MACROM: An Optimal Control Model for Balancing Climate Change Abatement and Damage Trade-offs

The current pace of global emissions reduction is inadequate to meet the Paris Agreement temperature target of 1.5°C. While carbon dioxide removal (CDR) is increasingly viewed as necessary to meet these targets, questions remain about the optimal scale and timing of deployment when both costs and climate damages are considered. Here we present MACROM, an optimal control climate-economic model that identifies cost-optimal pathways for deploying emissions reduction and CDR to achieve the 1.5°C target by 2100. MACROM minimises the combined costs of climate action and temperature-related economic damages, while targeting a specific temperature goal and year. We examine cost-optimal pathways across five Shared Socioeconomic Pathways, finding that immediate, large-scale deployment of both emissions reduction and CDR minimises costs across all scenarios. Optimal solutions require 492-1,894 GtCO2 of emissions reduction and 394-1,374 GtCO2 of CDR by 2100, with CDR volumes well beyond current feasibility estimates. Temperature-related damages comprise 66-90% of total costs, far exceeding abatement expenditure. Sensitivity analysis reveals that discount rate and economic damage coefficients are the primary drivers of cost variability, while CDR and emissions reduction costs exert minimal influence. Our results demonstrate that even with unlimited CDR capacity, immediate emissions reduction remains essential for cost-effective climate action to achieve the Paris Agreement target this century.