Large-Scale Carbon Dioxide Removal: The Problem ofPhasedown

Most scenarios that achieve present climate targets of limiting global heating to1.5°–2.0°C rely on large-scale carbon dioxide removal (CDR)to drive net emissions negative after mid-century. Scenarios that overshoot andreturn to a future temperature target, or that aim to restore some priorclimate, require CDR to be rapidly deployed, operated for a century or so, thengreatly reduced or phased out. This need for future phasedown presentschallenges to near-term policies that have been underexamined. A CDR enterpriseof climate-relevant scale will require financial flows of billions to trillionsof dollars per year. The enterprise and supporting policies will create risks oflock-in via mobilized actors whose interests favor continuance as well as othermechanisms. The future phasedown need implies suggestive guidance for near-termdecisions about removal methods and design of associated policy and businessenvironments. First, variation among methods’ scale constraints and coststructures suggests a rough ordering of methods by severity of future phasedownchallenges. Second, of the three potential means to motivateremovals—profitable products incorporating removed carbon, extendedemissions-pricing policies, or public procurement contracts—publicprocurement appears to present the fewest roadblocks to future phasedown.