Hubris and Humility in the Face of Climate Overshoot

Cutting through the noise on negative emissions

Carbon dioxide removal (CDR) is a critical component of climate change mitigation strategies, and ocean afforestation via macroalgae cultivation has been touted as a promising CDR approach due to its high productivity and favourable carbon-to-nutrient ratio. However, global CDR models largely overlook iron limitation, a potential bottleneck for sustainable macroalgae cultivation. Iron is an essential micronutrient for macroalgal growth and metabolism and also limits phytoplankton production in many ocean regions. Large-scale macroalgae cultivation could intensify nutrient competition, affecting both macroalgal CDR potential and broader marine ecosystems. Additionally, nutrient stoichiometry and uptake affinities influence whether macroalgae can outcompete phytoplankton in carbon fixation, determining the benefit of replacing phytoplankton production with macroalgae. However, these dynamics are often neglected in global macroalgal cultivation models, leaving significant gaps in our understanding of macroalgae’s CDR potential.Our study is the first to assess the combined effects of nutrient demand, uptake affinities, and iron limitation on macroalgal CDR efficiency. Using an ocean biogeochemical model, we simulated 25 years of macroalgae cultivation followed by 50 years of cessation in the Exclusive Economic Zones (EEZs) of the global ocean under a high-mitigation scenario. The impact on macroalgal production potential and CDR efficiency was assessed using an ensemble of simulations that integrate published values of macroalgae stoichiometry and nutrient affinities.Our results reveal that iron limitation reduces macroalgal CDR potential, decreasing the production potential three-fold after accounting for N and P limitation. CDR efficiency declined by 40% after 25 years of cultivation, with regions previously showing high CDR potential under N and P limitations exhibiting negative CDR when iron limitation is additionally accounted for. In these regions, cultivation reduced ocean carbon uptake rather than enhancement. Iron limitation also amplified ecosystem impacts, reducing phytoplankton production by up to 81% in iron-limited areas such as the Southern Ocean and upwelling zones. This reduction in primary production could weaken the biological pump, diminishing the overall CDR outcome and impacting broader ecosystems. Additionally, variability in macroalgal stoichiometry and nutrient uptake affinities contributed to substantial uncertainty in projections of CDR efficiency, with global CDR efficiency ranging from -43% to +53%.Our findings underscore the need for iron demand and affinities to be included in projections of ocean afforestation. Failing to account for such nutrient dynamics may result in overestimating the efficacy of macroalgae-based CDR as a mitigation strategy.

This Perspective describes the various dimensions of Carbon Dioxide Removal (CDR) durability and interprets them in the context of current policy making. Durability – together with scalability and sustainability – is an essential condition of CDR. It depends on (i) the duration of CO2 storage and (ii) the risk of reversing such storage. The risk profile of durability varies widely across CDR methods. Because engineered, novel CDR methods involve more stable forms of CO2 storage than nature-based CDR, these methods are often promoted as a priority for CDR mitigation investments. However, shorter-term CDR plays an essential role in balancing sources and sinks of greenhouse gases in the second half of this century. Decision makers must also consider CDR policies in a larger context that takes into account readiness and feasibility, policy alignment and co-benefits of different CDR methods. They must also address durability in CDR policies and contracts, which tend to span much shorter timeframes than those contemplated by science when discussing durability. We argue that nature-based conventional CDR and novel engineered CDR that show complementary timing and risk profiles can be deployed in synergistic CDR portfoliosto balance the conditions of durability, feasibility and social and environmental sustainability.

In 1998, computer scientist Ehud Shapiro returned to the Weizmann Institute in Rehovot, Israel, as a group leader after a five‐year break as a software entrepreneur. At the peak of the Internet boom, it would have been easy to find an exciting topic to pursue in computer science. Instead, Shapiro became interested in the origin of life and began to train himself in molecular biology, which eventually sparked his idea to build computers from biological molecules. His team first constructed a molecular Turing machine based on DNA, restriction nuclease and ligase to perform simple computations (Benenson et al , 2001), soon followed by a more sophisticated system that performs stochastic computations using mRNA molecules as input (Benenson et al , 2004). What seems merely to be the intellectual interest of an Israeli computer scientist—using biological compounds and systems to create logical circuits—has in fact become the hottest area in the biological sciences: synthetic biology. Other engineers are also dropping their soldering guns for micropipettes to rewire genes and genomes with the aim of reprogramming living organisms. “Synthetic biology is the other side of the coin of systems biology,” commented Victor de Lorenzo, Vice Director of the National Centre of Biotechnology in Madrid, Spain. “What you want is to create or recreate systems that have some properties of life from engineering principles.” This includes a range of techniques from recombinant cloning, to synthesizing genomes de novo , to creating completely new entities such as Shapiro's artificial systems. However, more interesting than the technology itself is the ability to create artificial metabolic and regulatory pathways and to test their viability in living systems. It allows scientists to probe the complexity of an organism's innards and thus derive further insights into how cells work. As George Church, Professor of Genetics at Harvard Medical School …

We attempt to justify the use of synthetic biology in response to the climate crisis, based on the premise that it is impossible to avert runaway climate change without sequestering sufficient greenhouse gases (GHG), which could only become possible through Negative Emissions Technologies (NETs). Then, moving from a consequentialist standpoint, we acquiesce to how the consequences of using NETs through synthetic biology are preferable to the catastrophic consequences of runaway climate change. In conclusion, we show how our analysis of synthetic biology resonates with a zoecentric view of climate science and ethics.

According to Henry Shue, everyone alive today is part of the ‘pivotal generation’, meaning that they have a special responsibility to mitigate climate change. One way to mitigate climate change is through carbon dioxide removal (CDR) technologies. CDR technologies include a range of measures, from afforestation to filtering carbon dioxide from the ambient air. What role CDR should play in climate policy is Shue’s main concern in chapter 4 of The Pivotal Generation: Why We Have a Moral Responsibility to Slow Climate Change Right Now and the focus of my commentary. I begin by presenting Shue’s account of CDR. To define the role CDR should play in climate policy, Shue distinguishes three purposes of CDR: CDR that enhances mitigation efforts; CDR that remedies inadequate past mitigation; and CDR that is undertaken to ‘rescue’ the stranded assets of fossil fuel companies. Shue provides arguments for why we should prioritise enhancement CDR over remedial CDR, and dismisses asset-rescue CDR. The purpose of CDR is essential to our ethical inquiry and to answering the question of how CDR should be undertaken. But our generation is not only pivotal in determining how CDR is done. It is also pivotal in deciding whether CDR is undertaken at all. The moral challenges of CDR must be taken seriously and at the same time the urgency of mitigating climate change may demand that CDR be implemented. The urgency of mitigating climate change provides a rationale for implementing CDR as well as theorizing the non-ideal circumstances of the current situation. This is subject of part 2 of my commentary. This consideration leads me to argue, in part 3, that ‘asset-rescue CDR’ needs to be included in our ethical discussion of CDR. Unlike Shue, I argue that we need to morally examine the case of CDR being undertaken by agents to save their assets. This is especially pressing for corporations from the oil and gas industry. In part 4, I argue that this requires a more nuanced understanding of the moral agency of corporations within the pivotal generation. Instead of dissolving the responsibilities of separate agents into the responsibility of one generation, we should differentiate the responsibilities for carbon removal. I then draw my conclusions.

ABSTRACTIn principle, many climate policymakers have accepted that large-scale carbon dioxide removal (CDR) is necessary to meet the Paris Agreement’s mitigation targets, but they have avoided proposing by whom CDR might be delivered. Given its role in international climate policy, the European Union (EU) might be expected to lead the way. But among EU climate policymakers so far there is little talk on CDR, let alone action. Here we assess how best to ‘target’ CDR to motivate EU policymakers exploring which CDR target strategy may work best to start dealing with CDR on a meaningful scale. A comprehensive CDR approach would focus on delivering the CDR volumes required from the EU by 2100, approximately at least 50 Gigatonnes (Gt) CO2, according to global model simulations aiming to keep warming below 2°C. A limited CDR approach would focus on an intermediate target to deliver the CDR needed to reach ‘net zero emissions’ (i.e. the gross negative emissions needed to offset residual positive emissions that are too expensive or even impossible to mitigate). We argue that a comprehensive CDR approach may be too intimidating for EU policymakers. A limited CDR approach that only addresses the necessary steps to reach the (intermediate) target of ‘net zero emissions’ is arguably more achievable, since it is a better match to the existing policy paradigm and would allow for a pragmatic phase-in of CDR while avoiding outright resistance by environmental NGOs and the broader public.Key policy insightsMaking CDR an integral part of EU climate policy has the potential to significantly reshape the policy landscape.Burden sharing considerations would probably play a major role, with comprehensive CDR prolonging the disparity and tensions between progressives and laggards.Introducing limited CDR in the context of ‘net zero’ pathways would retain a visible primary focus on decarbonization but acknowledge the need for a significant enhancement of removals via ‘natural’ and/or ‘engineered’ sinks.A decarbonization approach that intends to lead to a low level of ‘residual emissions’ (to be tackled by a pragmatic phase-in of CDR) should be the priority of EU climate policy.

With the Paris Agreement’s ambition of limiting climate change to well below 2 °C, negative emission technologies (NETs) have moved into the limelight of discussions in climate science and policy. Despite several assessments, the current knowledge on NETs is still diffuse and incomplete, but also growing fast. Here, we synthesize a comprehensive body of NETs literature, using scientometric tools and performing an in-depth assessment of the quantitative and qualitative evidence therein. We clarify the role of NETs in climate change mitigation scenarios, their ethical implications, as well as the challenges involved in bringing the various NETs to the market and scaling them up in time. There are six major findings arising from our assessment: first, keeping warming below 1.5 °C requires the large-scale deployment of NETs, but this dependency can still be kept to a minimum for the 2 °C warming limit. Second, accounting for economic and biophysical limits, we identify relevant potentials for all NETs except ocean fertilization. Third, any single NET is unlikely to sustainably achieve the large NETs deployment observed in many 1.5 °C and 2 °C mitigation scenarios. Yet, portfolios of multiple NETs, each deployed at modest scales, could be invaluable for reaching the climate goals. Fourth, a substantial gap exists between the upscaling and rapid diffusion of NETs implied in scenarios and progress in actual innovation and deployment. If NETs are required at the scales currently discussed, the resulting urgency of implementation is currently neither reflected in science nor policy. Fifth, NETs face severe barriers to implementation and are only weakly incentivized so far. Finally, we identify distinct ethical discourses relevant for NETs, but highlight the need to root them firmly in the available evidence in order to render such discussions relevant in practice.

Ocean negative carbon emissions: A new UN Decade program

This article explores the impact that synthetic biology approaches may have on Negative Emissions Technologies (NETs). Synthetic biology has both altered and created biological pathways inspired by nature to develop new NETs that sequester greenhouse gases into industrially useful chemicals, such as biomass and calcium carbonate. However, synthetic biology continues to encounter difficulties when implementing and scaling up production due to a combination of hard limits (within biology) and ‘soft’ limits (of social and economic costs). Additionally, NETs, along with Ecosystem Technologies in general, operate as climate technofixes, wherein insufficient thought is given to the ethical quandaries arising from releasing designed organisms into the environment, even under controlled conditions. In this paper, we provide a technological and ethical appraisal of synthetic biology approaches to NETs, in the context of climate change mitigation through Ecosystem Technology.

Climate change mitigation strategies informed by Integrated Assessment Models (IAMs) increasingly rely on major deployments of negative emissions technologies (NETs) to achieve global climate targets. Although NETs can strongly compliment emissions mitigation efforts, this dependence on the presumed future ability to deploy NETs at scale raises questions about the structural elements of IAMs that are influencing our understanding of their deployment. Model inter-comparison results underpinning the IPCC’s special report on Global Warming of 1.5oC were used to explore the role that current assumptions are having on projections and the way in which emerging technologies, economic factors, innovation, and tradeoffs between negative emissions objectives and UN Sustainable Development Goals might have on future deployment of NETs. Current generation IAM scenarios widely assume we are capable of scaling up NETs over the coming 30 years to achieve negative emissions of the same order of magnitude as current global emissions (tens of gigatons of CO2/year) predominantly relying on highly land intensive NETs. While the technological potential of some of these approaches (e.g., direct air capture) is much greater than for the land-based technologies, these are seldom included in the scenarios. Alternative NETs (e.g., accelerated weathering) are generally excluded because of connections with industrial sectors or earth system processes that are not yet included in many models. In all cases, modeling results suggest that significant NET activity will be conducted in developing regions, raising concerns about tradeoffs with UN Sustainable Development Goals. These findings provide insight into how to improve treatment of NETs in IAMs to better inform international climate policy discussions. We emphasize the need to better understand relative strength and weaknesses of full suite of NETs that can help inform the decision making for policy makers and stakeholders.

The Value of Bioenergy Sequestration

Generating negative emissions by removing carbon dioxide from the atmosphere is a key requirement for limiting global warming to well below 2 °C, or even 1.5 °C, and therefore for achieving the long-term climate goals of the recent Paris Agreement. Despite being a relatively young topic, negative emission technologies (NETs) have attracted growing attention in climate change research over the last decade. A sizeable body of evidence on NETs has accumulated across different fields that is by today too large and too diverse to be comprehensively tracked by individuals. Yet, understanding the size, composition and thematic structure of this literature corpus is a crucial pre-condition for effective scientific assessments of NETs as, for example, required for the new special report on the 1.5 °C by the Intergovernmental Panel on Climate Change (IPCC). In this paper we use scientometric methods and topic modelling to identify and characterize the available evidence on NETs as recorded in the Web of Science. We find that the development of the literature on NETs has started later than for climate change as a whole, but proceeds more quickly by now. A total number of about 2900 studies have accumulated between 1991 and 2016 with almost 500 new publications in 2016. The discourse on NETs takes place in distinct communities around energy systems, forests as well as biochar and other soil carbon options. Integrated analysis of NET portfolios—though crucial for understanding how much NETs are possible at what costs and risks—are still in their infancy and do not feature as a theme across the literature corpus. Overall, our analysis suggests that NETs research is relatively marginal in the wider climate change discourse despite its importance for global climate policy.

Climate change mitigation strategies consistent with the Paris Agreement’s temperature targets rely heavily on future carbon dioxide removal (CDR). Although such strategies have drawn considerable critique for long, e.g., that they are ‘betting on negative emissions’, the risks from this betting have not been quantified nor addressed properly. We use a lightweight integrated assessment model SCORE to explore possible scenarios using CDR for limiting global warming to 1.5 °C by 2100. Particularly, we quantify the impacts of relying on CDR when accounting for 1) possible under- and overestimation of the cost, potential, and availability (feasibility) of future CDR and 2) the compound effect with uncertainty in climate sensitivity.All scenario results unquestionably show that aggressive near-term mitigation is required for limiting warming to 1.5 °C by 2100 for all levels of climate sensitivity, but that some amount of CDR is likely required in the future even if climate sensitivity turns out to be extremely low. If uncertainty in climate sensitivity is disregarded, initial assumptions on the CDR feasibility have only minor effects on the total cumulative mitigation cost. However, taking the uncertainty in climate sensitivity into account changes this conclusion. Wrong assumptions on CDR feasibility can, surprisingly, even lead to lower costs under extreme realizations of climate sensitivity, especially in scenarios where CDR feasibility is underestimated. Assuming low feasibility for CDR eliminates the possibility of sky-rocketing costs associated with overestimating CDR feasibility in combination with a high climate sensitivity. Therefore, a prudent climate policy should assume a low feasibility of CDR to reduce the risk of leaving runaway mitigation costs to future generations.

To achieve net-zero emissions by mid-century, the removal of carbon dioxide from the atmosphere through negative emission technologies (NETs) will play an integral part. With renewable energy technologies (RETs), there has already been the introduction and expansion of a clean technology that faced similar obstacles as NETs—high up-front costs, limited competitiveness, and low public perception. This article compares NET policy proposals with the lessons learned from RET support. For NETs, the use of R&D support for innovation is unequivocal due to its nascency, yet the demand-pull instrument differs whether NETs are used as an alternative mitigation strategy, as a bridging technology or as a last resort. As an alternative mitigation method, a market-based approach by integrating NETs into emission trading systems is applicable because the use of NETs has no additional environmental benefit compared to abatement. Using NETs as a bridging technology requires restricting the demand for NETs to control the volume, and possibly type of NETs. This can be achieved via mandates or auctions. As a last resort, the removal via NETs requires heavy state involvement as emission removal constitutes a pure public good. This warrants public procurement or even state-led NET operation.