Carbon Dioxide Removal and warming reversal in the light of uncertain Earth System processes

Current and projected emissions trends suggest a likely reliance on carbon dioxide removal (CDR) to achieve the long-term temperature goal of the Paris Agreement. Studies using Integrated Assessment Models have considered scenarios limiting global warming to 1.5°C in 2100 that include cumulative net removal of up to ~1000 GtCO2 over the 21st century, although also scenarios deploying substantially less CDR have been considered. Large CDR deployment beyond achieving net zero emissions is often assumed to achieve a Global Mean Temperature (GMT) decline after peak warming of several tenths of a degree by the end of the century. The feasibility of achieving CDR on such a large scale has been strongly contested for environmental, technological, geophysical, economic, institutional or socio-cultural reasons. Beyond these considerations, it is important to assess the possible realisation of a GMT decline in the context of uncertainties in the Earth System response. Here we attempt to review and compare these various sources of uncertainty. Some relevant Earth System feedbacks are not represented in climate models or only in a limited manner, such as carbon release from permafrost melting, wetlands and wildfires, which could release ~100 GtCO2 over the 21st century even in stringent mitigation scenarios, but there is low confidence around this number. Another source of uncertainty is the Zero Emissions Commitment, which quantifies how much global warming would occur after global CO2 emissions reach net zero. Its best estimate over 50 years is close to zero, but uncertainties were quantified to be as high as +/-0.3°C for cumulative emissions of 1000 PgC and uncertainties remain on the potential for warming on multi century timescales. Given this, additional post net-zero warming countering CDR induced cooling cannot be ruled out. Furthermore, it is not clear whether the TCRE assessed for increasing CO2 emissions would remain the same once we reach net-negative emissions. In light of these various sources of uncertainty we argue that a broader perspective on CDR and future GMT evolution beyond net zero is advisable. Acknowledging the uncertain efficacy of CDR-induced GMT decrease is important for comprehensive risk assessments and reflections on scenario design.

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