Replenishing and protecting the world’s soil carbon stores could help to offset up to 5.5bn tonnes of greenhouse gases every year, a study finds.

This is just under the current annual emissions of the US, the world’s second largest polluter after China.

Around 40% of this carbon offsetting potential would come from protecting existing soil carbon stores in the world’s forests, peatlands and wetlands, the authors say.

In many parts of the world, such soil-based “natural climate solutions” could come with co-benefits for wildlife, food production and water retention, the lead author tells Carbon Brief.

Ground up

The top metre of the world’s soils contains three times as much carbon as the entire atmosphere, making it a major carbon sink alongside forests and oceans.

Soils play a key role in the carbon cycle by soaking up carbon from dead plant matter. Plants absorb CO2 from the atmosphere through photosynthesis and this is passed to the ground when dead roots and leaves decompose.

But human activity, in particular agriculture, can cause carbon to be released from the soil at a faster rate than it is replaced.

Few countries record data on soil-carbon loss directly from agriculture, according to the Intergovernmental Panel on Climate Change’s most recent assessment report, making it difficult to understand the degree to which soil carbon losses are contributing to climate change.

The new analysis, published in Nature Sustainability, takes a look at how protecting and replenishing soils – both in agricultural and natural landscapes – could instead help to combat warming.

If finds that, if techniques to improve soil carbon were rolled out at the maximum assumed level worldwide, they could remove up to 5.5bn tonnes of CO2e a year.

The study’s lead author Dr Deborah Bossio, lead soil scientist at the Nature Conservancy, an environmental NGO that advocates for nature-based solutions to climate change, tells Carbon Brief:

“I talk about soil as being the forgotten solution. What we’re really trying to emphasise is that soil is important and so it should not be ignored, but also not exaggerated.”

Counting carbon

For the analysis, the authors built on an earlier study which looked at the global greenhouse gas removal potential of all “natural climate solutions”. The term is used to describe a range of negative emissions techniques that aim to enhance the ability of natural ecosystems to remove CO2 from the atmosphere.

The study framework assumes natural climate solutions are deployed on a global scale. However, the methodology does not allow natural climate solutions to compromise land that is currently used by wildlife or food production.

Bossio explains: “We’re asking: ‘What’s the land area available for change?’”

The research finds that a quarter of all the greenhouse gas removal ability of natural climate solutions comes from soil-based techniques, such as protecting and restoring forest soils, peatlands and wetlands.

The chart below shows the greenhouse gas removal potential of various soil-based natural climate solutions. The figures are shown in billion tonnes of CO2e per year.

The global greenhouse gas removal potential of various soil-based natural climate solutions (CO2e in billions of tonnes per year). Data source: Bossio et al. (2020). Chart by Carbon Brief using Highcharts.

The research shows that the largest greenhouse gas removal potential comes from protecting existing forests and reforestation. This technique could offset 1.2bn tonnes of CO2e a year, when only forest soil carbon is considered.

Forests soils are a globally important carbon store. They can be particularly carbon-rich because they absorb high densities of dead plant matter. Forest soils also play a significant role in absorbing methane.

Another soil technique with large potential is “biochar”, according to the research. Biochar is a carbon-rich charcoal which, when sprinkled on land, can boost soil carbon storage.

A mound of biochar, including charred wood, soil, wet weeds, dead leaves and dry sticks. Credit: Ron Emmons / Alamy Stock Photo

A mound of biochar, including charred wood, soil, wet weeds, dead leaves and dry sticks. Credit: Ron Emmons / Alamy Stock Photo

It is often suggested that biochar should be spread across agricultural land because, as well as enhancing soil carbon storage, it could enhance crop productivity.

Other agriculture-based soil carbon techniques with large potential include “cover cropping”, which is the practice of planting crops to cover soil rather than for being harvested, as well as including more trees in cropland.

Both of these techniques could help protect and build on soil carbon stocks because the roots of plants can act as anchors, lessening the impact of soil erosion, Bossio explains.

Such agricultural techniques can be seen as “no-brainer” options for boosting soil carbon, she adds:

“Protecting what’s still in the ground and rebuilding the soil carbon in our agricultural systems is pretty much a no-brainer, because of all the multiple benefits that we get. In a lot of our farming systems, soil carbon levels are at a state where, if you improve them, you get benefits in terms of water regulation, water quality, stabilising production and resilience in the systems.”

Restoring and protecting peatlands could also help to sink large amounts of greenhouse gases, the study says.

Peatlands are carbon-dense boggy environments made up of partially decomposing organic matter. They cover just 3% of the world’s surface, but hold up to a third of its soil carbon.

Restoring wetlands could also have an important role in removing greenhouse gases from the atmosphere, according to the research.

Like peatlands, wetlands contain water-logged carbon-rich soils. A recent study found the Amazon’s wetlands are twice as carbon rich as its rainforests, with soils holding the majority of this carbon.

Overall, it is worth noting that the estimates are global – and it is likely that different countries will benefit most from different soil-based solutions, says Bossio.

For example, Mongolia, a country with vast grassy plains, could benefit most from stemming the conversion of grassland to agricultural land, she says. Whereas Indonesia, a country with vast peatlands and wetlands, could benefit more from protecting those environments.

Co-benefits and costs

The study also explores the likely costs and co-benefits of each soil-based natural climate solution.

The chart below shows the proportion of CO2e removal for each technique that would be low-cost (black), cost-effective (grey) and not currently cost-effective (white). The techniques are grouped into three categories: forests (top), agriculture and grasslands (middle) and wetlands (bottom).

A colour key indicates if the technique is likely to have co-benefits for air (yellow), biodiversity (green), water (blue) and food (red).

A summary of the costs and co-benefits of various soil-based natural climate solutions. The chart shows the proportion of CO2e removal for each technique that would be low cost (black), cost-effective (grey) and not currently cost-effective (white). A colour key indicates if the technique is likely to have co-benefits for air (yellow), biodiversity (green), water (blue) and food (red). Source: Bossio et al. (2020)

A summary of the costs and co-benefits of various soil-based natural climate solutions. The chart shows the proportion of CO2e removal for each technique that would be low cost (black), cost-effective (grey) and not currently cost-effective (white). A colour key indicates if the technique is likely to have co-benefits for air (yellow), biodiversity (green), water (blue) and food (red). Source: Bossio et al. (2020)

The chart shows how avoiding the degradation of forests, peatlands and wetlands would be the most low-cost way to mitigate greenhouse gas emissions on a global scale.

However, it is worth noting that ecosystems still face major threats in many parts of the world. For example, recent satellite data shows that Amazon deforestation could reach a record high in 2020. Meanwhile, the world’s largest tropical peatland is being threatened by a plan to drill for oil.

Other low-cost and cost-effective options for enhancing soil carbon could come from agricultural systems, the results show. However, large-scale change is needed to encourage farmers to pursue such options, says Bossio:

“There’s a lot of barriers right now to change in our agricultural systems. One of the biggest is disincentives in agricultural policy. There’s also a lack of knowledge about changing current practice. We tell farmers ‘just plant cover crops’ – but that means a farmer needs to know when to plant them, which to plant and how to manage them.”

Reforming agricultural policies around the world could be a way to encourage farmers to take up soil-based solutions, she adds:

“We need to be removing the disincentives in our current agricultural support systems…so farmers could be acknowledged for a range of societal benefits that they can provide.”

The study “does a good job of fleshing out the challenges to implementing schemes to protect and enhance soil organic carbon”, says Trisha Gopalakrishna, a research student in ecosystems at the University of Oxford who was not involved in the research. She tells Carbon Brief:

“It is a good first step and I would be further interested in refined analyses for each of the specific activities and regional analyses for different countries.”

Bossio et al. (2020) The role of soil carbon in natural climate solutions, Nature Sustainability, https://nature.com/articles/s41893-020-0491-z