New York's Tropical December Days: Climate Change or El Nino?
A strong El Nino is under way across the equatorial Pacific Ocean, and that upsets things in a way “that is much less favorable for outbreaks of cold Canadian or Arctic air,” said Kevin Trenberth, senior scientist at the National Center for Atmospheric Research in Boulder, Colorado. In addition, a pattern called the Arctic Oscillation is favoring warmth in the northern U.S.
That said, climate change also plays a role in setting the larger stage, Trenberth said.
There is evidence that the amount of carbon dioxide in the atmosphere compared with what existed in preindustrial times has elevated the chances of a mild December in Central Park, said [Bob Henson of Weather Underground]. And that demonstrates why carbon and emissions play such a big role in discussions like the one that just ended in Paris.
“So, in a nutshell, this month’s eastern warmth strikes me as the kind of dramatic event that one might expect in a strong El Nino, with record-warm temperatures at least a small bit higher as a result of the overall warming of our climate,” Henson said...
Sorting out all the moving parts affecting December’s New York warmth would require an in-depth look, Henson said.
“You would no doubt find the odds of an extreme-warm month like this one have gone up,” Henson said. “So to that extent, you could say that this month is partially a result of climate change -- but there’s no good way to estimate quantitatively how much a role the human factor played until you do the attribution study.”