According to NASA's monthly analysis of global temperatures last month was the warmest September in 136 years. Sea levels are also believed to have risen more in the 20th century than previously thought.
The temperature last month was just 0.004 degrees Celsius (32 degrees Fahrenheit) warmer than the previous record-breaking September in 2014, but still enough to make it the hottest September on record.
Compared to the mean September temperature from 1951 to 1980, last month was 0.91 degrees warmer. Eleven of the past 12 months have also set new high-temperature records, Nasa added.
Responding to the news, Gavin Schmidt, director of NASA's Goddard Institute for Space Studies, predicted that 2016 would be the hottest year since records began:
Ocean water levels
News of this year's rising temperatures came as a new NASA and university study also found that tide gauges - the longest and highest-quality records of historical ocean water levels - may have previously underestimated the extent of global average sea-level rise that occurred during the 20th century.
A research team which included experts from various institutes evaluated how various processes that cause sea level to change differently in different places may have affected past measurements.
The study found that it's highly unlikely that the global average sea level rose less than 14 centimeters (5.5 inches) during the 20th century. The increase was more likely to be closer to 17 centimeters, the report said.
"These results suggest that our longest records are most likely to underestimate past global mean change and allow us to establish the minimum amount of global sea level rise that could have occurred during the last century," Project team leader and associate director of the University of Hawaii Sea Level Center in the School of Ocean and Earth Science and Technology Philip Thompson said.