WASHINGTON -- It's a Friday afternoon and Michael Mann is at a downtown coffee shop. The climate scientist became famous in 2001 for his "hockey stick" graph that showed 900 years of relatively stable temperatures veering sharply higher in the 20th century. But now, standing in line at a crowded Starbucks at 17th Street and Pennsylvania Avenue, the Penn State professor is decidedly out of the spotlight.

Mann is in town to promote his new book, "The Hockey Stick And The Climate Wars: Dispatches From The Front Lines." While he had a strong turnout at the local bookstore, Politics and Prose, the topic that once made Mann a household name is now conspicuously absent from the national conversation.

President Barack Obama had just embarked on a cross-country energy tour, touting his "all-of-the-above energy strategy" in visits to Colorado, Nevada, New Mexico and Oklahoma. But he never once mentioned the single greatest consequence of America's reliance on oil and gas: global warming.

He wasn't the only one curiously silent on the issue.

Newspaper headlines announced the spike in gas prices, and Republicans on the campaign trail praised the merits of drilling. Rachel Maddow invited famous climate denier Sen. James Inhofe (R-Okla.) onto her show, where he ran through his usual talking points. "I expect that on Fox News," Mann said after his visit to Starbucks, where he ran into HuffPost editorial director Howard Fineman and agreed to an interview.

Mann had no pressing appointments, as Washington whirred with talk of energy policy. So he walked with Fineman back to the HuffPost Washington bureau, where he spent the afternoon drinking beer and explaining how the country's pre-eminent climatologist came to be standing invisibly at a coffee shop next to the White House at a time when his issue is more important than ever.

Mann's temperature graph was cited in the 2001 United Nations' Intergovernmental Panel on Climate Change seminal report, which concluded that "the balance of evidence suggests a discernible human influence on the global climate." His research would later make him the target of the infamous "climategate" email scandal of 2009, in which more than 1,000 emails were leaked from the Climatic Research Unit at the University of East Anglia in the UK and used by climate deniers to falsely allege that scientists had manipulated data. "Mann-made global warming," they called it at the time.

Excerpts from HuffPost's conversation with Mann:

The Huffington Post: Obama has been talking about America’s energy future on the campaign trail a lot this past week, yet he never mentions climate change. What do you make of that?

Michael Mann: I thought there was some irony to Obama going to Oklahoma, the state that maybe has been most devastated thus far by the emerging effects of climate change, to present a vision of our energy future that really did seem to ignore climate change. I was disappointed by that frankly.

HuffPost: Disappointed, but were you surprised?

Mann: In Obama’s second State of the Union address, he actually seemed to concede the scientific evidence as a weakness. He argued that we need to pursue a more enlightened energy strategy in spite of the doubts about the science of climate change. … We’ve actually made negative progress from where we were 10 years earlier, when Clinton gave his final State of the Union address. We've gone from the science being the primary reason to move forward to the argument that we should move forward in spite of supposed weaknesses in the science of climate change. So we've retreated to a position of weakness on this issue with Obama relative to Bill Clinton.

HuffPost: How about his appointees? How have they done?

Mann: It’s been a mixed bag really when it comes to Obama and climate change because he appointed all the right people to the various agencies. You know, no complaints about Steven Chu, Jane Lubchenco, John Holdren. … It’s like he had this all-star basketball team, but he wouldn’t let them go out on the floor and play. With the exception maybe of Chu, who’s been out there talking about these issues, they all seem to have been on a very short leash in terms of speaking up publicly about the energy challenge and the need to deal with climate change.

I know that they all have very enlightened view on this because I know some of them and I know what they’ve written and where they’ve stood on this issue for some time. So he appointed the right people, but then he didn’t really let them move forward on this issue. He says the right things sometimes, but then appears to retreat from them. The bottom line is that at the very least he appears conflicted between where his own heart is -– my guess is that his own heart is with moving forward and dealing with the problem of climate change -- and his instincts as a politician.

HuffPost: Do you feel like the book tour is allowing you to get your message out?

Mann: I had some quality interviews with some media outlets. I think it's difficult in this 24/7 constant news cycle to gain attention for an issue like this. And in a book tour, what you're of course hoping for is to sustain the attention and the interest in the topic. You're just competing with so much, especially during an election year, but at the same time this is an issue that keeps coming up and you have people like my former senator, Rick Santorum, who on the campaign trail has espoused the view that climate change is an elaborate hoax just like James Inhofe claims. And so in a sense it is a timely issue I suppose because it keeps on coming up in the political campaign and because people are asking about this ridiculously warm winter and this hot spring and they're starting to wonder, is this climate change? And it actually provides us an opportunity to talk about it.

Too many of my colleagues are so scared of drawing any relationship between the two because they know they’ll be attacked by climate change deniers, so they're unwilling to even talk about the possibility that climate change is influencing the weather. From a scientific point of view, if you take a sober look at what we're seeing, it is very much consistent with the impact of climate change on our weather the models have predicted -- that we will see a substantial increase in the number of warm days in the U.S. and that’s what we’re seeing.