This story was updated most recently at 10:05 p.m. ET on June 17.

Two of the leading centers that track global surface temperatures have reported their data for May, and they both found it to be the warmest such month on record for the planet. NASA found that May had an average global temperature that was 1.38 degrees Fahrenheit above average, which would make it the warmest such month, coming out far ahead of May 2012. The Japanese Meteorological Agency's separate analysis also found both May and the meteorological spring months of March through May to be the warmest on record.

These results are preliminary, with the data from both agencies subject to revision. NASA posted a note on its surface-temperature data website Tuesday that said "missing data" from China has not yet arrived, and that their data is not "directly comparable" to previous records.

Later this week, the National Oceanic and Atmospheric Administration (NOAA) will release their global numbers, which typically closely match the other centers, but sometimes differ slightly in rankings. According to the World Meteorological Agency, all but one of the 10 warmest years on record have occurred in the 21st century.

Global average spring temperatures have been increasing in recent years. Image: JMA

The data indicates that a developing El Niño event in the equatorial tropical Pacific Ocean may already be boosting global average surface temperatures slightly. Such events tend to add to manmade global warming, resulting in record warm years, such as those of 1998 and 2005, depending on whose data you trust more.

Monthly average temperature departure from average during May 2014. Image: NASA GISS

May featured several noteworthy heat waves. Searing heat gripped a large part of Asia during the middle to end of the month, with all-time monthly record-high temperatures hitting Japan, China and Mongolia. According to Weather Underground, Beijing "shattered" its May monthly record with a reading of 106 degrees Fahrenheit on May 30, beating the previous record by six degrees, and came close to its all-time high temperature record for any month.

Studies show that heat waves are already becoming more intense and long-lasting globally, as average temperatures warm in response to increasing amounts of greenhouse gases in the air.

Warmer than average temperatures also predominated during May across Australia, Europe and the western U.S.