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 emissionspricing policies, or public procurement contracts—public procurement appears to present the fewest roadblocks to future phasedown.