Scientists used tree rings to compare last year’s extreme heat with temperatures over the past two millenniums.

Unpaywalled version of the paper

Extra greenhouse gases in the atmosphere from the burning of fossil fuels are responsible for most of the recent increases in Earth’s temperature, but other factors — including El Niño, an undersea volcanic eruption and a reduction in sulfur dioxide aerosol pollution from container ships — may have contributed to the extremity of the heat last year.