Some NETs come under the heading of Natural Climate Solutions. For example, reforestation can be a NET, because trees take carbon dioxide from the air: the carbon ends up stored in their tissues for decades or even centuries. Reforestation could remove 3-18 GtCO2 per year. Other natural climate solutions include restoring wetlands and kelp forests, which are also carbon sinks. Taken as a whole, natural climate solutions could mop up at most 23.8 Gt GtCO2 per year.10
In contrast, other NETs are highly artificial. “Enhanced weathering”, for instance, entails crushing minerals like olivine into powder, which reacts with CO2 and water to form a new mineral that can be used or buried. Lime can be thrown into the ocean to react with dissolved CO2 to produce calcium carbonate that sinks to the seabed, providing a route to carbon sequestration. Alternatively, some regions of the ocean could be fertilised with iron particles, which drive the growth of phytoplankton that draw in CO2. There is also Direct Air Capture (DAC), in which machines are constructed that contain chemical “sponges”, which draw CO2 out of the air. In carbon capture and storage (CCS), this CO2 is sequestrated in underground repositories such as aquifers or depleted oil and gas reservoirs. In carbon capture and use (CCU), the carbon is used for enhanced oil recovery or re-purposed through conversion into forms such as fuels, fertilisers or construction materials.11 Because the carbon remains in use, CCU does not necessarily reduce emissions.
All these technologies face two immediate problems: scale and cost. Many of them would require huge areas of land or sea to have an appreciable effect, and they are all expensive: in the absence of a global price on carbon it is hard to see how they could be economically viable. Furthermore, they create jurisdictional and ecological issues: for the most part, their effects (desired or otherwise) are not constrained by territorial boundaries, and raise the potential for inter-nation conflict. However, recent uptake of AI in materials development has the potential to accelerate solutions for CCS and CCU.