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.