The UN’s long-awaited climate report on Monday offered a stark reminder that removing massive amounts of carbon dioxide from the atmosphere will be essential to prevent the gravest dangers of global warming. But it also highlighted that the necessary technologies barely exist—and will be tremendously difficult to deploy.
Global temperatures will continue to rise through midcentury no matter what we do at this point, according to the first installment of the Intergovernmental Panel on Climate Change’s sixth assessment report. How much hotter it gets, however, will depend on how rapidly we cut emissions and how quickly we scale up ways of sucking carbon dioxide out of the air.
Climate scientists say we’ll need to do carbon removal, in part, to balance out the emissions sources we still don’t know how to eliminate or clean up—like flights and fertilizer—and, in part, to pull the planet back after it blows through dangerous temperatures thresholds.
The UN report found greenhouse gases are likely to push global temperatures to at least 1.5 ˚C above pre-industrial conditions within the next 20 years, under almost every climate scenario, fueling more common and more severe heatwaves, flooding and droughts. But carbon dioxide persists in the atmosphere for hundreds to thousands of years, so once that happens, carbon removal is essentially the only way to bring the climate back to a safer zone. (A last alternative is, perhaps, some form of geoengineering that reflects heat back into space, but that subject creates all sorts of different controversies and 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. (Those figures are based on an analysis of earlier data by Zeke Hausfather, a climate scientist at The Breakthrough Institute and contributing author to the UN assessment.)
That requires ramping up technologies and techniques capable of pulling as much CO2 out of the atmosphere every year as the entire US emitted in 2020, in just the next 30 years.
In that scenario (known as SSP1-1.9), nearly all of 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 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.
A smaller amount 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 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 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, rainforests and certain species as well, depending on how much warmer the world gets before we cut emissions and scale up carbon removal.