Nations worldwide have agreed to limit carbon dioxide emissions in hopes of preventing global warming from surpassing 2 degrees Celsius by 2100. But countries will not manage to meet their goals at the rate they’re going. To limit warming, nations will also likely need to physically remove carbon from the atmosphere. And to do that, they will have to deploy “negative emissions technology”—techniques that scrub CO2 out of the air.

Can these techniques, such as covering farmland with crushed silica, work? Researchers acknowledge that they have yet to invent a truly cost-effective, scalable and sustainable technology that can remove the needed amount of carbon dioxide, but they maintain that the world should continue to look into the options. “Negative emissions technologies are coming into play because the math [on climate change] is so intense and unforgiving,” Katharine Mach, a senior research scientist at Stanford University. Last week at the American Geophysical Union conference in San Francisco, researchers presented several intriguing negative emissions strategies, as well as the drawbacks.

Enhanced Weathering with Agriculture

Earth’s surface naturally removes carbon dioxide from the atmosphere through the chemical breakdown of rocks, but the phenomenon occurs extremely slowly. Scientists have proposed speeding up this process—which is called “weathering”—with man-made intervention. At the AGU conference, David Beerling, director of the Leverhulme Centre for Climate Change Mitigation, explained an agricultural technique that could quicken weathering and theoretically benefit crops as well.

In this method, farmers would apply finely crushed silicate rocks to their land. The roots of crops and fungus in the soil would accelerate the chemical and physical breakdown of the silicate rocks, and at the same time, carbon dioxide would be pulled from the air into the soil due to a chemical reaction that occurs as part of the weathering process. Grinding the silicate rocks into the size of pellets or sand grains would speed up natural weathering because it increases the amount of rock surface area available to react.

In addition to capturing carbon dioxide, the weathered rocks would release valuable nutrients such as phosphorus and potassium into the soil, which would help crops grow. The rocks would provide plants with silica as well, which Beerling says could help them build stronger cells to better fend off pests. “You could reduce fertilizer and pesticide use, which would reduce the cost to the farmers as well,” he explains. The enhanced weathering may also help with ocean acidification, according to Beerling. Some of the carbon dioxide that’s captured would stay in the soil, but much of it would get flushed into the ocean as a compound called bicarbonate. Bicarbonate is basic, which means it could potentially balance out the increasingly acidic oceans.

This technique has major drawbacks, though. Researchers are skeptical of the method because it would cost a lot to grind and transport rocks, and both those steps would require a lot of energy, which could create more emissions. There are also possible impacts on the ecosystem to consider. “I’m concerned about [environmental] disturbance,” says Rob Jackson, professor of earth sciences at Stanford. “This would essentially be a massive mining operation.” Jackson does like the potential benefits, though—like fertilizing soils—in addition to removing carbon dioxide.

Carbon Capture with Ocean Thermal Energy

A different negative emissions technique would take advantage of the ocean’s vast temperature differences: ocean thermal energy conversion. In this approach, cold water is pumped from the ocean’s depth up to the warmer surface, and the temperature difference is used to generate electricity. Researchers have already demonstrated the technique on a small scale. Now Greg Rau at the University of California at Santa Cruz wants to combine it alongside a chemical reaction that would suck carbon dioxide from the air at the ocean’s surface and also generate hydrogen at the same time. The reaction would be helpful in several ways: it would capture CO2, and convert the thermal-generated electricity to an energy form—hydrogen—that could be transported by tanker to land from offshore. And like the enhanced weathering method, this approach would turn CO2 into bicarbonate that would sift down into the ocean, helping to counteract acidification. Rau has also proposed modifying the ocean thermal energy system to avoid any CO2 release that could happen when deep ocean water is pumped to the surface.

So far, Rau has only demonstrated his process in the lab. “His idea is very much in the conceptual stages,” says Chris Field, founding director of the Carnegie Institution's Department of Global Ecology at Stanford. “It has the potential to be something, but it’s still very much a niche solution at this point.”

Bio-Energy with Carbon Capture and Storage

One of the most developed negative emissions technologies is known as bio-energy with carbon capture and storage, or BECCS. The process entails growing trees and plants such as switchgrass that suck up carbon dioxide as they grow, burning them for energy in power plants, and then capturing and storing the carbon dioxide released during the burning. The capture and storage would be done by putting a filter in the smokestack, compressing the CO2, and then injecting it underground. BECCS generates energy and removes carbon from the air, which sounds appealing.

But scientists say BECCS would take up a massive amount of land. According to one estimate, BECCS would require about a third of the world’s arable land in order to capture enough carbon dioxide to keep the temperature from rising above two degrees. “I'm skeptical you can ever reach the necessary scale, because you'd need a huge amount of land to keep up with human emissions, and in a world where you have to feed more people than ever before,” says Klaus Lackner, director of the Center for Negative Carbon Emissions at Arizona State University. Plus, some are concerned that BECCS isn’t as effective as it appears—they say that if factors like land-use changes that result from the process are counted, BECCS may not reduce emissions as much as people think. For instance, if people cut down a tropical rainforest to burn its wood for BECCS, they would ultimately create more emissions—at least in the short-term. Or if BECCS takes over land previously used for purposes like agriculture, it could push people to deforest other land for their needs. Also, storing the carbon created by burning biomass requires a lot of energy, which costs money and can generate more emissions.

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There are other negative emissions strategies as well—major ones that have gotten a lot of attention, such as directly capturing CO2 from the air with large panels coated with chemicals, and restoring forests so more trees can absorb more CO2. Lesser known ideas are out there, too, such as using wood to build homes and offices so that carbon dioxide is locked away in the walls of buildings. Yet none of the technologies so far have proven to be viable and cost-effective on the scale that’s needed.

Experts say more money and research should go into investigating a broad range of technologies and determining the best options—and they say the work needs to start happening now. In Chris Field’s AGU lecture, he said that if the world wants to use negative emissions technologies to significantly draw down carbon emissions by the latter part of this century, they need to start being deployed as early as 2020 or 2030. “All the negative emissions technologies, except for growing forests, are in the very early stages of development,” explains Field. “If these technologies are going to make a difference, they’re going to have to go from essentially nothing now to a massive scale in decades.”