May 2017 was the second warmest May in 137 years of modern record-keeping, according to a monthly analysis of global temperatures by NASA scientists.
The two top May temperature anomalies have occurred during the past two years as May 2016 was the hottest on record.
May 2017's temperature was 0.05 degrees Celsius cooler than May 2016. It was just 0.01 degrees Celsius warmer than the third warmest May, which occurred in 2014, NASA said in a statement.
The monthly analysis by the scientists at NASA's Goddard Institute for Space Studies (GISS) in New York 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 did not cover enough of the planet.
Monthly analyses are sometimes updated when additional data becomes available, and the results are subject to change, NASA said.