2015 on track to be Earth's warmest year since at least the 600s

Fueled by a combination of man-made global warming and the natural El Niño climate cycle, 2015 remains on pace to be the warmest year since records began in 1880, federal scientists announced Wednesday, smashing a record set just last year.

But going back even farther, before instrumental records, 2015 will almost certainly be the Earth's warmest year since at least the 600s, likely further — that's before the Crusades, the fall of the Mayan Civilization and the Viking invasions of Europe.

Current temperatures are unprecedented for at least 1,400 years, according to the most recent United Nations' Intergovernmental Panel on Climate Change, which met last year. Climate scientists can tell that by analyzing ice cores, lake sediments and tree rings to determine the air and sea temperatures of past centuries and millennia.

"This will be the warmest year, of the warmest decade, of the warmest multi-decade-long period in at least the past millennium," Penn State University meteorologist Michael Mann said in an e-mail.

Last month was the hottest September ever recorded, NOAA reported, agreeing with a separate analysis from the Japan Meteorological Agency. (NASA data indicated September was second-warmest).

September's temperature was also the greatest rise above average for any month in the 136-year historical record, according to NOAA. The agency also reported that through September, seven months, including the past five, have been record warm this year.

While the natural El Niño cycle, marked by warmer-than-average water in the central Pacific Ocean, is the main driver of the warmth, it's being augmented by non-stop emissions of greenhouse gases.

The emissions come from the burning of fossil fuels such as oil, coal and natural gas, which release carbon dioxide and other greenhouse gases into the atmosphere and oceans, warming them to levels that cannot be explained by natural variability.

"Little doubt about it, the planet is warming at an unprecedented rate," said University of Arizona climate scientist Jonathan Overpeck. Temperatures in upcoming decades are forecast to blow past anything seen in the past 11,000 years, if greenhouse gas emissions are not curtailed.