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.

Our study reveals that with suitable high-quality temperature measurements, a “discernible human influence on global climate” could have been detected by the end of the 19th century. It is unclear whether such early knowledge of the climate-altering consequences of fossil fuel burning would have prompted human societies to follow a more environmentally sustainable greenhouse gas emissions pathway.

Today, however, we know with high confidence that sustainable pathways must be followed to avoid dangerous anthropogenic interference with climate.

Humanity is now at the threshold of dangerous anthropogenic interference. Our near-term choices will determine whether or not we cross that threshold.