Tucson, Arizona: Operation Deep Freeze in effect Thursday and Friday nights

The Salvation Army has launched its Operation Deep Freeze cold weather shelter program for the homeless for the first time this winter.
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"With so many on the street, especially women and kids, tonight is going to be a cold, difficult night for many," McElwee said.

Last year's program averaged almost 200 people a night who sought shelter, McElwee said.

Glut of allegedly hot years a coincidence? Allegedly fat chance

The recent glut of unusually hot years is incredibly unlikely to happen in a stable climate.

Eduardo Zorita of Germany's Institute for Coastal Research and colleagues calculated the probability of this happening in a range of scenarios.

A key consideration is that the weather one year is not independent of the weather the year before. If it were, the odds of having any given temperature would be the same each year, and the likelihood of getting a such a 17-year cluster would be tiny – on the order of 1 in 10 trillion.

"An anomalous warm year tends to be followed by a warm year," says Zorita, because of the way oceans store heat and release it slowly. "A devil's advocate could argue that the clustering of warmest years at the end of the record could be simply due to chance, since the climate system has a natural memory."

However, even when Zorita included this natural feedback in his model, but excluded global warming, the odds of observing the cluster of record-breaking years was still about 1 in 10,000.

"We cannot ascribe the anomaly to any particular physical factor, like anthropogenic greenhouse gases," says Zorita. "But our conclusions are consistent with those of the fourth IPCC report," which states there is a very high probability that human emissions are causing global warming.