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Temperatures hit record highs globally. El Nino or global warming?

The first half of 2010 was the hottest six-month period recorded globally with temperatures around the globe 2 to 3 degrees Fahrenheit above averages.

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"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."

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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.

IN PICTURES: Beating the summer heat