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.
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.
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.
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:
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.)