What type of negative emissions technologies should be prioritised?

Combating global warming will require removing carbon and not just reducing new emissions. A new study co-authored by Professor David Reiner of Cambridge Judge Business School identifies the most promising approaches.

Restricting global warming to 1.5 degrees Celsius above pre-industrial levels will require not only emission reductions but also significant contributions from negative emissions technologies and practices (NETPs). Many NETPs are put forward as possible solutions, but which are the most promising in this emerging field?

A new study co-authored by David Reiner at Cambridge Judge Business School that looked at 36 NETP “configurations” concludes that a portfolio of approaches drawing on their complementary strengths is the best way forward, because no single technique outscores others on all indicators simultaneously.

The 6 most promising technologies for carbon removal

The study finds, however, that 6 of the approaches are deemed particularly promising. These are:

• forestation
• soil carbon sequestration (farming techniques that absorb and hold more carbon)
• enhanced weathering with olivine (which spreads fine rock material onto land areas to remove carbon dioxide)
• three types of direct air carbon capture and storage, or DACCS (capturing carbon directly from the atmosphere and then permanently storing the CO₂) – which produce significantly fewer negative side-effects and could be relatively quickly scaled up with new investment.

“The decisions and investments made during this decade will determine the course of GGR (greenhouse gas removal) throughout the 21st century,” concludes the study published in the journal Environmental Research Letters. “We should regard the 2020s as a training period to develop and upgrade NETPs, and ensure that they can be sustainably deployed at the needed scale.”

Why 2 parallel strategies are needed to achieve net zero

The study acknowledges that both the effectiveness and sustainability implications of NETPs remain uncertain, and that deployment of NETPs could raise other environmental concerns. But the vast magnitude of the cumulative carbon dioxide removal needed for the rest of this century makes the evaluation of various NETPs an essential part of combating global warming.

“The study proposes 2 parallel investment plans designed to tap these approaches as a vital part of achieving net zero, since even under aggressive plans to reduce emissions most studies find there will still be a significant need for greenhouse gas removal,” says co-author David Reiner, Professor of Technology Policy at Cambridge Judge Business School. “These parallel paths are commercialising the more advanced NETPs while simultaneously prioritising R&D into the most promising NETPs.

“Although it is possible to disagree over the specific options put forward, we hope that our approach can provide a framework to governments and investors, and hopefully the study can help identify opportunities for developing sustainable business strategies to achieve carbon neutrality.”

Five key performance indicators of negative emissions technologies and practices

The study is based on 5 key performance indicators of NETPs:

1. the technology readiness level
2. the maximum annual greenhouse gas removal potential
3. the cost of removing 1 t CO2-eq (the equivalent of one tonne of carbon dioxide)
4. the number of negative side effects including substantial resource use
5. the number of positive side-effects such as a decrease in ocean acidification or improved crop yields.

“We found that none of the assessed NETPs simultaneously outperformed all the others in terms of the 5 KPIs, supporting the thesis that a portfolio of NETPs will likely be needed,” says the study.

“Our analysis indicates that terrestrial and chemical NETPs are the most promising. Forestation and soil carbon sequestration (SCS) practices show an overall good performance level and are currently ready for deployment, offering potential co-benefits that could incentivise their implementation. However, they also pose risks, chiefly the possible release of the stored carbon.

Direct air carbon capture and storage can be sustainably scaled

“By contrast, most NETPs based on chemical processes can reduce the unintended impacts of negative emissions. Among them, direct air carbon capture and storage (DACCS) presents particularly appealing KPI values; thus, investing in DACCS appears to be a good strategy to accelerate the sustainable scale-up of CDR (carbon dioxide removal).”

Other approaches looked at in the study include: marine NETPs (practices that seek to maximise the long-term storage of carbon in the ocean), bioenergy with carbon capture and storage, and enhanced weathering.

Crop productivity an added benefit to soil capture approach

“Our findings point towards terrestrial and chemical NETPs as the most attractive GGR options,” the study concludes. “Forestation and SCS (soil carbon sequestration) constitute easy-to-implement solutions that are ready for deployment. SCS practices are particularly appealing because of their potential co-benefits for crop productivity.” The study cautions, however, that the permanence of terrestrial NETPs is difficult to ensure because of the potential release of stored carbon through human action, so careful monitoring is needed to minimise such risks.