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.
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.