Agriculture has historically released almost as much carbon into the atmosphere as deforestation, a new study suggests — and that’s saying something.
In a paper published this week in Proceedings of the National Academy of Sciences, researchers found that land use changes associated with planting crops and grazing livestock have caused a loss of 133 billion tons of carbon from soil worldwide over the last 12,000 years, amounting to about 13 years of global emissions at their current levels. And at least half of those losses have probably occurred in the last few centuries.
“Historically, I think we’ve underestimated the amount of emissions from soils due to land use change,” said lead study author Jonathan Sanderman, an associate scientist with the Woods Hole Research Center, a climate change research organization based in Massachusetts.
“We have known that extensive agricultural practices are responsible for depleting soil carbon stocks, but the full extent of these carbon losses has been elusive,” said soil expert Thomas Crowther, who will be starting a position as a professor of global ecosystem ecology at the Swiss Federal Institute of Technology in Zurich in October, in an email to The Washington Post. “In this study, the authors do a really good job of quantifying how humans have altered the Earth’s surface soil carbon stocks through extensive agriculture, with direct implications for atmospheric CO2 concentrations and the climate.”
Previously, studies on global soil carbon losses have varied wildly in their conclusions, suggesting historical losses of anywhere from 25 billion to 500 billion tons of carbon, Sanderman noted. In general, based on the average findings from multiple studies, scientists have often assumed a total loss of around 78 billion tons, he added.
Many of these past studies have relied on “simple bookkeeping estimates,” according to Sanderman, which involve calculating the carbon losses from one plot of land and then multiplying the results to get a value for the entire world.
But for the new study, the researchers were able to employ a large data set containing specific information on different soils from all around the world. They applied this data set to a model, along with another database on human land use and agricultural activity over the last 12,000 years, and added information on various other physical factors like climate and topography. Then they ran the model to see how soil carbon content has changed.
The model suggested that agricultural changes are responsible for the loss of a total of 133 petagrams, or 133 billion metric tons, of carbon from the top six-foot-deep layer of soil all over the world. The most intense losses per unit of land have been caused by the planting of crops — however, more land worldwide is devoted to grazing livestock than cropping. As a result, the study suggests that cropping and grazing are responsible for roughly equal shares of global soil carbon losses.
These losses have varied over time and in different locations as well, the study suggests. On a global scale, soil carbon losses have been speeding up since the industrial revolution, particularly in the 19th century. In the past 100 years, losses have tapered slightly, but still remain high, with the most significant emissions coming from new-world countries, such as Brazil, where large-scale agriculture is still expanding.
The researchers suggest that their findings could be used to help inform global efforts to improve soil carbon storage by pinpointing the parts of the world where losses have been highest — generally, places that have experienced the most intense agricultural conversion. And Crowther, the Netherlands Institute for Ecology researcher, added that “modifying large-scale agricultural practices to restore some of these lost soil carbon stocks might be a valuable strategy in our efforts to dampen climate change.”
That said, the researchers note that it’s essentially impossible to replace all 133 billion tons of lost carbon.
“If we allow natural vegetation to take over the world, we may eventually get close to that,” Sanderman suggested. “But obviously we need to feed 7 billion people, going up to 10 billion by the middle of the century, so the reality is we are not going to be abandoning agricultural land and restoring it to its native state in any large way.”
But, he added, there’s plenty of research to suggest that land can be managed in a more sustainable way.
“There’s a lot of studies showing that if you adopt recommended best management practices, you could slowly regain some fraction of that lost carbon,” he said.
Overall, the researchers suggest that with modified agricultural practices — which could include everything from more efficient crop rotation strategies to changes in the way land is plowed and tilled — we could realistically regain anywhere from 8 billion to 28 billion tons of the carbon that’s been lost.
And in the meantime, the study sheds some new light on our current climate situation, suggesting that human land use was likely a much more significant factor in the carbon emissions warming our planet than previously thought.
“We know how much carbon is in the atmosphere now,” Sanderman pointed out. “So that just changes how we apportion the blame historically.”
from Carbon Brief, 25 Aug 2017
The world’s soils have lost a total of 133bn tonnes of carbon since humans first started farming the land around 12,000 years ago, new research suggests. And the rate of carbon loss has increased dramatically since the start of the industrial revolution.
The study, which maps where soil carbon has been lost and gained since 10,000 BC, shows that crop production and cattle grazing have contributed almost equally to global losses.
Understanding how agriculture has altered soil carbon stocks is critical to finding ways to restore lost carbon to the ground, another scientist tells Carbon Brief, which could help to buffer the CO2 accumulating in the atmosphere.
Soil as a carbon sink
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 pass carbon 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. This net release of carbon to the atmosphere contributes to global warming.
New research, published in the Proceedings of the National Academy of Sciences (pdf), estimates the total amount of carbon that has been lost since humans first settled into agricultural life around 12,000 years ago.
The research finds that 133bn tonnes of carbon, or 8% of total global soil carbon stocks, may have been lost from the top two metres of the world’s soil since the dawn of agriculture. This figure is known as the total “soil carbon debt”.
Around two-thirds of lost carbon could have ended up in the atmosphere, while the rest may have been transported further afield before being deposited back into the soil.
And since the industrial revolution, the rate of soil carbon loss has increased, says lead author Dr Jonathan Sanderman, a scientist at the Woods Hole Research Center in Massachusetts. He tells Carbon Brief:
“Considering humans have emitted about 450bn tonnes of carbon since the industrial revolution, soil carbon losses to the atmosphere may represent 10 to 20% of this number. But it has hard to calculate exactly how much of this has ended up in the atmosphere versus how much has been transported due to erosion.”
‘Hotspots’ for carbon loss
As part of the study, the researchers designed an artificially intelligent model that used an existing global soil dataset to estimate past levels of soil carbon stocks, Sanderman says.
“We used a dataset which defines 10,000BC as a world without a human footprint. What we did was develop a model that could explain the current distribution of soil carbon across the globe as a function of climate, topography [physical features], geology and land use. Then we replaced current land use with historic reconstructions including the ‘no land use’ case to get predictions of soil carbon levels back in time.”
To calculate an overall soil carbon debt, the researchers subtracted the amount of current global soil carbon from the amount of soil carbon predicted to have existed in the era before human agriculture. The model also allowed the researchers to estimate global soil carbon stocks at different points throughout history, including at the advent of the industrial revolution.
The results allow scientists to get a clearer picture on how 12,000 years of human agriculture have affected the world’s soil stocks, says Sanderman.
“More carbon has been lost due to agriculture than has generally been recognised and a lot of this loss predated the industrial revolution. This loss isn’t equally distributed across agricultural land. Some regions stand out as having lost the most carbon.”
Map B below shows the regions that have experienced the most soil carbon loss, and includes the US corn belt and western Europe. The red shading represents the very highest level of soil carbon loss since 10,000BC, while blue shows the highest level of carbon gain.
The US corn belt and western Europe are likely to have experienced high levels of soil carbon loss as a result of long periods of intense crop production, says Sanderman.
However, the analysis also reveals a number of regions which have seen high levels of soil carbon loss despite having relatively little farming. These “hot spots” – including the rangelands of Argentina, southern Africa and parts of Australia – are considered to be particularly vulnerable to land degradation driven by agriculture, says Sanderman.
“Semi-arid and arid grasslands [the hotspots] are particularly vulnerable to potentially irreversible degradation if grazing intensity is too high. That’s because there isn’t a lot of soil carbon to start with and there can often be a complete shift in vegetation cover leading to lots of erosion.”
Map A shows the distribution and intensity of crop production (red) and cattle grazing (green) across the world. Both have contributed almost equally to loss of soil carbon stocks, Sanderman says.
Repaying the debt
Identifying how much carbon has been lost from the soil could also help scientists understand how much could be replenished, if soils were managed so that they took up more carbon from the atmosphere than they released into it.
Soil carbon management is one of a number of negative emissions technologies (NETs) that could help to remove greenhouse gases from the air. Research suggests that NETs will be key to meeting the Paris Agreement, which aims to keep warming “well below” 2C above pre-industrial temperatures, while striving to limit increases to 1.5C.
In theory, soils could be managed in a way that would allow them to reabsorb all of the carbon that has been lost since the agricultural revolution. In practice, however, this is highly unlikely, Sanderman explains. “This figure [133bn tonnes of carbon] is likely a maximum potential if we were willing to give up agriculture and completely restore natural ecosystems. That is obviously not going to happen, so the real potential – giving the constraint of needing to feed 10 billion people by 2050 – is going to be a lot lower.”
“Modifying large-scale agricultural practices to restore some of these lost soil carbon stocks might be a valuable strategy in our efforts to dampen climate change. If regenerative agriculture can restore some of the carbon that we have lost, then it might be a really valuable tool in our fight against climate change.”
However, the study lacks clarity on how it considers peat soils, says Prof Meine van Noordwijk, chief science advisor at the World Agroforestry Centre in Kenya, who also wasn’t involved in the study. Peat is a type of soil made up of waterlogged partially-decomposed plant material such as moss, which builds up over thousands of years in wetland environments including bogs.
Peat soils are thought to contain up to half of global soil carbon stocks, van Noordwijk explains to Carbon Brief, and so are of particular concern:
“Peat soils require and currently receive separate attention. Water management [of wetland soils] is a relevant part of agricultural use, leading to [carbon] losses, but also indicating opportunities for restoration.”
Sanderman, J. et al. (2017) Soil carbon debt of 12,000 years of human land use, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.1706103114