President Obama’s legacy will be defined by a number of issues: the stimulus package; killing Osama bin Laden; his endless battles with Congress; his support for gay marriage.

But arguably the biggest issue he’ll be judged on — one that is still rarely mentioned in Washington policy circles — is climate change.

It is true that this President has done more than any other leader in American history to promote clean energy. But that will only be one piece of his legacy. The rest of his climate record will be marked by his decision to allow drilling in the rapidly melting Arctic ocean, his choice to approve or deny the Keystone XL pipeline, and his ability to once again lead on pricing carbon. These are all still very much up in the air.

Van Jones, the former White House “Green Jobs Czar” and a progressive moment builder, is warning the President that he needs to find the “courage” to stand up and deal with these issues in a climate context. In an interview with science writer Chris Mooney for Mother Jones Magazine, Jones says he believes that climate will be “the issue he’s judged on”:

Mother Jones: What would real climate leadership look like? You gave President Obama a “B” or “B-” on the environment in his first term, what would he have to do to earn an “A” in the second one?

Van Jones: An “A” would be a major energy and climate bill as a centerpiece of his legacy. He obviously has to deal with the economy and the budget issues that the Tea Party keeps trying to politicize. And there’s a question of immigration reform, which is critical as a major part of the progressive coalition. But, ten years from now, twenty years from now, the only thing people are going to be asking of this president is either, why he didn’t find the courage to do something on climate change, or they’re going to be asking how he found the courage. I think from the viewpoint of history, this is going to be the issue that he’s judged on. We’ve seen a lot of conversation about this fiscal cliff, which is an invented, manufactured crisis, but very little talk about the climate cliff, which is a real, unavoidable crisis.

So if we can have the president of the United States on TV every day talking about the manufactured fiscal cliff, then he can use all of those resources to put pressure on Congress to do something about the real climate cliff.

After his re-election last month, President Obama hinted that he might once again talk about climate change in his second term: “We want our children to live in an America that isn’t…threatened by the destructive power of a warming planet.”

However, in his first post-election press conference, the President backed away from acting on climate and attempted to separate the environment from the economy: “If the message is somehow we’re going to ignore jobs and growth simply to address climate change, I don’t think anybody is going to go for that. I won’t go for that.”

The White House likes to point to the stimulus package, EPA regulations, and other executive orders as evidence of the President’s work on climate — often without really talking about climate. While these actions do set us on a pathway toward emissions reductions, they are still not nearly close to what scientists say we need, and not quite bold enough to convince countries like China and India that the U.S. is leading internationally on climate.

In the last few weeks, we’ve seen a range of new research telling us that we are on the brink of catastrophic climate change — right now. In the next twenty years, everything we know will be influenced and transformed by climate. And when people look back, they won’t be nitpicking about which policies did more to incrementally reduce emissions.

Assuming that Obama stays mum on climate, they’ll be asking why the President of the United States didn’t use the opportunity to rally the country when the science demanded it.