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What's all but certain to be the warmest year on record is pouring on additional steam on its way out the door.
Dreams of a white Christmas are melting away in the reality of a wet — and very warm — Christmas for most of the eastern United States. Forecasters project holiday temperatures in the 50s Fahrenheit (10-16 degrees Celsius) as far north as Maine, with shirtsleeve weather in New York and Philadelphia and rain from Boston to Austin.
And in an era of climate change, it's fair to ask whether this is the new normal. Or as Ebenezer Scrooge, the haunted miser of Dickens' "A Christmas Carol," once put it: "Are these the shadows of the things that will be, or are they shadows of things that may be, only?"
It's a bit of both, according to Pennsylvania State University climatologist Michael Mann. What's warming the East is a combination of the fickleness of ordinary weather, the cyclical Pacific warming phenomenon known as El Niño and the planetary warming already resulting from human-produced carbon emissions.
"The first two factors come and go, but the third is there for posterity, and will only be exacerbated by additional carbon emissions," Mann said.
"We've had very strong El Niños in the past. We've had unusual weather fluctuations in the past," he said. "But the fact that we're talking about not just extreme but unprecedented conditions — mid- to upper 70s over a large swath of the eastern US on Christmas Eve, something we haven't seen before — speaks to the exacerbating effects of human-caused climate change."
Forecasters say Christmas Eve highs in New York could top 70 degrees — about 30 degrees above normal. Meanwhile, in the Southeast, the warm, wet weather presents a risk of severe thunderstorms and tornadoes. Tornado warnings were posted Wednesday morning just east of Mobile, Alabama, where a tornado ripped through the heart of that Gulf Coast city on Christmas Day in 2012.
It's a bit different in the West, where the jet of warm, wet air streaking across the continent will bring snow to the Sierra Nevadas and the Rockies — a holiday blessing to drought-stricken California and its neighbors, which depend on the mountain snowpack for water the rest of the year.
Much of the Sierras were snow-free at the beginning of spring, leading the state to impose sharp cuts in water usage on homeowners. But two of California's mountain snowpack is more than 10 percent above its average for this time of year, state water monitors reported this week. But much of the additional precipitation is falling as rain, resulting in flooding in coastal Oregon and Washington.
The year is already headed for the record books, with month after month of global average temperatures setting new high marks, according to the National Oceanic and Atmospheric Administration. The current El Niño will extend that hot streak into 2016, said Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research in Boulder, Colorado.
"It's a very large and persistent anomaly, and it tends to lock these patterns into place," Trenberth said. Long-range forecasts show a warmer-than-average January for most of the US northern tier, as well as Alaska, while the deep South is likely to see a cooler, wetter winter as the season goes on.
While there's still "a tremendous amount of variety" in day-to-day weather, this year's El Niño is building on a foundation of a warmer sky and ocean that's causing headaches worldwide, as warmer air carries more moisture. The combination fueled one of the most active Pacific hurricane seasons on record and sent shock waves across the tropics.
"The Bay of Bengal got quite warm, and the delayed southwest monsoon came in there and caused extensive flooding in Chennai, in southeast India. The magnitude of that flooding was certainly unprecedented," he said. But other countries nearby, like Indonesia saw a deep drought that worsened wildfires, shrouding the country and some of its neighbors in choking haze.
"In some places where it's hot and dry, global warming makes it a bit hotter and drier and things dry out a little quicker, and the risk of wildfires go up," Trenberth said.
The effects are spilling over across the Atlantic as well. The Mediterranean region has been drier than normal, and western Europe is seeing a warmer winter: Forecasters at Britain's national weather agency, the Met Office, report that December started off as the mildest in 45 years in southern England and Wales.
And as uncharitable as 2015 has been, 2016 is likely to be worse, Mann said. The cumulative effect of the carbon dioxide and other heat-trapping gases released since the dawn of the Industrial Revolution are likely to add another half a degree Celsius (0.9F) to warming that's already occurred, "So what we are seeing is the veritable tip of the iceberg," he said.
"With even a modest warming of the globe comes dramatic increases in the likelihood of extreme individual warm spells and heat waves, like the one we are seeing," he said. "Add global warming to the mix and you get a veritable 'perfect storm' of conditions favoring heat spells like the one we're seeing right now."
Follow Matt Smith on Twitter: @mattsmithatl
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