The United States – among the top three emitters of carbon dioxide and other greenhouse gases, along with China and India – cannot hope to confront climate change unless our political leaders stop tiptoeing around the issue.

Few scientists doubt that the Earth's climate is changing and growing warmer. Only a small number of skeptics dispute that humans are a prime cause of the problem.

The ranks of the skeptics just got smaller, however, with UC Berkeley physics professor Richard Muller joining the overwhelming scientific consensus that global warming is real, and that human-caused pollution is a major culprit.

Describing his "total turnaround," Muller wrote in a Sunday column for the New York Times: "Three years ago I identified problems in previous climate studies that, in my mind, threw doubt on the very existence of global warming. Last year, following an intensive research effort involving a dozen scientists, I concluded that global warming was real and that the prior estimates of the rate of warming were correct. I'm now going a step further: Humans are almost entirely the cause."

This summer offers a sense of the consequences of inaction. We've seen massive drought, Colorado on fire, and Atlanta recording its hottest day in history. While it's impossible to tie specific events to climate change, these are the kinds of extremes we will increasingly see unless emissions are brought under control.

On the campaign trail, there is plenty of vague talk about "energy independence" or "clean energy," with both presidential candidates ducking what policies they will pursue to reduce greenhouse gases.

Presumptive Republican nominee Mitt Romney has been a major flip-flopper, depending on the audience, providing little confidence to date in where he really stands.

In 2003, as governor of Massachusetts, Romney said climate change "is beginning to affect our natural resources and that now is the time to take action." In 2005 he supported the Regional Greenhouse Gas Initiative, a cap-and-trade system among Massachusetts and eight other states, calling it a "great thing" for Massachusetts' jobs and economy. Then he got cold feet and withdrew his support.

Fast forward to the current campaign, and Romney has been all over the map.

In June last year, he said, "I think it's important for us to reduce our emissions of pollutants and greenhouse gases that may well be significant contributors to the climate change and the global warming that you're seeing." A few months later, he said the opposite: "My view is that we don't know what's causing climate change on this planet. And the idea of spending trillions and trillions of dollars to try to reduce CO2 emissions is not the right course."

Obama made climate change a priority in running for president in 2008, saying future generations would look back and say, "This was the moment when the rise of the oceans began to slow and our planet began to heal." But he has backed off in the face of Republican opposition in Congress. On the campaign trail this year, he talks about "energy." Addressing climate change is a promise Obama has failed to deliver upon.

Both candidates must be pressed on what policies they would pursue, both nationally and globally. If Muller can change his tune on climate change, Obama and Romney can at least discuss the issue and offer solutions. The stakes involved couldn't be higher.

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