State dependence and an emergent constraint on warming after zero emissions

Abstract Global warming is expected to stabilise once carbon emissions are brought down to – and remain at – zero. Reaching net zero is thus an obligatory minimum to stay below a specific temperature threshold. But not all net zero futures are created equal. Here, we present an emergent constraint on the Zero Emissions Commitment (ZEC): the additional global temperature change in a world that reaches and sustains zero carbon emissions. We build a linear regression model on a handful of predictors derived from real-world observables that can be measured prior to net zero, and validate its performance on the behaviour of Earth System Models. Applying this constraint to present day observations, we find a best estimate of carbon-only ZEC₅₀ of 6% cooling [19% cooling to 7% warming] for a world that instantaneously achieves and sustains zero carbon emissions. This estimate shifts if climate action wavers. A warmer planet portends a world at higher risk of extra warming under zero emissions. Conversely, the more we restrain global warming on the way to zero carbon, the more likely we are to see temperatures fall without intervention. This win-win, lose-lose relationship fortifies the benefits of staying well within the limits of the Paris Agreement.