The UN report found that greenhouse gases are likely to drive worldwide temperatures at least 1.5 ˚C above preindustrial conditions within the next 20 years, fueling more common and more severe heat waves, floods, and droughts. Once that happens, carbon removal is essentially the only way to bring the climate back to a safer zone, because the greenhouse gas persists for hundreds to thousands of years in the atmosphere. (A last alternative is, perhaps, some form of geoengineering that reflects heat back into space, but that controversial idea presents all sorts of concerns.)
The model used to create the most optimistic scenario in the report, which limits warming to 1.5 ˚C, assumes the world will figure out ways to remove about 5 billion tons of carbon dioxide a year by midcentury and 17 billion by 2100. (The scenario is known as SSP1-1.9, and those figures are based on an analysis of earlier data by Zeke Hausfather, a climate scientist at the Breakthrough Institute and contributing author of the UN assessment.)
That requires ramping up technologies and techniques capable of pulling as much CO2 out of the atmosphere every year as the US economy emitted in 2020. In other words, the world would need to stand up a brand-new carbon-sucking sector operating on the emissions scales of all America’s cars, power plants, planes, and factories, in the next 30 years or so.
In that model, nearly all the carbon removal is achieved through an “artificial” approach known as bioenergy with carbon capture and storage, or BECCS. Basically, it requires growing crops that consume 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.
The smaller remaining amount of removal in the model is done through “natural” solutions like reforestation and tree planting (see the the illustration below).
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 proved challenging to develop systems to reliably incentivize and measure carbon removal through natural systems like forests and soil as well.
The IPCC assessment noted numerous other limitations and difficulties.
For one thing, while carbon removal does reduce the level of greenhouse gases in the 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 our coastal cities, the report stresses. There could be all but irreversible damage to ice sheets, coral reefs, rain forests, and certain species as well, depending on how much warmer the world gets before we deeply cut emissions and scale up carbon removal.