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Climate change is a paradigmatic example of systemic risk. Recently, proposals for large‐scale interventions—carbon dioxide removal (CDR) and solar radiation management (SRM)—have started to redefine climate governance strategies. We describe how evolving modeling practices are trending toward optimized and "best‐case" projections —portraying deployment schemes that create both technically slanted and politically sanitized profiles of risk, as well as ideal objectives for CDR and SRM as mitigation‐enhancing, time‐buying mechanisms for carbon transitions or vulnerable populations. As promises , stylized and hopeful projections may selectively reinforce industry and political activities built around the inertia of the carbon economy. Some evidence suggests this is the emerging case for certain kinds of CDR, where the prospect of future carbon capture substitutes for present mitigation. Either of these implications are systemic: explorations of climatic futures may entrench certain carbon infrastructures. We point out efforts and recommendations to forestall this trend in the implementation of the Paris Agreement, by creating more stakeholder input and strengthening political realism in modeling and other assessments, as well as through policy guardrails.

Negative emissions technologies (NETs), especially bioenergy with carbon capture and storage and direct air capture and storage, have been invoked as necessary to achieve the aspirational 1.5°C target of the Paris Agreement. However, currently their costs are estimated to be very high, NETs do not seem to offer co-benefits besides mitigating climate change and there are significant concerns regarding possible negative impacts of their large-scale implementation on sustainable development. Costs can vary significantly due to locational factors such as availability of biomass resources and geological storage capacity. It will be up to progressive industrialized countries to take first steps to mobilize the mitigation potential of NETs. In order to understand whether NETs can provide a significant contribution to mitigation, financial incentives are needed that allow implementing the most attractive NET activities at the global scale. We see the market mechanism under Article 6.4 of the Paris Agreement – colloquially called 'Sustainable Development Mechanism' – as a possible cornerstone of such a policy instrument. While initially NETs will not be competitive on the free market, the mechanism can facilitate bilateral financial transfers for NETs, where mitigation units accrue to the financier. We discuss the functions and design elements that an international policy instrument may need to fulfil to successfully mobilize NETs. This includes in particular robust quantification of removed carbon under international oversight and preventing social and environmental conflicts particularly on land and water use by NETs to ensure long-term acceptability.

Carbon dioxide removal (CDR) poses a significant and complex public policy challenge in the long-term. Presently treated as a marginal aspect of climate policy, addressing CDR as a public good is quickly becoming essential for limiting warming to well below 2 or 1.5°C by achieving net-zero emissions in time – including by mobilization of public and private finance. In this policy and practice review, we develop six functions jointly needed for policy mixes mobilizing CDR in a manner compatible with the Paris Agreement's objectives. We discuss the emerging CDR financing efforts in light of these functions, and we chart a path to a meaningful long-term structuring of policies and financing instruments. CDR characteristics point to the need for up-front capital, continuous funding for scaling, and long-term operating funding streams, as well as differentiation based on permanence of storage and should influence the design of policy instruments. Transparency and early public deliberation are essential for charting a politically stable course of action on CDR, while specific policy designs are being developed in a way that ensures effectiveness, prevents rent-seeking at public expense, and allows for iterative course corrections. We propose a stepwise approach whereby various CDR approaches initially need differentiated treatment based on their differing maturity and cost through R&D pilot activity subsidies. In the longer term, CDR increasingly ought to be funded through mitigation results-oriented financing and included in broader policy instruments. We conclude that CDR needs to become a regularly-provided public service like public waste management has become over the last century.

Despite extensive efforts, greenhouse gases continue to be emitted in vastamounts, with potentially devastating consequences around the world. This iswhy targeted interventions in the climate system, known collectively as 'climateengineering', are receiving increased attention. Proposed approaches are oftendivided into two groups: those intended to remove carbon dioxide from theatmosphere and those intended to reduce the amount of solar energy that reachesthe Earth's surface or is trapped in the atmosphere. There are some similaritiesbetween the two classes of activities, but they often raise different physical,political, and governance concerns. This series provides an introduction to eachset of approaches.

Despite extensive efforts, greenhouse gases continue to be emitted in vastamounts, with potentially devastating consequences around the world. This iswhy targeted interventions in the climate system, known collectively as 'climateengineering', are receiving increased attention. Proposed approaches are oftendivided into two groups: those intended to remove carbon dioxide from theatmosphere and those intended to reduce the amount of solar energy that reachesthe Earth's surface or is trapped in the atmosphere. There are some similaritiesbetween the two classes of activities, but they often raise different physical,political, and governance concerns. This series provides an introduction to eachset of approaches.

Sustainable Development Goals and the Paris Agreement stand as milestone diplomatic achievements. However, immense discrepancies between political commitments and governmental action remain. Combined national climate commitments fall far short of the Paris Agreement's 1.5/2°C targets. Similar political ambition gaps persist across various areas of sustainable development. Many therefore argue that actions by nonstate actors, such as businesses and investors, cities and regions, and nongovernmental organizations (NGOs), are crucial. These voices have resonated across the United Nations (UN) system, leading to growing recognition, promotion, and mobilization of such actions in ever greater numbers. This article investigates optimistic arguments about nonstate engagement, namely: (a) "the more the better"; (b) "everybody wins"; (c) "everyone does their part"; and (d) "more brings more." However, these optimistic arguments may not be matched in practice due to governance risks. The current emphasis on quantifiable impacts may lead to the under‐appreciation of variegated social, economic, and environmental impacts. Claims that everybody stands to benefit may easily be contradicted by outcomes that are not in line with priorities and needs in developing countries. Despite the seeming depoliticization of the role of nonstate actors in implementation, actions may still lead to politically contentious outcomes. Finally, nonstate climate and sustainability actions may not be self‐reinforcing but may heavily depend on supporting mechanisms. The article concludes with governance risk‐reduction strategies that can be combined to maximize nonstate potential in sustainable and climate‐resilient transformations.