All excess emissions must be removed

 Large historic emitters do not have any remaining carbon budget.  Countries with outsized historic emissions have long since exceeded their fair share of the global 1.5°C carbon budget. All of their ongoing emissions add to an ever-accumulating carbon debt.

 Emissions in excess of carbon budgets need to be removed.  The need for carbon removal includes past, present and future emissions, not only the “residual emissions” remaining at the year of net zero. Policymakers should begin accounting for this growing carbon debt, ultimately adjusting climate ambition to ensure these excess emissions are addressed in time to avoid unsafe temperature overshoot.

 Emission reductions still take precedence in most cases, but some emissions are “CDR-optimal”.  As with other abatement options, neutralising excess emissions with carbon dioxide removal (CDR) may or may not be appropriate or currently cost-effective. For most sources of emissions, the immediate priority remains elimination rather than removal. However, for some limited emissions, carbon removal will be the most appropriate and sustainable mitigation option – what we define as “CDR-optimal”. This terminology reflects the fact that all removals will count against some emissions and that all emissions can be either eliminated, sustained, or removed. We must therefore optimise which removals are applied against which emissions.

 The terms “excess” and “CDR-optimal” emissions provide more precision than “residual emissions,” which is used inconsistently.  “Residual emissions” can refer to either all emissions remaining when net zero is achieved, emissions remaining at a specific target year, or emissions from specific hard-to-abate sectors at any point in time. If the term “residual emissions” is used, it’s essential to specify whether it refers to a particular time period, a category of emissions, or those emissions best addressed by carbon removal – what we call “CDR-optimal”.

 Carbon removal is at limited risk of creating stranded assets. There is no shortage of emissions to address.  Investing in high-durability carbon removal capacity is essential, regardless of future emission trajectories, due to the growing carbon debt that needs to be paid off to avert or reverse overshoot of 1.5°C. Countries and corporations need to act with much greater urgency and start building the foundation for CDR today while planning for future removals to meet their obligations.