RENEE MONTAGNE, HOST:
Just this last winter, Boston broke its all-time record for snowfall - piles and piles of snow. This last week, people in Boston were playing tennis outdoors. Meteorologists say this year will very likely be the planet's warmest year since modern record-keeping began about a century ago. As NPR's Christopher Joyce reports, experts say the reason is both natural and human caused.
CHRISTOPHER JOYCE, BYLINE: Jake Crouch lives in North Carolina. Winters are pretty mild there, but this year is a little ridiculous.
JAKE CROUCH: The temperature here in western North Carolina, it's been about 20 degrees above average in the daytime.
JOYCE: Twenty degrees above average. At night - 30 degrees above average.
CROUCH: So definitely a lot more people out and about than we would typically see this time of year.
JOYCE: Crouch knows why because he's a meteorologist with the National Oceanic and Atmospheric Administration.
CROUCH: It's a result of both long-term warming and the El Nino that we've seen develop during the year.
JOYCE: First, the warming.
BRENDA EKWURZEL: We fully expect that 2015 will be the hottest ever on record.
JOYCE: Brenda Ekwurzel is a climate scientist at the environmental group Union of Concerned Scientists. She's not alone on this. NOAA scientists say the same thing. Ekwurzel notes that the oceans are absorbing a lot of that extra heat.
EKWURZEL: There were points there in the summertime where I was looking at the ocean temperatures and they were the hottest ever over a century of record-keeping.
JOYCE: And that leads us to the second reason for our weird weather - El Nino. El Nino is a natural warming cycle in the water of the western Pacific Ocean that happens every few years. That extra-warm water sloshes around the Pacific and influences weather over huge parts of the world. In many places, parts of the U.S. for example, that means warm and wet. Ekwurzel says this year's El Nino is a humdinger in part because the whole planet is, on average, getting warmer. She says global warming is supercharging this year's El Nino.
EKWURZEL: The El Nino conditions, which is a natural phenomenon, is starting at a higher sea surface temperature than it would be if it were completely natural conditions. Therefore we can have a higher risk of a more powerful El Nino.
JOYCE: And this one is already among the top three strongest El Ninos in the past 65 years. Global warming affects more than El Nino, though. The Arctic, for example, is much warmer than it used to be. That can alter the behavior of the polar jet stream that flows from west to east across North America, pushing it farther south. The result is the wonderfully named but frigidly unpleasant Arctic vortex. Ekwurzel says scientists are struggling to predict where else the effects of a warmer planet will show up in local weather and what they will look like.
EKWURZEL: We have more wacky weather with climate change than what our historical experience has taught us.
JOYCE: Which makes predicting the weather even harder than it used to be. Christopher Joyce, NPR News. Transcript provided by NPR, Copyright NPR.