Soil: It helps feed the world, but could it also help our efforts to keep it cool?
Soil is a store for carbon and moisture, and changing the way it is managed could help mitigate or even counteract global warming, according to two studies presented at the recent European Geosciences Union General Assembly in Vienna, Austria.
Hannah Cooper of the University of Nottingham in the United Kingdom is investigating the effect of no-till farming on the amount of carbon that is captured by the soil. No-till farming is currently used on about 10% of arable land worldwide.
Cooper took cores from 80 conventionally farmed fields in the United Kingdom’s East Midlands region and from no-till fields right next to those. Some hadn’t been under the plow for a few years; others hadn’t been plowed for up to 15 years.
After 1–5 years, Cooper found the nontilled soil was less porous than tilled soil, and carbon content was about the same.
After 5 years, she found water and roots had an easier time penetrating nontilled soil, and it contained more carbon. The carbon was also increasingly bound in organic compounds, such as ethers and aromatics, which are less readily released into the atmosphere.
Combining these data with the release by the soil of nitrous oxide, another greenhouse gas, Cooper concluded that the emissions from no-till soil had a global warming potential that was almost 6 times lower than that of tilled soil.
Her results were met with a bit of skepticism by Dani Or, a soil scientist and environmental physicist at the Swiss Federal Institute of Technology in Zurich, who was not involved with the study.
“I would say that no-till has tremendous ecological justification, and when it works, it is actually a good thing. The problem is that it is not a solution for all climates, or all soils, or all crops,” he told Eos. “I’m sure their work is very good. But the climate in the U.K. and the climate in the Sahel are quite different—there is a danger of generalization.”
On the other hand, Or said, “People have been plowing their field to change the structure from the dawn of civilization. The tillage of arable land is probably the biggest civil engineering operation on the planet, year by year. And yet the scientific basis for why we do it and what benefit it derives is very vague.”
Radical Climate Modeling Around Irrigation Practices
Whether soil is tilled or not tilled, the climate might benefit enormously by irrigating as much as possible, diverting all available water for that purpose, said Thomas Raddatz, a meteorologist of the Max Planck Institute for Meteorology in Hamburg, Germany.
Raddatz is not really proposing that, he reassured his listeners at the conference, but he did it in a computer model of the climate to see what effect irrigation may have on the climate now and in the future.
In Raddatz’s experiment, in a model of the world not yet burdened by human-triggered greenhouse gas emissions, he diverted all available water on all land masses to reservoirs, from which it was gradually released onto the local soil. To do all that, 41,000 cubic kilometers of water were needed each year, 50 times the amount used for irrigation today.
Surprisingly, Raddatz told Eos, diverting all that water didn’t mean that rivers stop flowing.
“You bring the water to the surface of the land, this enhances infiltration, and after some time you have more drainage again, and you pump this water back to the reservoir. So you cycle it probably several times until it is evaporated to the atmosphere. And even then, for large parts of the Northern Hemisphere, you still keep it likely on the continent, because you also enhance precipitation.”
The global effects of this radical piece of geoengineering would be impressive. The evaporating water takes heat from the surface, causing a 2.1°C cooling over land. Once in the air as vapor, the water acts as a greenhouse gas but also ends up in clouds that radiate energy into space as infrared radiation. On balance, there is a global cooling of 1.1°C and an increase of 2.5 million square kilometers in sea ice in the Arctic. Raddatz also notes a strengthening by 15% of the Meridional Overturning Circulation, the current in the North Atlantic that has a strong influence on Earth’s climate and is thought to be vulnerable to global warming.
Raddatz said attention must be paid to the climate-related consequences of policies that involve irrigation. This concern is motivated by runs of his model in which only some parts of the world were irrigated.
“If the EU decides to have a massive irrigation program, to increase crop yields, to grow biofuels to reach carbon targets, to develop rural areas, they may conclude it is cost-effective. So over decades, you increase irrigation,” Raddatz explains. “But it turns out this decreases the precipitation in the Sahel by 100 to 200 millimeters per year. Then we have a large catastrophe there. And all these 300 million people living there will try to come to the EU. So no one profits. We should care about this, before we do it.”
—Bas den Hond (email@example.com), Freelance Journalist
den Hond, B. (2019), Looking for climate solutions down in the dirt, Eos, 100, https://doi.org/10.1029/2019EO121241. Published on 17 April 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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