A major consideration is which crops you will grow to feed a large-scale BECCS system.that could be switchgrass or Miscanthus, another grass, neither requires as much water or added nutrients as corn. “They are very effective,” said David Lawrence, a climate scientist at the National Center for Atmospheric Research and co-author of the new paper. They are also perennial crops, so you don’t have to plant and till them all the time. “But in the context of the study, we found that despite this, we are still seeing increased water stress and deteriorating water quality,” added Lawrence. “This is because of the scale of BECCS implementation: in this case, it requires a very large increase in the amount of bioenergy.”

In order for the U.S. to do its fair share of reducing atmospheric carbon to keep global warming to 2 degrees Celsius — in addition to drastically reducing greenhouse gas emissions — it would need to add 460,000 square miles of bioenergy crops if BECCS was used, while renewing Afforestation requires only 150,000 square miles.With this extra space, BECCS can sequester 11.4 to 31.2 gigatons of carbon dioxide2 By 2100, similar to the 19.6 to 30.2 Gt reforestation. (For reference, humans as a whole are currently emitting nearly 40 gigatons per year.) This means that reforestation would be a more carbon-negative option because it uses less land to achieve the same effect. This and all these extra crops divert water from other needs, like hydrating people. On the other hand, the forest should be able to take care of itself.

However, growing should. Forests are a powerful carbon sequestration tool because it brings a whole bunch of benefits at the same time: let a person grow and you increase biodiversity, locals can use it to make money from tourism, and healthy forests allow an area Cool down as plants release water vapor. But forests around the world are threatened by rapidly rising temperatures, raising questions about their ability to persist for centuries to come.

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In other words: if humans don’t drastically cut emissions, temperatures will continue to soar and we will lose our forests as carbon sequestration powerhouses. Especially in the western US, climate change is exacerbating wildfires, so if you put in a lot of effort to restore forests and set them on fire, all the carbon goes straight back into the atmosphere. (Forests adapt to burning from time to time, but only slightly—the fires we’ve seen in recent years are far from natural.) If it’s still too hot for the forest to regrow in a healthy way, you can’t sequester that carbon anymore. “Can we find enough climates to support healthy forest growth?” Lawrence asked. “That’s a very difficult question to answer. If that forest is likely to be burned, does it make sense to try to reforest? It’s really going to be very location-dependent.”

Bioenergy crops could also struggle as the world warms.switchgrass and Miscanthus are great bioenergy species, in part because they are drought tolerant, but heat stress is still a serious problem – just as our bodies struggle with extreme temperatures, so do plants. Scientists need to tailor specific species to specific environments: In a humid climate like Florida, perhaps a crop like sugar cane would be better. “Finding bioenergy production plants that are climate-friendly and that don’t consume more and more water is a bigger challenge than thinking. Miscanthus Praveen Kumar, a hydrologist at the University of Illinois who studies bioenergy crops but was not involved in the new study, said he will deploy switchgrass across the country as a solution.