Equity is more important for the social cost of methane than climate uncertainty

The social cost of methane (SC-CH₄) measures the economic loss of welfare caused by emitting one tonne of methane into the atmosphere. This valuation may in turn be used in cost-benefit analyses or to inform climate policies^1-3. However, current SC-CH₄ estimates have not included key scientific findings and observational constraints. Here we estimate the SC-CH₄ by incorporating the recent upward revision of 25 per cent to calculations of the radiative forcing of methane⁴, combined with calibrated reduced-form global climate models and an ensemble of integrated assessment models (IAMs). Our multi-model mean estimate for the SC-CH₄ is US$933 per tonne of CH₄ (5-95 per cent range, US$471-1,570 per tonne of CH₄) under a high-emissions scenario (Representative Concentration Pathway (RCP) 8.5), a 22 per cent decrease compared to estimates based on the climate uncertainty framework used by the US federal government⁵. Our ninety-fifth percentile estimate is 51 per cent lower than the corresponding figure from the US framework. Under a low-emissions scenario (RCP 2.6), our multi-model mean decreases to US$710 per tonne of CH₄. Tightened equilibrium climate sensitivity estimates paired with the effect of previously neglected relationships between uncertain parameters of the climate model lower these estimates. We also show that our SC-CH₄ estimates are sensitive to model combinations; for example, within one IAM, different methane cycle sub-models can induce variations of approximately 20 per cent in the estimated SC-CH₄. But switching IAMs can more than double the estimated SC-CH₄. Extending our results to account for societal concerns about equity produces SC-CH₄ estimates that differ by more than an order of magnitude between low- and high-income regions. Our central equity-weighted estimate for the USA increases to US$8,290 per tonne of CH₄ whereas our estimate for sub-Saharan Africa decreases to US$134 per tonne of CH₄.