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.)
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A major consideration is the kind of crop you’d grow to feed a wide-scale BECCS system. That would probably be switchgrass or Miscanthus, another kind of grass, neither of which need as much water or added nutrients as a crop like corn. “They’re quite efficient,” says David Lawrence, a climate scientist at the National Center for Atmospheric Research and coauthor of the new paper. They’re also perennial crops, so you don’t need to plant and till the ground all the time. “But in the context of the study, we found that despite that, we still are seeing increases in water stress and degraded water quality,” Lawrence adds. “And that is because of the scale of the implementation of BECCS: In this scenario it requires a very large-scale increase in the amount of bioenergy.”
For the US to do its fair share in reducing atmospheric carbon to keep global warming to 2 degrees Celsius—in addition to big cuts in greenhouse gas emissions—it would need to add 460,000 square miles of bioenergy crops if using BECCS, while reforestation would require just 150,000 square miles. With this extra space, BECCS could sequester between 11.4 and 31.2 gigatons of CO2 by 2100, similar to the 19.6 to 30.2 gigatons for reforestation. (For reference, humanity as a whole currently emits almost 40 gigatons a year.) That means reforestation would be a more efficient carbon-negative option because it uses less land to get the same effect. That and all those extra crops would divert water from other needs, like hydrating people. Forests, on the other hand, should be able to take care of themselves.
Increasingly, though, that’s a big should. A forest is a powerful carbon sequestration tool because it comes with a whole bunch of simultaneous benefits: Let one grow and you get a boost in biodiversity, locals can use it to make money from tourism, and a healthy forest cools a region because plants release water vapor. But forests the world over are threatened with rapidly rising temperatures, calling into question their ability to persist over the coming centuries.
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Put another way: If humanity doesn’t massively reduce emissions, temperatures will continue to skyrocket and we’ll lose forests as carbon-sequestration powerhouses. In the American West, in particular, climate change is supercharging wildfires, so if you put a bunch of effort into restoring a forest and it goes up in flames, all that carbon heads straight back into the atmosphere. (Forests are adapted to burn from time to time, but only mildly—the mega-blazes we’ve been seeing in recent years are far from natural.) And if it remains too hot for the forest to grow back in a healthy way, you can’t sequester that carbon again. “Can we find enough locations where the climate supports the growth of a healthy forest?” asks Lawrence. “That is a very difficult question to answer. Does it make sense to put your efforts into reforestation if that forest is likely to burn? It really is going to be very location-dependent.”
Bioenergy crops may also struggle as the world warms. Switchgrass and Miscanthus are good bioenergy species in part because they’re drought-resistant, but heat stress is still a serious concern—just as our bodies struggle with extreme temperatures, so do plants. Scientists would need to tailor a particular species to a specific environment: In a wetter climate like Florida’s, perhaps a crop like sugarcane would be better. “Finding the right plant for bioenergy production, that is suited to the climate and doesn’t draw more and more water, is a better strategy than thinking that Miscanthus and switchgrass are going to be deployed all across the country as a solution,” says hydrologist Praveen Kumar, who studies bioenergy crops at the University of Illinois but wasn’t involved in the new research.
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At the same time, higher temperatures mean more water usage, further straining supplies. And as this new modeling shows, the extra fertilization required to scale up BECCS would pollute water meant for human consumption. “The easiest thing would be not to fertilize,” says climate scientist Fabian Stenzel, who studies the potential water impacts of BECCS at the Potsdam Institute for Climate Impact Research but wasn’t involved in the new research. “Then the question is: Do we have enough yield—or still have enough biomass—to get to negative emissions to then offset fossil fuels?”
The other outstanding question is how much carbon BECCS itself would emit. Agricultural machinery spews emissions, and disturbing the land emits carbon from the soil as well. And BECCS isn’t a centralized process, so there’s transport involved: You grow the crops in one place and burn them in a power plant somewhere else, where the geology might not be right to pump the carbon underground for storage (you can inject it into emptied petroleum reservoirs, for instance) so you have to ship it elsewhere as liquefied gas. “I think in most regions, we won’t get around that,” says Stenzel. “That also costs money, and it also costs emissions.” This highlights yet another benefit of reforestation, which all happens in one place and doesn’t require constant watering.
It’s important, all of these scientists agree, to think of BECCS not as the negative-emissions tech to end all others, but as a potential tool in a portfolio. Perhaps there are regions where BECCS ends up working well, where there’s abundant water and crops grow fine without heaps of fertilizer, and where the farms, power plants, and storage geology can be sited close together. But elsewhere, particular forests may be resilient enough to weather climate change—if only we’d stop degrading them—sequestering carbon the old-fashioned way. And maybe someday DAC technology will get to the point where it’s able to make a dent in atmospheric CO2 concentrations.
All of this is no substitute for quickly decarbonizing our way of life so there’s not so much carbon in the atmosphere to get rid of. “There is never going to be one, or even two, dominant ways that we mitigate against climate change—it’s going to require direct carbon capture, it’s going to require some BECCS, it’s going to require reforestation,” says Lawrence. “And what we’re trying to do in this research is help put that strategy into context and make sure we are asking the right questions about what are the other potential consequences of choosing whichever path in whichever location.”