It appears the search is on to find ways to capture carbon right from thin air. Nations worldwide have just agreed to limit carbon dioxide emissions in hopes of preventing global warming from surpassing 2-degrees Celsius by 2100.

However, countries will not manage to meet their goals at the rate they're going. Nations may need to physically remove carbon from the atmosphere, and they may have to deploy "negative emissions technology" - techniques that scrub CO2 out of the air.

According to Scientific American, numerous methods exist that try to do this. They were all emphasized in the American Geophysical Union conference in San Francisco. 

According to Stanford University, an example is enhanced weathering with agriculture. Earth's surface naturally removes carbon dioxide from the atmosphere through the breakdown of rocks, but this occurs slowly.

Scientists proposed speeding up the process, called "weathering," with a man-made invention. David Beerling, director of the Leverhulme Cenre for Climate Change Mitigation, explained a new technique that could quicken weathering and theoretically benefit crops. 

In this new method, farmers could 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 pull carbon dioxide from the air into a soil. 

The weathered rocks would then release valuable nutrients such as potassium into the soil and help the crops grow. However, drawbacks to the method include a lot of cost to grind and transport rocks, and even need more energy, thus more emissions.

According to Nature Zone, another method is to capture carbon with ocean thermal energy. Cold water is pumped from the ocean depth up to warmer surface and the temperature difference generates electricity. 

Greg Rau at the University of California, Santa Cruz, wants to combine it with a chemical reaction that would suck carbon dioxide from the air at the ocean's surface and generate hydrogen at the same time. 

Another method is bio-energy with carbon capture and storage. This is one of the most developed negative emissions technologies and is known as bio-energy, 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 CO2 released during the burning.

However, scientists say this could take up a massive amount of land. BECCS may need about a third of the world's arable land to capture enough carbon to keep the temperature from rising above two degrees.

A lot of other methods exist as well, such as directly capturing CO2 from the air with large panels coated with chemicals and even restore forests. 

However, 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.