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2012 was warmest on record for the United States

Last year was the warmest on record in the United States and brought the second most extreme weather in more than a century — causing droughts, wildfires, and storm damage — according to a report Tuesday from the National Oceanic and Atmospheric Administration.

The news continues a dramatic warming trend in the mainland United States: Seven of the nation’s 10 warmest years have taken place in the past 15 years, according to
NOAA. Last year’s average temperature of 55.3 degrees Fahrenheit beat the previous 1998 record by 1 full degree, a dramatic increase in climate data where records are often broken by far smaller amounts.

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New England, too, had its own record-breakers with every state but Maine experiencing record warm temperatures last year. Boston, Hartford, Providence, and other major New England cities also experienced record or near-record average temperatures. And the ocean was not immune: While a final analysis still needs to be conducted, waters off the Northeast in 2012 were probably the hottest, or nearly the hottest, on record.

“We will have to get used to living with this new baseline,’’ said Raymond S. Bradley, director of the Climate System Research Center at the University of Massachusetts Amherst. He said the previous record in 1998 was associated with El Nino, which tends to drive warm temperatures, but 2012 was not. “We have an underlying rising trend in temperature due to greenhouse gases,” Bradley said.

Scientists cannot tie temperatures in any one year — or one weather event — to climate change, so it is unclear how much global warming and how much natural variability contributed to the heat-packing 2012. However, since NOAA began keeping records in 1895, many of the hottest years in the United States have come recently, leading many scientists to conclude much of the warming is coming from the release of heat-trapping carbon dioxide and other greenhouse gases from power plants, cars, and factories.

Global temperatures for 2012 are not yet available, but they are expected to be among the hottest since record-keeping began.

Environmentalists and some politicians seized on the latest data to push President Obama to more aggressively tackle climate change. While he and other global leaders in 2009 pledged to contain world temperatures from rising more than 3.6 degrees Fahrenheit above pre-industrial levels, some climate specialists say not enough is being done to meet that goal, or question whether it can even be reached.

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“Our planet is warming, our oceans are rising, and our storms are strengthening. Congress can no longer afford to watch the devastation from an air-conditioned perch,’’ Representative Edward J. Markey said in a statement. “We must make 2013 a year for climate action. Waiting around for the next superstorm to flood Boston’s Faneuil Hall or the Boston Garden is not an option.”

The annual temperature data are compiled from hundreds of locations throughout the United States by using average highs and average lows for each month. Last year was also a year of extremes in the United States, with 11 weather disasters that caused more than $1 billion in damage, including Superstorm Sandy, Tropical Storm Isaac, and tornado outbreaks in the Great Plains, Texas, the Southeast, and the Ohio Valley. However there were fewer tornadoes overall than normal.

NOAA scientists evaluate extremes in temperature and precipitation as well as tropical cyclones, such as hurricanes and tropical storms, that make land to create an index of extreme weather. The most extreme year for the nation was in 1998.

While the drought was the big news throughout a large part of the nation, the Northeast was wetter than average, though the winter was drier.

NOAA scientists say that in addition to global warming, the record-breaking year was associated with a pressure pattern in the North Atlantic Ocean that kept the jet stream — bands of strong winds in the upper levels of the atmosphere — to the north, preventing cold air from getting into the lower 48 states. That northern jet stream also limited precipitation, contributing to the summer drought. Droughts tend to warm daily temperatures, and with 60 percent of the country experiencing drought over the summer, daytime average temperatures were higher than average over a good part of the United States.

Scientists focused on the enormous 1 degree difference from 1998, the year with the previous record-breaking average annual temperature, to 2012. All previous 117 years of data fit within 4 degrees, said Jake Crouch, climate scientist with NOAA’s National Climatic Data Center, so to have 2012 be a full degree higher is significant.

It was “a huge exclamation point,’’ said Deke Arndt, chief of the climate monitoring branch at Climatic Data Center. “This is consistent with what we would expect in a warming world.’’

Beth Daley can be reached at bdaley@globe.com.

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