(Image Credit: Brendan Smialowski/AFP/Getty Images)

New York City Mayor Michael Bloomberg's endorsement of President Obama Thursday put the spotlight on an issue that has been largely absent from this presidential campaign and mainstream political discourse for nearly two years: climate change.

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For Obama, the turnaround - from advocating for comprehensive cap-and-trade legislation in Congress to rarely mentioning a global solution to climate change as a candidate for re-election - has been striking.

Obama campaigned for the presidency in 2008 with a clear pledge to make the United States a global leader in addressing climate change. Then, he linked the issue to high gas prices at the pumps and the country's security. He rated the threat of climate change as on par with other global scourges such as disease and poverty.

"No single issue sits at the crossroads of as many currents as energy," Obama said in a speech at the Clinton Global Initiative in September 2008. "This is a security threat, an economic albatross, and a moral challenge of our time.

"The time to debate whether climate change is man-made has passed. It's time, finally, for America to lead," Obama said.

Four years later, a starkly different Obama came to the Clinton Global Initiative stage, again as a candidate for the nation's highest office.

In his speech, the phrases "climate change," "global warming," and even global energy concerns were nowhere to be found.

The New Yorker's Ryan Lizza, who notably penned a piece chronicling the death of the Obama-backed, cap-and-trade legislation in 2010, reported this year that Obama believes "the most important policy he could address in his second term is climate change."

But judging from his campaigning in recent months, it might be hard to tell.

On the stump, candidate Obama 2012 does mention "climate change," but almost exclusively as a justification for more domestic investments in green-energy technology, not as a call for leadership on the world stage.

Obama made a rare reference to global warming in Manchester, N.H., Oct. 18, touting his second-term intention to shift $4 billion worth of oil-company subsidies to green-energy firms.

"And, by the way, those investments not only create good jobs here at home, create new industries here at home, but it also reduces the carbon pollution that's heating our planet," Obama said. "Climate change is not a hoax. Droughts and floods and fires, they're not a joke. They are a threat to our children's future, and we've got to deal with it in a serious way that also grows our economy."

Obama as president has made strides on at least some of his 2008-era promises to address global warming. His administration has raised fuel-efficiency standards for vehicles for the first time in 30 years, and the stimulus legislation he passed in his first year in office was an unprecedented windfall for clean-energy companies, injecting $90 billion into the industry.

But the strides are relatively small, especially by comparison to the failed, 2010 effort by congressional Democrats to pass cap-and-trade legislation, which would have restricted the level of greenhouse gases that can be emitted nationally.

That effort, combined with the heavy lifting needed to pass comprehensive health-care overhaul, effectively drained Obama of what political capital he had accumulated coming into office.

The results for the political discourse have been dramatic.

Climate change is absent from the 2012 presidential election, in stark contrast to 2008 when Obama and Republican nominee Sen. John McCain could agree that cap-and-trade legislation was a necessary step to address global warming.

And neither Obama nor Mitt Romney was asked about the issue in any of the presidential debates, and it has not featured prominently in any of the plans for their presidencies.

In his endorsement of Obama, Bloomberg gave the president credit for taking steps to reduce the country's carbon consumption, even as he gave Romney credit for supporting, then abandoning cap-and-trade legislation. But he pointed to Superstorm Sandy as the push he needed to bring "the stakes of Tuesday's presidential election into sharp relief."

And former Vice President Al Gore, whose advocacy on behalf of global climate-change awareness made him doubly famous, also blamed climate change for the deadly storm. New York Gov. Andrew Cuomo agreed.

But absent from the line of public figures talking about global warming - at least so far - in the wake of the historic storm: Obama.

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