AMY GOODMAN: Scientists are warning the planet has now reached a grim climate milestone not seen for two or three million years. The National Oceanic and Atmospheric Administration says the amount of heat-trapping carbon dioxide in the atmosphere has topped 400 parts per million. The 400 ppm threshold has been an important marker in U.N. climate change negotiations, widely recognized as a dangerous level that could drastically worsen human-caused global warming.

The environmentalist group 350.org takes its name after the 350 parts per million threshold that scientists say is the maximum atmospheric concentration of carbon dioxide for a safe planet. In a statement on the parts per million number hitting 400, 350.org co-founder Bill McKibben said, quote, “The only question now is whether the relentless rise in carbon can be matched by a relentless rise in the activism necessary to stop it.”

To find out more about the impacts of crossing the threshold, we’re joined now by leading climate scientist Michael Mann, distinguished professor of meteorology at Penn State University, author of the recent book, The Hockey Stick and the Climate Wars: Dispatches from the Front Lines.

We welcome you to Democracy Now! Thank you for joining us from the public television station in State College, WPSU. Thanks so much, Michael Mann. Talk about the significance of this threshold being passed.

MICHAEL MANN: Thanks. It’s great to be with you.

So, this number, 400 parts per million, what does it mean? It’s the number of molecules of CO2 for every million molecules of air; 400 of them are now CO2. Just two centuries ago, that number was only 280 parts per million. So if we continue to add carbon to the atmosphere at current rates, we’ll reach a doubling of the pre-industrial levels of CO2 within the next few decades.

Now, 400, what does that round number, 400, mean? Well, what it means is that, as you alluded to, we have to go several million years back in time to find a point in earth’s history where CO2 was as high as it is now. And, of course, we’re just blowing through this 400 ppm limit. If we continue to burn fossil fuels at accelerating rates, if we continue with business as usual, we will cross the 450 parts per million limit in a matter of maybe a couple decades. We believe that with that amount of CO2 in the atmosphere, we commit to what can truly be described as dangerous and irreversible changes in our climate.

AMY GOODMAN: Like what?

MICHAEL MANN: So, what we are already witnessing, in fact, the effects of climate change. If we look at the past year here in the U.S., last summer, the record heat, the record drought, the record wildfire that destroyed large forest areas in Colorado, New Mexico. We saw, you know, tremendous damage to our crops in the breadbasket of the country. We saw Arctic sea ice diminish to the lowest level we’ve ever seen, and it’s on a trajectory where there will be no ice in the Arctic at the end of the summer in perhaps a matter of 10 years or so. We also saw the devastation of Superstorm Sandy. Now, we can’t say that Hurricane Sandy was caused by climate change, but many of its characteristics are precisely the kinds of characteristics that we predict tropical storms and hurricanes will have if we continue to warm the planet. We will see more destructive tropical storms. We’ll see more flooding. We’ll see more drought. And that’s just the tip of the iceberg, because, remember, we’ve only just crossed 400 now. We will reach 450 ppm in a matter of a couple decades if we continue with business as usual.

AMY GOODMAN: How can this be stopped, Professor Mann?

MICHAEL MANN: Well, ultimately, you know, we have to find a way to move away from our addiction to fossil fuels. As former President George W. Bush once put it, we’re addicted to fossil fuels. We need to find a way to transition away from those means of obtaining energy, fossil fuel energy, that we know is degrading the climate and degrading the planet. We have to find a way to level the playing field so that the marketplace will allow renewable energy sources to compete with fossil fuel energy. We’re currently providing subsidies to fossil fuels, the very sources of energy that are degrading the climate, and not providing the appropriate incentives to developing alternative energy.

AMY GOODMAN: Professor Mann, I want to turn to the Keystone XL pipeline, the controversial pipeline which would deliver tar sands oil from Canada to refineries in Texas. Earlier this year, a State Department report concluded the Keystone XL pipeline does not threaten the global climate. A number of environmental groups opposed the conclusion in a report called “Cooking the Books.” They said, quote, “In a world constrained by the realities of climate change, the proper measure of any project’s climate impact should not be based on the assumptions inherent in a business as usual scenario that guarantees climate disaster. … There is a climate impact from burning 830,000 barrels per day of any crude that cannot be ignored,” they wrote. Michael Mann, what do you think of the climatic effects of the Keystone XL pipeline, if it is approved by President Obama and the State Department?

MICHAEL MANN: Sure. And my understanding was that, in fact, that report was criticized by folks within the EPA, and so there was some question about the accuracy of that report.

But that aside, you know, the larger picture here, the Keystone XL pipeline, developing these tar sand oils in Canada, there have been some calculations that argue that, you know, even if we develop the full petroleum reserves from the tar sands, that we wouldn’t add to our global carbon emissions nearly as much as if we continue, for example, to burn coal. There’s a lot more coal available to burn than there is these tar sand oils.

On the other hand, I think it represents what’s wrong about our current prioritization. If we are to invest heavily in the infrastructure so we’re subsidizing efforts to get at this increasingly difficult reservoir of fossil fuel energy, if we are to incentivize that effort through certain government subsidies, then we’re going in exactly the opposite direction of where we need to be going. We’re actually simply developing more and more of the available fossil fuel reserves at a time when we have to be ramping them down. We need to be bringing our global carbon emissions to a peak within the next few years, and we need to ramp them down dramatically in the decades ahead, if we are going to avoid crossing that next sobering milestone, 450 parts per million in the atmosphere.

And so, it’s really not so much precisely how much carbon we’ll add to the atmosphere by building the Keystone XL pipeline as it is it’s an example of how we’re going the wrong direction. We need to follow what the rest of the world is doing. If you look to India and China, the developing world, they’re investing far more in renewable energy than we are here in the U.S. And this is in fact a matter of global competitiveness. We in the U.S. are falling behind because we’re letting the rest of the world move ahead and recognize that the future of our global economy is going to be in renewable energy.

AMY GOODMAN: You know, last week we reported that an environmental activist met Vice President Joe Biden in South Carolina. Her name was Elaine Cooper. And she wrote for the Sierra Club, saying, “I asked him” — she’s talking about Vice President Joe Biden— “about the administration’s commitment to making progress climate and whether the president would reject the pipeline.” She said, “He looked at the Sierra Club hat on my head” — Biden did — “and [he] said, 'Yes, I do—I share your views—but I am in the minority.'” What does that suggest to you, Michael Mann? What message do you have for President Obama?

MICHAEL MANN: Yeah, I saw that. I read that interview, and it was a bit disturbing. You know, the president has talked a good game in the past few months since he won re-election. In his State of the Union address, I think he outlined very clearly the threat of continuing to worsen the climate change problem through our burning of fossil fuels. He acknowledged the threat that it represents to us, whether you’re talking about human health, food resources, water resources, national security. Across the board, if we continue to burn fossil fuels and elevate greenhouse gas concentrations, the cost to society is going to be far greater than any cost of action. And the president has talked a good game in recent months. On the other hand, there are some—to those who read the tea leaves, there appear to be some signals that suggest that he may approve the Keystone XL pipeline. And if he were to do that, in my view, that would be a big mistake. Like I said before, it would be moving us in exactly the opposite direction of where we need to be moving.

AMY GOODMAN: Very quickly, Professor Mann, you have been targeted by climate skeptics for—climate change deniers for years. What is your message to them? And can you talk about the title of your book?

MICHAEL MANN: Sure thing. So, the title of the book, The Hockey Stick and the Climate Wars: Dispatches from the Front Lines. And as a climate scientist who published an iconic graph called “the hockey stick,” which shows how unusual recent warming is, I found myself in the crosshairs of the efforts to discredit the science of climate change, many of those efforts funded by vested interests who don’t want to see us move away from our addiction to fossil fuels. And so I recount my experiences in the book.

But I also talked about the larger issues involved. You know, for example, you alluded to the word “skeptic.” Well, many of those who simply deny that climate change exists, we don’t call them skeptics, because that’s not skepticism. That’s just denial or contrarianism. Now, skepticism is a good thing in science, but it means looking at all sides of an issue. And so, when you talk about the uncertainties, there are uncertainties in, for example, the precise projections of how much sea-level rise we will see in the next century. But it turns out those uncertainties are not a reason for inaction, for the same reason that we purchase fire insurance—

AMY GOODMAN: We have 10 seconds.

MICHAEL MANN: —not because we think our houses are going to burn down, but because we need to hedge against that potentially catastrophic, low-probability outcome. Mitigating climate change is in fact a planetary insurance policy. And so, there’s room for discussion of uncertainty, and there’s room for a good-faith, worthy debate about what to do about this problem. There are valid points of view.

AMY GOODMAN: Michael Mann, we’re going to have to leave it there.

MICHAEL MANN: There are valid points of view across the spectrum. Thank you.

AMY GOODMAN: I thank you so much for being with us, leading climate scientist, distinguished professor of meteorology, speaking to us from Penn State University. His book, The Hockey Stick and the Climate Wars.