Time series of Earth’s average temperature during September. (NOAA)

For an unprecedented 16 straight months, the Earth’s average temperature established new record highs. The streak came to an end this September, NOAA reports, as the planet’s monthly temperature was a hair cooler than 2015’s record-breaking mark.

September 2016 ranked second warmest in records going back to 1880, 0.07 degrees cooler than September 2015 according to NOAA. It may mark the start of a period in which global temperatures hover somewhat below record-setting levels.

Even so, 2016 year-to-date ranks as the warmest on record by a large margin and the long-term rise in temperature is set to continue despite present or future short-term hiccups.

A year ago, the global temperature was racing upward due to a strengthening El Niño event, which would rank among the most intense on record. During El Niño events, excess heat in the tropical Pacific Ocean flows into the atmosphere, elevating the planet’s temperature.


The exceptionally warm waters of the tropical Pacific pushed El Niño close to the strongest on record in 2015 and pushed global temperatures upward. Blue shades are cooler than normal, and yellow and red shades are warmer than normal. (earth.nullschool.net)

Since the spring, the El Niño event has faded away. Last week, NOAA issued a La Niña watch as ocean temperatures in the central and eastern tropical Pacific cooled during September. There’s a 70 percent chance La Niña conditions will develop this fall, NOAA says, and they may linger through winter.

Because the huge reservoir of Pacific heat is depleted during La Niña events, the global temperature tends to step back some.


Sea surface temperature difference from average in the Pacific Ocean in October 2016. Blue shades are cooler than normal, and yellow and red shades are warmer than normal. (earth.nullschool.net)

Even so, because the first two-thirds of 2016 were so unusually warm, 2016 is “locked in” to be the warmest on record, according to NASA climate scientist Gavin Schmidt. Assuming this holds true, it would mark the third consecutive year the planet has set a new record high.


Global temperature difference from average during 2016. (Climate Central)

But it is possible or even likely 2017 may not continue the streak of record warm years if La Niña develops and persists, because the warmth in upcoming months would likely fall a notch below levels reached in 2015 and 2016.

Yet the cessation of record warm months would not mean global warming has stopped.

El Niño events tend to speed global warming up while La Niña events slow it down. But if you analyze temperature trends separately for El Niño and La Niña in recent decades, you see a rise in temperatures over time for both of these phenomena:


Temperature difference from normal over time. El Niño years in red, La Niña years in blue. (World Meteorological Organization)

Just as the El Niño-induced peaks in temperature are rising, so are the La Niña-induced valleys. In other words, even as El Niño and La Niña push and pull on the long-term temperature trend, global warming continues to force it upward, unambiguously.

(Note: Although NOAA found the past month ranked as the second warmest September on record, NASA found September 2016 was the warmest in its analysis, as did satellite analyses of the month’s temperature.)