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Burning Crops to Capture Carbon? Good Luck Finding Water

Thank Mother Nature for saving us from ourselves. Plants on land and phytoplankton in the sea absorb CO2 as they photosynthesize, sucking planet-warming carbon out of the atmosphere. Vegetation has canceled a quarter of humanity’s emissions, and the oceans absorb still more, helping keep warming so far to 1.2 degrees Celsius above pre-industrial levels.Our civilization, though, is still on track to barrel past 1.5 degrees warming (the optimistic goal set by the Paris Agreement) in the early- to mid-2030s . So the UN’s Intergovernmental Panel on Climate Change (which authors all those damning climate reports ) stresses that it’s not enough to dramatically reduce greenhouse gas emissions—and fast—we should also use negative-emissions techniques to suck carbon out of the atmosphere. Engineers might do that with “direct air capture ” (DAC) machines that scrub the air of CO2. But others are turning back to Mother Nature, exploring ways to use plants’ powers of carbon sequestration.“It's a huge challenge to decarbonize the whole energy sector in 20 to 30 years, which is what would be necessary to reach 1.5, or I think even 2 degrees [warming],” says climate scientist Vera Heck of the Potsdam Institute for Climate Impact Research. “So there will be a variety of tools needed to counterbalance the remaining emissions.”One controversial idea is known as bioenergy with carbon capture and storage, or BECCS: You’d grow crops and burn them for energy, then capture the emissions coming out of the facility and pump them underground as liquefied gas. (We already get bioenergy from plants by burning wood pellets or by producing ethanol from corn , but both are done without the carbon-capture-and-storage bit.)“BECCS is the only technology that removes carbon from the atmosphere that also gives you sort of a free energy source,” says Heck, who studies the process. It’s essentially a natural version of direct air capture (DAC), which instead uses membranes to absorb CO2 from the air. Only unlike DAC, BECCS requires lots of land and water to grow the requisite crops—on a planet with a ballooning human population that itself needs more food and water. That is not to mention the fact that climate change is already driving more intense droughts across the world.Writing this week in the journal Science Advances, scientists imagined a scenario in which bioenergy crops were massively scaled up across the United States, and what that would mean for both carbon capture and water use compared to bolstering regular forests. The good news is that large-scale BECCS would sequester about as much carbon as reforestation. But the bad news is it would expose 130 million Americans to water stress by 2100 because of the water required to grow all those crops and because the extra fertilizer would pollute rivers with nitrogen.The researchers used socio-economic models that incorporated a range of variables—population growth, water and energy needs for people and agriculture, how land is used, and others—that simulated how the US might change up to 2100. Based on all those variables, the models predicted where in the US it would be best to site bioenergy crops or reforest. The researchers then fed this into an Earth system model, which projected the environmental consequences—specifically on water availability and quality—of changing the land to accommodate BECCS or reforestation. (The two scenarios weren’t exclusively BECCS or reforestation—the BECCS version included a little bit of reforestation, and vice versa.)