Human influence on climate detectable in the late 19th century

Significance When could scientists have first known that fossil fuel burning was significantly altering global climate? We attempt to answer this question by performing a thought experiment with model simulations of historical climate change. We assume that the capability to monitor global-scale changes in atmospheric temperature existed as early as 1860 and that the instruments available in this hypothetical world had the same accuracy as today’s satellite-borne microwave radiometers. We then apply a pattern-based “fingerprint” method to disentangle human and natural effects on climate. A human-caused stratospheric cooling signal would have been identifiable by approximately 1885, before the advent of gas-powered cars. Our results suggest that a discernible human influence on atmospheric temperature has likely existed for over 130 y.