Overshoot and recover? On the problem of substitution between negative emissions and emissions reductions

Overview

This article examines a critical tension in climate mitigation strategies between carbon dioxide removal (CDR) measures and immediate deep decarbonization. Given the constrained global carbon budget, many scenarios limiting warming to well below 2°C incorporate hundreds of gigatons of CO2 removal. However, the availability of large-scale CDR in mitigation pathways may create incentives to defer or reduce the urgency of emissions reductions, potentially leading to carbon budget overshoot. The work focuses on the substitution problem whereby reliance on future negative emissions technologies could displace or delay the implementation of aggressive decarbonization measures. This substitution raises concerns about both the practical feasibility of achieving climate targets and the ethical dimensions of mitigation strategies that defer action.

Methods and approach

The article structures its analysis around three components: explicating the structure of the substitution problem between CDR and emissions reductions, articulating the ethical issues that emerge from this substitution dynamic, and identifying three specific conditions under which substitution is likely to occur. The approach is philosophical and analytical rather than empirical, examining the logical and normative dimensions of how CDR availability influences decarbonization decision-making. The investigation considers how different mitigation pathway scenarios incorporate varying assumptions about the scale and timing of both emissions reductions and negative emissions deployment, and how these assumptions relate to carbon budget constraints and justice considerations.

Results

The analysis demonstrates that mitigation pathways heavily dependent on large-scale CDR create structural conditions for substitution effects that could result in substantial carbon budget overshoot. The article identifies that this substitution problem generates severe justice concerns, as overshooting carbon budgets through delayed emissions reductions imposes greater climate impacts and risks on vulnerable populations. The investigation reveals specific conditions under which the substitution of CDR for immediate decarbonization becomes more probable, suggesting that pathway design choices have significant ethical implications. The findings indicate that while CDR may be necessary given remaining carbon budget constraints, pathways that rely on extensive future negative emissions risk enabling inadequate near-term mitigation action.

Implications

The research highlights fundamental governance and policy challenges in climate mitigation planning. The substitution problem suggests that integrating large-scale CDR into climate scenarios without adequate safeguards may inadvertently weaken political and economic commitment to rapid decarbonization. From a justice perspective, the analysis indicates that pathways involving carbon budget overshoot followed by CDR-based recovery impose disproportionate climate risks on present and near-future populations, raising questions about intergenerational and international equity. The work implies that mitigation strategies must carefully balance the role of CDR against the imperative for immediate emissions reductions, potentially requiring institutional mechanisms or policy frameworks that prevent substitution effects. The findings suggest that scenario development and climate policy implementation need more explicit consideration of how negative emissions assumptions influence decarbonization trajectories and their associated ethical dimensions.