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This kind of confident assertion rarely makes its way into the scientific literature. Yet it appeared in three studies included in a special annual edition of the Bulletin of the American Meteorological Society (BAMS) dedicated to attributing the causes of extreme weather events. If these results hold up, the implications would be profound and unsettling: humanity has already pushed the global climate into a new regime. To be clear, natural variability will always have a major role, but the blame for some of the most extreme weather phenomena — as well as some of the resulting impacts — would rest squarely on our own shoulders.
Released on 13 December, the research in question focused on 2016, the hottest year on record. One modelling study, led by scientists at the US National Oceanic and Atmospheric Administration, compared the temperature record to a simulated baseline climate without human greenhouse-gas emissions (T. R. Knutson et al. Bull. Am. Meteorol. Soc. 99, S11–S15; 2018). In baseline simulations of some 24,000 years of weather from seven climate models, nothing like the record warmth of 2016 ever occurred. Greenhouse-gas emissions, chiefly those from fossil-fuel use, are a prerequisite for this kind of heat. What’s more, the paper indicates that greenhouse gases began to push the climate outside the realm of natural variability around 1980.