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.