Two of America's top science agencies confirmed last week that 2017 was one of the hottest years on record. But the news came with some anomalies — the kind that skeptics might use to muddy the evidence that the planet is growing dangerously warmer.
The reports from NASA and the National Oceanic and Atmospheric Administration (NOAA) confirmed that global temperatures remain markedly above the Earth's pre-industrial levels. But they also showed that the upward heat trajectory is not constant, or uniform, around the globe. And the studies revealed that serious cold spates, and big snowfalls, have not disappeared.
One counter-intuitive finding from the two agencies was that Earth recorded its second- or third-warmest year in recorded history, in the same year that storms left the Northern Hemisphere with more snow than at any time in the last three decades.
Another twist: The warming forces couldn't entirely overwhelm that age-old weather trickster — the Pacific Ocean's El Niño/La Niña temperature oscillation.
Scientists say weather quirks should not distract from the more worrisome, long-term trend: Temperatures that are inexorably increasing as mankind's burning of fossil fuels loads the atmosphere with carbon dioxide and other greenhouse gases.
"Every time we have a cold snap, some people say 'Are we done with that? Are we done with the whole warming thing?'" said Gavin Schmidt, chief of NASA's Goddard Institute for Space Studies. "The answer is 'No, we are not.'"
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The NASA institute and NOAA announced on Thursday that 2017 was the second or third hottest year on record, depending on the method of calculating average temperatures. Both agencies reported that the five warmest years ever recorded have taken place since 2010, with 2017 temperatures 1.62 degrees Fahrenheit warmer than the average between 1951 and 1980.
With those high temperatures, wouldn't it stand to reason that the Earth would be covered with less snow? Not necessarily.
Analyzing data from NOAA, the Rutgers University Snow Lab found that the Northern Hemisphere was covered with a total of nearly 10 million square miles of snow during 2017. That's roughly 360,000 square miles more than the average snow cover from 1981 to 2010. It's also the most snow to cover land north of the equator since 1985.
"That really leaves people scratching their heads," said David Robinson, a Rutgers climatologist. "On average in recent years, we are seeing less snow, particularly in the spring. And the snow tends to melt earlier. But last year was an exceptional year."
The key was that the high atmospheric winds known as the jetstream moved to the south and weather troughs — lower pressure areas that carry moisture-packing storms — fell over land (instead of the oceans) more than they had in years past.
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"Even though we had warmer temperatures over the planet, the cold air and snow aligned themselves," said Robinson. "There was more precipitation and cold air over the land masses … so we ended up with more snow cover."
While scientists are convinced of the impacts of climate change, they say some people may mistakenly believe that it human-driven changes obliterate other weather patterns. They don't.
That was demonstrated in 2017, they say, when the El Niño/La Niña weather phenomenon — believed to be in existence for hundreds of years, or more — tamped down the globe's overall average temperature.
El Niño is a pattern of unusual warming of the Pacific Ocean in the tropics that drives up temperatures and moisture in the surrounding air. An El Niño condition helped push the globe's temperature to a record high in 2016, aided by the greenhouse gas forces, the NOAA study said.
But in 2017, the pattern reversed and water temperatures in the mid-Pacific dropped roughly 1.8 degrees Fahrenheit, a phenomenon known as La Niña. In an analysis that removed that lower ocean temperatures as a factor, NOAA found that 2017 would have been the warmest year on record.
"So that cooler water surface takes extra heat in from the atmosphere and that lowers the global mean surface temperature," said Richard Seager, a research professor at Columbia University's Lamont-Doherty Earth Observatory. "So we got this extremely high temperature in 2017 in the absence of an extra push from an El Niño event and with the surface ocean temperatures actually going down."
Schmidt of the NASA Goddard institute said that the smaller the geographic area and the shorter the time span, "the more noise there will be" in any analysis and "the harder to detect forced climate change."
"So if people want to get distracted by noise, they will always be able to find some," he concluded. "That is independent of the growth of the [warming] signal at the continental, hemispheric or global scales."