In that model, nearly all of the carbon removal comes through a process known as bioenergy with carbon capture and storage, or BECCS. Basically, it requires growing crops that suck up CO2 and then using the harvested biomass to produce heat, electricity or fuels, while capturing and storing any resulting emissions. But despite the billions and billions of tons of carbon removal that climate models are banking on through BECCS, it’s 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 take up and store away CO2. It’s proven challenging to develop systems to reliably incentivize and measure carbon removal through natural systems like forests and soil as well.
The IPCC assessment on Monday noted there are numerous other limitations and difficulties.
For one thing, while carbon removal does reduce the level of greenhouse gases in atmosphere, the report notes that this effect may be offset to some degree. Modeling studies have found that the oceans and land start releasing more CO2 in response to that shifting atmospheric chemistry over certain time periods, undermining the benefits.
In addition, while carbon removal could gradually ease temperature increases and ocean acidification, it doesn’t magically reverse all climate impacts. Notably, it would still take centuries to bring oceans back to the levels around which we’ve built out coastal cities, the report stresses. There could be all-but irreversible damage to ice sheets, coral reefs, rainforests and certain species as well, depending on how much warmer the world gets before the world cuts emissions and scales up carbon removal.
Chapter five of the report lays out a variety of other tradeoffs and unknowns with pretty much every potential approach to wide-scale carbon removal.
Carbon-sucking machines require large amounts of energy and materials. Planting more trees for carbon sequestration or crops for fuels will compete with growing food for an expanding global population.
Climate change itself will undermine the ability of forests to suck up and store carbon dioxide, as the risks of droughts, wildfires and insect infestations grow with rising temperatures. And there’s still considerable scientific uncertainty about the side-effects of various ocean-based approaches on marine ecosystems.
The good news is 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, cheaper methods. But as Monday’s report makes clear, we’re falling far behind in a race with very high stakes.