From NASA (Leslie McCarthy):
September 2017 was the fourth warmest September in 137 years of modern record-keeping, according to a monthly analysis of global temperatures by scientists at NASA’s Goddard Institute for Space Studies (GISS) in New York.
Last month was +0.80 degrees Celsius warmer than the mean September temperature from 1951-1980. The warmest months of September according to the analysis happened in 2016 and 2014 (+0.87 degrees Celsius) and 2015 (+0.82 degrees Celsius).
The monthly analysis by the GISS team is assembled from publicly available data acquired by about 6,300 meteorological stations around the world, ship- and buoy-based instruments measuring sea surface temperature, and Antarctic research stations.
The modern global temperature record begins around 1880 because previous observations didn’t cover enough of the planet. Monthly analyses are sometimes updated when additional data becomes available, and the results are subject to change.