Large-Scale Carbon Dioxide Removal: The Problem of Phasedown

Most scenarios that achieve present climate targets of limiting global heating to 1.5°–2.0°C rely on large-scale carbon dioxide removal (CDR) to drive net emissions negative after mid-century. Scenarios that overshoot and return to a future temperature target, or that aim to restore some prior climate, require CDR to be rapidly deployed, operated for a century or so, then greatly reduced or phased out. This need for future phasedown presents challenges to near-term policies that have been underexamined. A CDR enterprise of climate-relevant scale will require financial flows of billions to trillions of dollars per year. The enterprise and supporting policies will create risks of lock-in via mobilized actors whose interests favor continuance as well as other mechanisms. The future phasedown need implies suggestive guidance for near-term decisions about removal methods and design of associated policy and business environments. First, variation among methods’ scale constraints and cost structures suggests a rough ordering of methods by severity of future phasedown challenges. Second, of the three potential means to motivate removals—profitable products incorporating removed carbon, extended emissions-pricing policies, or public procurement contracts—public procurement appears to present the fewest roadblocks to future phasedown.