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Carbon dioxide removal (CDR) technologies, which give a method of taking carbon out of the atmosphere, are one of the most hotly debated areas of climate research.

Last month, the last phase of the Intergovernmental Panel on Climate Change (IPCC) comprehensive evaluation of climate science sparked a controversy about whether and how CDR should be developed. The analysis discovered that, while expensive, methods of capturing and storing carbon dioxide may play a role in attempting to maintain global temperatures within acceptable ranges.

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However, scientists and policymakers are split. Some argue that technology should be the top focus for research right now. Others encourage caution and caution against putting confidence in untested technology before it has completely deployed the reliable low-carbon solutions currently available, such as renewable energy.

John Kerry, the United States' special presidential envoy on climate change, expressed that "Some scientists suggest that it's possible there could be an overshoot [of global temperatures, beyond the limit of 1.5C above pre-industrial levels that governments are targeting] and you could clawback, so to speak; you have technologies and other things that allow you to come back."

He elaborated that "we may be at or past several tipping points that they have been warning us about for some time [...] That's the danger, the irreversibility."

Sir David King, the former UK government's main scientific advisor, strongly disagrees. He believes that CDR of many types would be required, as well as methods to "repair" the climate, such as refreezing the ice caps because the globe is almost guaranteed to exceed the global goal limit of 1.5 degrees Celsius over pre-industrial levels.

"We are already at 1.35C above pre-industrial levels today," he said. "We are already experiencing massive warming in the Arctic, where it's more than 3C above the pre-industrial average."

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A flood of new technology businesses demonstrates the potential financial opportunity that many corporations and investors perceive in CDR. These start-ups are investigating everything from "scrubbers", which chemically remove carbon dioxide from the air, to "biochar", which produces fertilizer by burning wood waste without oxygen, and carbon capture and storage (CCS), which involves liquefying carbon dioxide and pumping it into underground geological formations. They have seen the IPCC report as a catalyst for investment and a seal of approval.

Timmermann, the chief technology officer of Andes, a company that plans to use micro-organisms to sequester carbon in soil says, "Growing carbon removal to be in line with the IPCC requires a massive scale-up in the next decade. Startups are meeting this climate challenge by developing a suite of approaches that can make a gigaton impact."

According to Ben Rubin, the executive director of the Carbon Business Council, which represents several CDR specialists, the window of opportunity is closing "quickly".

However, the crucial piece of the IPCC report that sparked the dispute was intensely battled over by scientists and governments until the publication was finalized. The few mentions of CDR in the final 36-page summary for policymakers, which distills the important points and is written by scientists with government officials from any UN member who wants to participate, came only after hours of debate.

Saudi Arabia and other oil-producing countries were adamant that CDR and CCS be included and highlighted. In the end, the summary contained nine references to CDR and five more to CCS.

Many scientists, activists, and environmental specialists are dissatisfied with the references. They are concerned that creating the idea that there are realistic solutions for eliminating CO2 would provide a false sense of security. Most CDR technologies are untested, will be restricted in scope, will require years to develop, and will be expensive.

Lili Fuhr, the director of the climate and energy program at the Center for International Environmental Law, says "We need to challenge the idea that we have to do less now because we can do more later, with technofixes. This is a dangerous idea."

A harmful diversion

Friederike Otto, a lead author of the IPCC, and senior lecturer at the Grantham Institute at Imperial College London says the scientific community should act as though CDR will never be achievable and that policies need to be made in accordance with that. 

She warned that pursuing CDR may be a harmful diversion, and questioned whether it was wise to invest money in technologies with extremely unclear future benefits while effective techniques of lowering emissions were not being adopted quickly enough. "CDR has already been used as an excuse to dither and delay," she pointed out.

Otto stated that "It's very important to highlight that we still can keep to 1.5C – we have the knowledge and the tools to do it. But what we do not have is a sense of urgency and political will."

Some scientists are concerned about CDR, but King feels it is necessary due to the failure to act earlier. "[Those who object] are taking the exact position I took in 2015 when I was leading global negotiations for the UK," he said. "But there is no time for messing about now."

King, who is collaborating with Cambridge University's engineering department to try to refreeze the Arctic, points out that the IPCC report found only a narrow opportunity for the world to limit warming to 1.5 degrees Celsius, which depends on massive reductions in greenhouse gases in the coming years, an unlikely scenario.

"The IPCC does not go nearly far enough on CDR," he said. "I believe it is more than likely we will hit 1.5C by the end of the decade. It's false thinking that the IPCC is saying we can manage [to stay below that level] by reducing emissions. The carbon we have put up [in the atmosphere] will have to be removed. It may cost a fortune, but we have to recognize that the alternative is to lose our civilization."