As the United States heads into the final three months of the year, 2012 is likely to end up as the warmest year on record for the lower 48 states, according to an analysis from the federal government's National Climatic Data Center.
That would mark the fourth time in the last 14 years that annual average temperatures for the continental US either set records or flirted with them – a cluster unmatched since the 1930s.
Since January, year-to-date temperatures for the continental US have consistently run well above the 20th-century average with each passing month – reaching a maximum of 6 degrees Fahrenheit above average for the period ending March 31, then declining steadily to 4 degrees F. above the 20th-century average for the period ending Aug. 31.
Still, that 4 degrees is at least a full degree higher than January-to-August averages in any of the five warmest years on record.
"That's quite a jump," says Jake Crouch, a climate researcher at the center, located in Asheville, N.C.
The year's unusual warmth for the US results from a one-two punch, many researchers say. Normal swings in large-scale climate features, such as El Niño and La Niña, are superimposed over a longer-term worldwide warming trend triggered by the buildup of atmospheric greenhouse gases that come from burning fossil fuels, as well as from land-use changes, researchers say.
Even if year-to-date temperatures were to fall back to the 20th-century average by the end of the year, 2012 would still be a record buster, Mr. Crouch says.
To pull off that feat of cooling, he explains, September through December would have to rank among the 20 coldest final four months on record – highly unlikely given the historical record as well as seasonal forecasts for the final four months of 2012.
Most of the continental US – from the Southwest up into the Rockies and eastward into Maine – is forecast to see warmer-than-normal temperatures throughout this final period, notes Huug van den Dool, with the National Oceanic and Atmospheric Administration's Climate Prediction Center in College Park, Md.
One key reason is the apparent shift in conditions from two consecutive years of La Niña to a weak El Niño this coming winter. La Niña and El Niño represent opposite states in sea-surface temperatures and wind patterns along the equatorial Pacific. Both can affect atmospheric circulation patterns well beyond the tropics.
For the US, La Niña tends to push average storm tracks across the continent farther north than usual, while El Niño tends to drive them farther south. The onset of El Niño is expected to bring welcome rainfall to the Southeast and mid-Atlantic states. But the more southerly path for storms could lead to a dearth of precipitation farther north in sections of the country already hit hard by drought.
As a result, forecasters are calling for drought to continue through the end of the year for much of the western two-thirds of the US, with drought conditions likely to extend into the Pacific Northwest.
Globally, June through August posted the third warmest temperatures on record for that three-month period. January through August was the ninth warmest first eight months on record.
Of keen interest has been the dramatic decline in summer sea ice in the Arctic Ocean, which this melt season hit a record low of 1.3 million square miles – 18 percent below the previous record set in 2007 and 51 percent below levels in 1979, when consistent satellite measurements of sea ice began.
Less ice means more open ocean to absorb sunlight during the summer, energy released as heat in the fall and winter. The decline not only affects local Arctic weather patterns in the fall – found in forecasts of warmer-than-normal temperatures along northern Alaska – but researchers are beginning to uncover ways in which the heat being released deeper into an Arctic fall and winter can affect atmospheric circulation patterns at lower latitudes.
Indeed, the decline in summer ice has occurred far faster than models of global warming's effect on the top of the world predicted. For seasonal-forecasting purposes, this is prompting forecasters to rely even more heavily on their computer-based forecasts than they might otherwise, suggests Dr. van den Dool.
"The reliance on models will probably increase, because what is happening right now in the Arctic appears to be so fast that we hardly have any time to take data points and declare that we now know what is going on in the present climate," he says.
Intuition and sound judgment will remain key features of forecasts, he adds, "but it's a challenging situation."