Global temperatures, both land and sea, were 1.6 degrees above the 20th century average of 59 degrees, according to the report. That is 0.07 degrees below NOAA's September 2015 record, which occurred during a strengthening El Nino.
But the year-to-date temperature is still 1.78 degrees above the 20th century average of 57.5 degrees. This was the highest for January-September since records began in 1880, surpassing 2015's record by 0.23 degrees.
NOAA's report appears to differ from one released by NASA on Monday, though only slightly. NASA showed September's temperatures breaking a 2014 record by a "razor-thin" 0.004 degrees.
The reason for the discrepancy between the two agencies likely results from their different ways of calculating temperatures in regions around the world that do not have weather stations that can measure temperatures directly.
"The key thing to remember is that it really doesn't matter whether the month is record warmest, tied as warmest, or second warmest, etc. — it's the overall trend of increasing global temperatures that matter," said Jessica Blunden, a climate scientist for NCEI. "Both NOAA and NASA wholeheartedly agree that this year is record warm so far and will end up as warmest or very close to warmest in the modern record."