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December 2021 was Philly’s second warmest on record. Climate change explains the trend.

Only December 2015 was warmer than the last month of 2021, according to 147 years of weather records.

People ice skate on a warm day at the Blue Cross Riverrink Winterfest on the Delaware River Waterfront in Philadelphia, Pa. on Tuesday, December 14, 2021.  It was the second warmest December on record.
People ice skate on a warm day at the Blue Cross Riverrink Winterfest on the Delaware River Waterfront in Philadelphia, Pa. on Tuesday, December 14, 2021. It was the second warmest December on record.Read moreMONICA HERNDON / Staff Photographer

As if wiping the slate clean for 2022, temperatures plunged and snow capped lawns in the Philadelphia region and Jersey Shore on Monday, leaving December to feel like a quickly fading memory.

But the last day of 2021 registered an average daily temperature of 51 degrees in Philadelphia — 16 degrees above normal.

In fact, the entire month averaged 45.3 degrees, the second-warmest December on record for the city stretching back to 1874, according to an Inquirer analysis of records kept by the National Oceanic and Atmospheric Administration. Six of the top 10 warmest Decembers have occurred in the 21st century; 2015 marked the warmest December on record.

As The Inquirer recently noted, temperatures do bounce around, often varying wildly from year to year. Of all the seasons, winter sees the most dramatic temperature swings, and, though climate is always changing, in recent decades temperatures have been trending in one direction -- warmer.

» READ MORE: Philly winters aren't what you think

Indeed, the top coldest Decembers all fell in the 19th or 20th centuries.

In December 2021, the average temperature in Philadelphia was 6.7 degrees above normal (it was also dry with only 1.64 inches of precipitation, or 41% of normal).

Dave Dombek, a senior meteorologist with Accuweather, said the weather in the region was dominated by a winter version of a Bermuda high — a subtropical area of high pressure in the Atlantic Ocean that migrated east.

“It certainly was an unusually warm month,” Dombek said. “We had a handful of cold days, but if you look at the low temperatures, a lot of those days didn’t even go below freezing at night. It was pretty extensive. It just wasn’t one stretch. One day that stands out is the 68 degrees that occurred on the 11th.”

The warmth fits in with a long-term trend under climate change, says Climate Central, a nonprofit composed of scientists and journalists. The group found that Decembers in Philadelphia are getting milder, leading to a rise of 4.4 degrees in the average temperature since 1970.

December is also the month with the second-fastest rate of warming in Philadelphia. January is the fastest warming month, and March is the third.

To calculate that average rise, Climate Central used linear regression, a standard statistical method to model change over time.

Though records for Philly date back to the 19th century, Climate Central chose 1970 for its analysis in order to be consistent with calculations for locations across the United States, some of which did not keep reliable records before that.

“I’m always a little cautious about fitting trends over the whole record” going back to 1874, said Andrew Pershing, director of climate science at Climate Central. “While we normally think of adding more data as giving you a more accurate picture, in the case of climate change, that isn’t always true. Basically, global warming is accelerating — it was weak at the start of the record and has gotten much stronger in the last few decades.”

For example, Pershing said the entire record of 147 available years for Philly shows a pretty standard pattern of ups and downs, with an increase of .02 degrees a year. However, a clear trend emerges around 1970 with a change of .09 degrees per year through the present, resulting in a 4.4-degree change over that 51-year period.

On Thursday, NASA and NOAA are set to announce how hot 2021 overall was with all seasons included. Though the agencies are not expected to announce a record warm year, they are expected to say it was much hotter than normal.