The scenarios modelled for the IPCC’s Fifth Assessment Report assume the large-scale deployment of technologies that achieve negative emissions that draw down carbon dioxide from the atmosphere and permanently store it. But whether such proposed methods could be deployed at a material scale is unproven. It would be more prudent to exclude these techniques from mitigation scenarios used by the IPCC, unless and until we have sufficient evidence of their availability and viability to support their inclusion.

Most of the modelled emissions pathways limiting warming to 2 °C (and all the ones that restrict the rise to 1.5 °C) require massive deployment of Biomass Energy with Carbon Capture and Storage (BECCS). This involves growing biomass which is used to generate power and geologically sequestering the carbon dioxide produced. While the constituent steps of this process have been demonstrated, there are but a few, small, examples of the combined process. To rely on this technique to deliver us from climate change is to demonstrate a degree of faith that is out of keeping with scientific rigour.

There is a distinct lack of evidence to determine whether BECCS is technically feasible, economically affordable, environmentally benign, socially acceptable and politically viable at a material scale. Technically, there are serious doubts about the ability to sequester the vast quantities of carbon dioxide that are implied in the models. Economically, without a substantial carbon price, the costs would be much higher than competing power-generation technologies. Environmentally, growing such volumes of biomass would have profound effects on biodiversity. Socially, the use of land for BECCS would restrict agriculture – contributing to substantial increases in food prices; while politically, the issue seems so toxic that the Paris Agreement carefully avoided mentioning negative emissions at all. Such impacts would not be material were BECCS to be deployed at a small scale, but the economic scenarios consistent with 1.5 °C (or even 2°C) assume that BECCS is deployed at a truly gargantuan scale, at which these adverse impacts would indeed be material.

For a technology to be deployable it needs not only to work, but also to possess a social licence to operate. For example, that Germany possesses the technical ability and financial means to build new nuclear power plants is not in question, but lacking the social and political will to do so makes the point moot.

The IPCC’s own scenario database suggests that the ambition of the Paris Agreement cannot be achieved without negative emissions technologies. Even with rapid decarbonisation, there will be a need to achieve net negative emissions during the second half of this century. That objective cannot be achieved from a standing start. Well-functioning methods would need to be developed and rolled out at a rate unprecedented in human history. Yet to model what you want to happen, rather than what there is evidence could happen, is to lose the thread of reality. It is redolent of a defeated leader issuing orders to armies that have long since ceased to exist – not so much vision, as delusion.

Should modellers be able to model what they like? Of course. Scenarios allow us to undertake useful thought experiments that provide us with the means to assess potentially novel approaches.

But it is hazardous to rely on science fiction in the development of the scenarios that are used to inform policymakers. To include scenarios for avoiding dangerous climate change that employ entirely speculative approaches seems reckless in the extreme.

Some will defend the use of these technological imaginaries in IPCC scenarios by arguing that without them hopes of avoiding dangerous climate change are forlorn and that this would generate a degree of despair that would undermine the will to act.

But that is not the role of models. “Fake it ‘til you make it” may work as a tactic, but it is a lousy strategy. As the dust settles on the Paris Agreement and policymakers face up to the challenge of achieving the ambition set out by their leaders, we need to reflect on what actually needs to happen. Policymakers can only hope to develop realistic plans, if the basis on which they are making those plans is itself realistic. While the boundary between ambition and delusion may be not be entirely sharp, the inclusion of negative emissions amounting to 600-800 billion tonnes of carbon dioxide (equivalent to 15-20 years of current annual emissions) is clearly more than a stretch goal. For this reason, negative emission techniques should be excluded from the mitigation scenarios used by the IPCC unless and until there is sufficient evidence to warrant their inclusion and then only on a scale that is demonstrably realistic.

The IPCC recently announced a Special Report on the 1.5 °C target. To be credible, this must include detailed assessment of proposed negative emission techniques, drawing on a wide range of expertise from natural sciences, engineering, social sciences and the humanities to assess to what extent, if any, such approaches could be deployed without creating countervailing side-effects.

On the basis of such a comprehensive assessment, policy makers will then have to make an explicit decision either to invest in the necessary research, development and demonstration of the technologies or to explain how they propose to meet their ambitious targets without such interventions. Policymakers cannot be allowed to hide behind the vague language of the Paris Agreement (“achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases”).

In the absence of comprehensive research and indications of political feasibility, it seems prudent to exclude from the models what is currently magical thinking. Only by undertaking research will it be possible to determine whether today’s science fiction could be transformed into tomorrow’s science reality.





Tim Kruger is a James Martin Fellow at Oxford University and manages the Oxford Geoengineering Programme. Oliver Geden is head of EU division at the German Institute for International and Security Affairs (SWP). Steve Rayner is James Martin Professor of Science and Civilization and Director of the Institute for Science, Innovation and Society (InSIS) in the School of Anthropology and Museum Ethnography at Oxford University.