A map shows how land and ocean temperatures around the globe this past September compared to the average for that month. (NOAA)

Last month again set a new mark for global heat. And U.S. meteorologists say Earth is now on pace to tie the hottest year ever recorded, or more likely, to break it.

The National Oceanic and Atmospheric Administration announced Monday that last month the globe averaged 15.7 degrees Celsius. That was the hottest September in 135 years of record keeping.

This is one of many indicators that climate change not stopped. - Donald Wuebbles, University of Illinois

It was the fourth monthly record set this year, along with May, June and August.

NASA, which measures temperatures slightly differently, had already determined that September was record-warm.

The first nine months of 2014 have a global average temperature of 14.7 degrees Celsius, tying with 1998 for the warmest first nine months on record, according to NOAA's National Climatic Data Center.

"It's pretty likely" that 2014 will break the record for hottest year, said NOAA climate scientist Jessica Blunden.

The reason involves El Nino, a warming of the tropical Pacific Ocean that affects weather worldwide. In 1998, the year started off super-hot because of an El Nino. But then that El Nino disappeared and temperatures moderated slightly toward the end of the year.

This year has no El Nino yet, but forecasts for the rest of the year show a strong chance that one will show up, and that weather will be warmer than normal, Blunden said.

If 2014 breaks the record for hottest year, that also should sound familiar: 1995, 1997, 1998, 2005 and 2010 all broke NOAA records for the hottest years since records started being kept in 1880.

"This is one of many indicators that climate change not stopped and that it continues to be one of the most important issues facing humanity," said University of Illinois climate scientist Donald Wuebbles.

Some non-scientists who are skeptical of man-made climate change have been claiming that the world has not warmed in 18 years, but "no one's told the globe that," Blunden said. She said NOAA records show no pause in warming.

No monthly record for cold since 1916

The record-breaking heat goes back to the end of last year — November 2013 broke a record. So the 12 months from October 2013 to September 2014 are the hottest 12-month period on record, Blunden said. Earth hasn't set a monthly record for cold since December 1916, but all monthly heat records have been set after 1997.

September's monthly record follows new records for May, June and August. (iStock)

September also marks the third month in a row that Earth's oceans broke monthly heat records, but these are all-time records for how much hotter than normal they were, Blunden said.

The U.S. as a whole was warmer than normal for September, but the month was only the 25th warmest on record.

While parts of the U.S. Midwest, Russia and central Africa were slightly cool in September, it was especially hotter than normal in the U.S. West, Australia, Europe, northwestern Africa, central South America and parts of Asia. California and Nevada set records for the hottest September.

If Earth sets a record for heat in 2014 it probably won't last, said Jeff Masters, meteorology director for the private firm Weather Underground. If there is an El Nino, Masters said, "next year could well bring Earth's hottest year on record, accompanied by unprecedented regional heat waves and droughts."