Last month was the hottest September in NASA's modern records, meaning 11 of the past 12 months have been record-setting.
Scientists say it's almost certain Earth will crack the annual record this year.
The September result continues a distinct warming pattern in NASA's Goddard Institute for Space Studies (GISS) data, which dates back to 1800.
GISS director Gavin Schmidt says with this latest data, an annual record seems "locked in".
June was the only month in the past year which didn't break a record, with temperature readings putting it at the third warmest behind 2015 and 1998.
Dr Schmidt says until the late data from Antarctica came in, June was also ranked as the hottest on record.
It would've meant an entire year of record-hot months, in NASA's data.
"We continue to stress that while monthly rankings are newsworthy, they are not nearly as important as long-term trends," Dr Schmidt says.
The Paris Agreement formally ratified earlier this month aims to keep global warming to under 1.5degC. Dr Schmidt suggests this year could be 1.25degC above pre-industrial times.
The US National Oceanic and Atmospheric Administration (NOAA) tracks its climate data separately from NASA and is expected to release its September information within the next week.
NOAA found August was the 16th consecutive month to set record-high temperatures.