You might have missed it if you live in many spots in the lower 48 states, but the first six months of 2010 were the warmest on record globally, according to preliminary data from the National Climatic Data Center in Asheville, N.C.
During the January-to-June period, combined land and ocean surface temperatures around the globe ran 1.22 degrees Fahrenheit above the 20th-century average.
Temperatures also ran above normal along the US East Coast. But in the the upper Midwest, the Pacific Northwest, the Rocky Mountain states, and the Southeast, temperatures ran from 2 to 3 degrees F below the long-term average.
"We haven't seen the warming in the 48 states, which is kind of nice," says David Pierce, a climate scientist at the Scripps Institution of Oceanography in La Jolla, Calif. "But the rest of the world has really seen warm conditions."
One of the drivers behind the warm first half was El Nino – a condition in which a deep pool of warm water in the tropical Pacific migrates from its hot spot in the western part of the ocean to the waters off of wester South America. The thunderheads that tower above that pool and help drive atmospheric circulation patterns move east as well, altering the circulation patterns as they travel.
But El Nino's reach also extended more indirectly to much of Alaska, Canada, and southern Greenland. Indeed, these areas experienced the most significant warming of any on the planet during the first half of the year, according to the NCDC's data.
As Washington takes a self-imposed political break from battles over energy and climate legislation, some researchers see a cautionary tale in these first-half temperatures.
To be sure, the fingerprint of global warming appears in long-term trends, not in single storms or a single season's worth of data, they agree. But human-triggered warming – through rising carbon-dioxide emissions from cars, factories, and power plants and through land-use changes – is a likely contributor to the six-month figures.
"Human-caused climate change contributed to the warm six months" the world experienced, Dr. Pierce says. "But it's also true that because of natural variability, it's not possible to say exactly how much of that warming was human, how much was El Nino, and how much was from other natural causes."
One clue pointing to a contribution from global warming comes from temperatures in the US and Canadian Arctic, he continues. Based on past records, the magnitude of the above-normal warming there was much larger than even the strongest El Nino has been observed to trigger.
The unusual warming is likely tied to the additional effect of a dramatic reduction of sea ice during the spring and summer melt season. With larger portions of the Arctic Ocean uncovered – and uncovered more quickly because much of the ice that built in winter is thin, vulnerable first-year ice – more ocean is exposed to soak up sunlight, which warms the water.
So far during this year's melt season, the extent of sea ice has been flirting with the declines seen in 2007, which saw the smallest extend of summer ice since 1979, when satellite records began building of the ice's status.
These days, when the melt season ends each September, the extent of summer ice is some 40 percent smaller than 30 years ago, says Kevin Trenberth, a climate researcher at the National Center for Atmospheric Research in Boulder, Colo.
That long-term ice loss as a clearer link to global warming than does six months of temperatures, he and others say, although even the ice loss also is affected by natural variations in wind patterns from one year to the next.
If the record for the first half of the year holds up after revisions, it still may not be enough to keep the year as a whole on track for record above-average warmth, according to Dr. Trenberth. El Nino is giving way to La Nina, it's opposite, he notes. La Nina's effects – direct an indirect – could put the global climate on a path to a cool second half.