Carbon accounting for negative emissions technologies

Negative emissions technologies (NETs) are an essential part of most scenarios for achieving the Paris Agreement goal of limiting warming to below 2°C and for all scenarios that limit warming to 1.5 °C. The deployment of these technologies requires carbon accounting methods for a range of different purposes, such as determining the effectiveness of specific technologies or incentivising NETs. Although the need for carbon accounting methods is discussed within the literature on NETs, there does not appear to be a clear understanding of the range of different accounting challenges. Based on a systematic literature review this study identifies five distinct accounting issues related to NETs: 1. estimating total system-wide change in emissions/removals; 2. non-permanence; 3. non-equivalence of ‘no overshoot’ and ‘overshoot and removal’; 4. accounting for incentives for NETs; and 5. the temporal distribution of emissions/removals. Solutions to these accounting challenges are proposed, or alternatively, areas for further research and the development of solutions are highlighted. One key recommendation is that carbon accounting methods should follow a ‘reality principle’ to report emissions and removals when and where they actually occur, and an important overall conclusion is that it is essential to use the correct accounting method for its appropriate purpose. For example, consequential methods that take account of total system-wide changes in emissions/removals should be used if the purpose is to inform decisions on the deployment or incentivisation of NETs. Attributional methods, however, should be used if the purpose is to construct static descriptions of possible net zero worlds. Key policy insightsNegative emissions technologies (NETs) raise a number of distinct carbon accounting challenges, the importance of which varies across different NETs.Attributional life cycle assessment is not an appropriate method for estimating the system-wide changes caused by the deployment of NETs.Consequential greenhouse gas accounting methods should be used to estimate system-wide changes, and should be used as much as possible for guiding incentives for NETs.Greenhouse gas accounting methods should follow a ‘reality principle’ to report emissions and removals when and where they actually occur. Negative emissions technologies (NETs) raise a number of distinct carbon accounting challenges, the importance of which varies across different NETs. Attributional life cycle assessment is not an appropriate method for estimating the system-wide changes caused by the deployment of NETs. Consequential greenhouse gas accounting methods should be used to estimate system-wide changes, and should be used as much as possible for guiding incentives for NETs. Greenhouse gas accounting methods should follow a ‘reality principle’ to report emissions and removals when and where they actually occur.负排放技术（Negative Emissions Technologies, NETs）是绝大多数旨在实现《巴黎协定》将全球变暖限制在2°C以内目标的情景，以及所有将全球变暖限制在1.5°C以内情景的核心组成部分。此类技术的部署需要适配多种不同用途的碳核算方法，例如评估特定技术的减排效果，或是对负排放技术进行激励。尽管NETs相关研究文献中已讨论过碳核算方法的必要性，但目前学界对其所面临的各类核算挑战尚未形成清晰认知。本研究通过系统文献综述，识别出与NETs相关的五类明确核算问题：1. 估算全系统范围内的排放/移除总量变化；2. 非持久性；3. “未超量排放”与“超量排放后移除”的非等价性；4. 负排放技术激励措施的核算；5. 排放/移除的时间分布特征。研究针对这些核算挑战提出了解决方案，同时也明确了需要进一步研究以开发解决方案的领域。一项核心建议是，碳核算方法应遵循“现实原则”，即在排放与移除实际发生的时间和地点进行相关报告；而一项重要的总体结论则是，针对特定用途选用恰当的核算方法至关重要。例如，若目标是为负排放技术的部署或激励决策提供参考，则应采用考量全系统排放/移除总量变化的后果法；反之，若目标是对潜在净零排放世界进行静态描述，则应使用归因法。关键政策洞察负排放技术（NETs）面临诸多差异化的碳核算挑战，其重要性因不同NETs类型而异。归因型生命周期评估（Attributional Life Cycle Assessment）并不适用于估算负排放技术部署所引发的全系统变化。后果型温室气体核算方法可用于估算全系统变化，并应尽可能用于指导负排放技术的激励工作。温室气体核算方法应遵循“现实原则”，在排放与移除实际发生的时间和地点进行相关报告。负排放技术（NETs）面临诸多差异化的碳核算挑战，其重要性因不同NETs类型而异。归因型生命周期评估（Attributional Life Cycle Assessment）并不适用于估算负排放技术部署所引发的全系统变化。后果型温室气体核算方法可用于估算全系统变化，并应尽可能用于指导负排放技术的激励工作。温室气体核算方法应遵循“现实原则”，在排放与移除实际发生的时间和地点进行相关报告。