In this model, almost all of the carbon removal goes through a process known as bioenergy with carbon capture and storage, or BECCS. Basically, this requires growing crops that suck in CO2, and then using the harvested biomass to produce heat, electricity, or fuels, while also capturing and storing the resulting emissions. But despite the billions and billions of tons of carbon removal that climate models rely on through BECCS, this has only been done in small-scale projects to date.
Other technical approaches are also immature, including carbon sucking machines and various ways of accelerating the natural processes by which minerals and the oceans capture and store CO2. This is proved difficult develop systems to incentivize and reliably measure carbon removal through natural systems such as forests and the ground as well as.
Monday’s IPCC assessment noted that there are many other limitations and challenges.
On the one hand, while removing carbon reduces the level of greenhouse gases in the atmosphere, the report notes that this effect can be offset to some extent. Modeling studies to have found as oceans and land begin to release more CO2 in response to this changing atmospheric chemistry over certain time periods, compromising the benefits.
Additionally, while removing carbon could gradually mitigate temperature increases and ocean acidification, it does not magically reverse all climate impacts. Notably, it would take centuries to bring the oceans back to the levels around which we built our coastal cities, the report said. There could be near irreversible damage to ice caps, coral reefs, tropical rainforests and some species as well, depending on global warming before we reduce emissions and increase carbon removal.
Chapter five of the report presents a variety of other tradeoffs and unknowns with just about every potential approach to large-scale carbon removal.
Carbon suction machines require large amounts of energy and materials. Planting more trees for carbon sequestration or crops for fuel will compete with food production for a growing world population.
Climate change itself will compromise the ability of forests to suck up and store carbon dioxide, as the risks of droughts, forest fires and insect infestations increase with rising temperatures. And there is still considerable scientific uncertainty about the side effects of various ocean approaches on marine ecosystems.
The good news is that there are a variety of ways to remove carbon from the air, and a growing number of research groups and companies are working to develop better and cheaper methods. But as Monday’s report makes clear, we are way behind in a very high stakes race.