entirely fossil fuel-free by 2035, and that includes gas. His preference for backing up renewables when the wind stops blowing, as it did for much of this summer, is nuclear. Parts of his audience when he gets to this point in his speech will applaud. Parts will sit on their hands. Some may have read a piece in the FT in which Philip Lambert, a veteran energy expert (and noted classical musician) warns of a potential backlash against the goal of net zero if a shortfall in baseload power leads to more gas price spikes and widespread energy poverty. Lambert takes particular aim at the IEA’s plea earlier this year for an end to oil and gas exploration. He accuses the agency of being captured by net zero ideologues, and relying in its pathways to net zero on unproven technologies. It’s true that batteries and green hydrogen haven’t been scaled up yet to the point that they can even out renewable supply curves, but that doesn’t mean they can’t be given the money and political will. They do exist. They are proven. 

Do share this around, and let us know what you think of it.

Thanks for reading.

Ella Hill
@_EllaHill

Giles Whittell
@GWhittell


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