All but official: 2015 world's warmest year on record

While the region braces for its first unofficial snow panic of the season, the government is about to announce that 2015 was the warmest year on record worldwide - probably by a comfortable margin.

Officials at the National Oceanic and Atmospheric Administration and NASA's Goddard Institute for Space Studies say they will release their annual reports Wednesday.

While the final piece - official December climate data - has not been made public, the first 11 months of the year were so historically balmy that the declaration of 2015 as the world's warmest year, surpassing 2014, is all but a certainty, weather watchers agree.

The average annual temperature last year was at least 1.5 degrees higher than those during the 20th century. And scientists at both agencies are expected to indict man-made greenhouse gases for the trend.

The announcement will come as a potential mega-snowstorm takes aim on the Northeast Corridor. Meteorologists on Tuesday warned of the potential for one to two feet of snow to pile up Friday and Saturday. New Jersey beaches are likely to take a pounding from potent onshore winds, with major flooding possible.

But the weekend forecasts are still evolving. In fact, on Tuesday the respected European computer model appeared to cut back on its initial projections of double-digit snowfall accumulations for the Philadelphia region.

While the juxtaposition between the snow threat and the global-temperature report might appear ironic, climate experts have long emphasized that climate trends transcend local, short-term events - the hurricanes, heat waves, cold spells, and blizzards that often get so much attention.

It is impossible, they say, to determine how a subtle increase in world temperatures could affect an individual storm. But the warming, they say, is very real.

NOAA and NASA maintain separate databases using slightly different - and quite complicated - methods. But their readings track closely.

The official database maintained by NOAA's National Climatic Data Center in Asheville, N.C., dates to 1880.

Taking the world's temperature isn't as simple as sticking a thermometer under the planetary armpit.

Daily high-low average temperatures at about 2,500 stations worldwide are taken on a month-by-month basis, according to government climate specialist Deke Arndt. The temperature is expressed relative to 20th century averages, rather than an absolute reading. The planet's thermometers do not constitute a homogeneous set. They are located at different elevations and above different terrains.

So, rather than attempting to average temperatures in different environments, Arndt has said, it is tidier to measure how readings at a given site deviate from average readings at the same site.

For 2014, the center's globally averaged temperature was 1.32 degrees Fahrenheit above the 20th century average, the warmest on record. NASA compares its annual temperature with that of the 1951-80 "base period." The 2014 temperature was about 1.2 degrees above the base period's.

Arndt noted that while world temperature databases, including those in Japan and the United Kingdom, are calculated by different methods, they generally agree.

"That's really reassuring, as a scientist," he said. "We're all competitive."

The warming has coincided with atmospheric increases in greenhouse gases, and the linkage "makes common sense," Bob Smerbeck, senior meteorologist at AccuWeather Inc., said Tuesday.

"There's always a debate on how much it's adding to the situation." But, he added, "we're still warming."

In all likelihood, the 2015 NOAA number will exceed 1.5 degrees Fahrenheit, which would beat the margin of error.

The December 2015 report isn't available yet, but from Jan. 1 through Nov. 30, the combined land-and-sea temperature was 1.57 degrees above the 20th century average.

And evidently December was quite warm globally, with abnormally high surface temperatures over a vast expanse of the Pacific, the result of the ongoing El Niño event.

For the contiguous United States, it was the warmest December on record. Just ask the outerwear retailers.

Winter, of course has since returned, and the people who run the Wegmans supermarkets are among those who have noticed.

Proving that some clichés are true, Wegmans is beefing up supplies of bread, milk, and eggs at its stores in the Philadelphia region and elsewhere in the Northeast, said spokeswoman Jo Natale.

"We certainly adjusted orders for the days ahead," she said. "Operating in the Northeast gives you that experience."

twood@phillynews.com 610-313-8210 @woodt15