The MSU and AMSU measurements are from three different research groups: Remote Sensing Systems (RSS), the Center for Satellite Applications and Research (STAR), and the University of Alabama at Huntsville (UAH). The grey and black horizontal lines are the 3σ and 5σ thresholds that we use for estimating the signal detection time. By 2002, all three satellite datasets yield signal-to-noise ratios exceeding the 3σ threshold. By 2016, an anthropogenic signal is consistently detected at the 5σ threshold. Further details of the model and satellite data and the fingerprint method are provided in the Supplementary Information.