According to NASA, May 2017 was the second hottest month in a span of 137 years when modern record-keeping of average global temperatures had commenced.
The temperature of May 2017 was 0.88 degrees Celsius warmer than the mean May temperature from 1951-1980.
May 2016 is the hottest on record with 0.93 degrees Celsius warmer than the May mean temperature.
May 2017 temperature was 0.05 degrees Celcius cooler than the temperature recorded in May 2016. It was only 0.01 degrees Celsius warmer than the third warmest May, which occurred in 2014.
NASA’s Goddard Institute for Space Studies (GISS) does the monthly analysis of global temperature by analysing the publicly available data acquired by about 6,300 meteorological stations around the world, ship- and buoy-based instruments measuring sea-surface temperature, as well as from the Antarctic research stations. The modern global temperature record commenced around the year 1880.