Speaking to a crowd in Philadelphia yesterday, former Massachusetts Governor Mitt Romney (R) suddenly changed his position on whether humans contribute to climate change, insisting that "we don’t know what’s causing climate change on this planet."

He added that "the idea of spending trillions and trillions of dollars to try to reduce CO2 emissions is not the right course for us."

Romney's comments might confuse those who listened in on him in June, when he told an audience in New Hampshire, "I believe that humans have contributed" to climate change.

Then he added: "It’s important for us to reduce our emissions of pollutants and greenhouse gases that may be significant contributors."

Despite what candidate Romney says about climate change, the scientific community is virtually unanimous on the issue: it is happening, and human activity is the main driver. Even the arch-conservative industrialist brothers Charles and David Koch know this, and a recent study they funded illustrated that in very clear terms.

In spite of all the efforts to drive down the public's belief that climate change is real, a recent survey by the Public Religion Research Institute found that 64 percent of respondents believed climate change is happening and humans contribute to it, while just 26 percent said they did not believe it was happening at all.

And strikingly, only 40 percent of the public said that scientists agree on the reality of human-driven climate change, when a scientific survey in June 2010 found that 97 percent of climate scientists say it is "very likely" that human activity is causing a shift in global temperatures and weather.

The United States is the second worst offender when it comes to greenhouse emissions, with China being the first. The U.S., however, has a much higher emissions-per-capita rate: a painful fact that led a key Chinese official to declare this week that if Chinese emissions were to grow as high per person as the U.S., it would be a "disaster for the world."

Reacting to that potential for disaster, China has rapidly accelerated the growth of their renewable energy sector and consolidated control over the rare earth elements that U.S. companies will need access to if they hope to compete in the coming decades.

Not only does this mean there is a compelling need for the U.S. to take actions against climate change, it also means the U.S. must adopt a national energy strategy that amplifies green technology production, or face being priced out of the future market -- not to mention growing pollution intensifying the severity of climate change and ever-climbing rates of inclement health effects on the public.

But try telling that to Mitt Romney.

This video is from a Romney campaign stop in Philadelphia on Thursday, Oct. 27, 2011, as snipped by Think Progress.