Should we stop the COPs?

More than a decade after COP21, carbon emission trajectories remain far above the 1.5° C threshold, due to lack of international consensus. Departing from cost-benefit approaches, we assess the maximum reduction in carbon emissions that could be accepted by all countries. We characterize the target-consistent mechanism that minimizes global emissions subject to the participation constraint of each country. The mechanism can be implemented either via a uniform carbon tax or as a cap-and-trade system. Calibrated to data from 69 countries, including GDP, carbon intensities, and observed tax rates, our model suggests — for our baseline scenario — that the maximum uniform carbon price politically acceptable for all countries is $250 per ton. It could reduce global emissions by 35%, but would require unprecedented international transfers: up to 3% of world GDP, with a large redistribution from high-income, low-emission countries to carbon-intensive emerging economies. Our analysis highlights the structural ambition gap imposed by voluntary cooperation and identifies two levers to overcome it: convergence in green technologies and stronger political support for mitigation. Without progress in these dimensions, international climate policy remains constrained to deliver only modest results.