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The planet will survive, but will we?

Paleoclimatologist Scott Stine shows a tree stump that is evidence of a past, severe drought near California's Sierra Nevada.

Gavin Schmidt, climate scientist at NASA's Goddard Institute for Space Studies.

TEXT OF STORY

KAI RYSSDAL: In about six weeks there's going to be a huge international conference on climate change in Copenhagen, Denmark. The United Nations is trying to cajole developing countries and the world's biggest economies to come to an agreement on how to slow down global warming. No easy task. And the closer the date gets, the more politicians and officials are trying to lower expectations.

That's the backdrop for our series, "The Climate Race," exploring the economic, the political, and the human dimensions of living on a warmer planet.

Today our sustainability reporters Sam Eaton and Sarah Gardner are going to look at how bad things might get if we don't start breaking our addiction to fossil fuels.

Sarah, to do that you're first going to take us way back?

SARAH GARDNER: That's right, Kai. I recently came back from a trip just east of the Sierra Nevada mountains in California.

SAM EATON: And Sarah I hear you were looking at tree stumps?

SARAH GARDNER: Right.

EATON: Should I be happy I didn't get that assignment?

SARAH GARDNER: Well, laugh all you want, Sam, but old, dead tree stumps are actually clues to climate past. Listen to this:

SCOTT STINE: These stumps tell us that California is capable of experiencing droughts more profound and more persistent than anything that we've seen during the last 150 years.

GARDNER: Now, that's paleoclimatologist Scott Stine. He looks at past climate to help figure out future climate. And scientists are really interested in this kind of work because, if they can understand climate shifts in the past, the hope is that that will help them more accurately project what may be in store for us this time around.

EATON: OK, so back to those tree stumps you were telling us about, Sarah?

GARDNER: OK, well, I met up with Stine just east of Yosemite National Park. Picture this: You've got 14,000-foot mountains to your west, you've got high desert to your east....

[Sound of river]

STINE: We're on the left bank of the West Walker River, the fourth-largest stream on the eastern side of the Sierra.

GARDNER: And what Stine showed me in the middle of this river -- and the river is fairly low these days -- were these huge, dead tree stumps, rising out of the water. They look sort of like log spires.

He's counted 130 all along the river bottom. If you were driving by, you wouldn't even think twice about them. But some are nearly 1,000 years old.

STINE: Take off a piece about this big right here, and that would be more than enough to radiocarbon date....

GARDNER: So to Stine these tree stumps are like ghosts of climate past. They're in the middle of a river today, but they couldn't have been when they were growing because trees can't grow in rivers -- it floods the root system. So they must have grown when this was a dry riverbed. In other words, during a period of severe drought. And Stine says that drought lasted two centuries. He can tell from the tree rings.

He's found other tree stumps in the same river that point to yet another mega-drought later on. And there's evidence of mega-droughts like this all over the West.

STINE: Look right over there and see how....

GARDNER: He also took me to the shores of Mono Lake. That's about an hour's drive to the south. Different trees, same climate story.

EATON: All right, so what's any of this have to do with future climate?

GARDNER: OK. Stine says these past mega-droughts are warning signs, because they occurred during an unusually warm period for the northern hemisphere. It gives you a sense of how California reacted to that wider warming trend.

Here's Stine again:

STINE: It has led a number of us to conclude, or at least entertain the idea that it is global warming -- for whatever reason, artificial or natural -- it is global warming that causes California to become very, very dry.

EATON: So, in other words, we think we've got drought now in the Southwest.... We ain't seen nothing yet.

GARDNER: Right. And that's unsettling, of course, given that today we've got tens of millions of people living in the Southwest. You've got huge cities, major industry, major agriculture, all assuming that the climate and the water supply we have now will stay the same.

EATON: OK, but say I don't believe humans are causing climate change. It sounds like I could easily point to this history and say, "Look, the climate has shifted in the past, and the shift we're experiencing now is nothing more than just another natural event."

GARDNER: Right. Right. I asked Stine that at the end of the day as we were driving back. And he says, yes, there has been climate change before. But he says this time around we're the ones with our hand on the thermostat and we're raising the temperatures very, very rapidly.

STINE: And therein lies the danger. It's a vastly different type of climate change than anything that has occurred naturally in the past thousands of years. So this, this scares me.

SAM: And others scientists trying to figure out what the climate could be like in the future are worried too. Some even suggest more severe water shortages in California could eventually make the state unfarmable.

SARAH: But I could see where people would hear a statement like that and think, "How do scientists really know what the future climate is going to look like?"

SAM: It's a good question. But the first thing people should understand is none of these scientists would ever say with 100 percent certainty what the future climate will look like. They talk instead about the likelihood of future climate scenarios. And the main tool they use to create those scenarios are computer models.

SARAH: Right, basically these giant computer programs that try to mimic the Earth's climate. On the front end, you have scientists plugging in data about things like the atmosphere, oceans, sea ice, and how they all interact. And then they tweak variables like carbon dioxide levels and see what happens.

SAM: Exactly. And this is what I was looking into while you were outside enjoying the Sierras, Sarah. And I can say the scenery isn't nearly as pretty. Some of these climate models run on computers so powerful they take up entire buildings.

One of the key people poring over all the data that comes out the other end is a guy named Gavin Schmidt. He's at NASA's Goddard Institute in New York. Here's what he's seen.

GAVIN SCHMIDT: The changes that we're projecting under kind of business-as-usual scenarios are enormous. I mean, they're much, much larger than the changes that we've seen over the 20th century.

SARAH: Now, "business as usual," he means if we did nothing to reduce CO2 and other gases.

SAM: Right.

SARAH: So, what are these models saying about how bad things could get? We've already heard a little bit about what could be in store for California.

SAM: Well, it's pretty sobering. Average temperatures could jump 7 degrees by 2050. Sea level could rise 2 feet by the end of the century. The Midwest could see as many as three major heat waves each summer. And the models project more intense rainstorms for the East -- storms like the one that flooded Georgia last September.

But again, it's hard to say exactly how fast, or even if, these scenarios will play out. There are just too many variables involved.

SARAH: But you know that's the reason these models have come under so much fire. Sometimes they just don't get it right.

SAM: That's true. For example, they missed the mark on air temperatures: they aren't rising as fast as originally projected. In fact, some studies even show temperatures have leveled off recently.

And sometimes the models are too conservative. Arctic sea ice, for example, is melting much faster. But other projections like rainfall patterns and heat waves have been dead on.

Here's Gavin Schmidt:

SCHMIDT: Models are not perfect. You know, they're just quantifications of what we think we know. Right? They're not perfect representations of the real world.

EATON: So, basically, these models are only as good as the assumptions that go into them, right?

SAM: Exactly. And that's why research like Scott Stine's is so important.

GARDNER: Because the more we understand about past climate shifts, the better the assumptions going into these climate models.

SAM: Right, and again, Gavin Schmidt says the issue isn't whether climate models can ever be 100 percent accurate. It's how much uncertainty are we comfortable with before we decide to act on the information they provide.

He likes to use the analogy of a doctor advising an overweight patient to go on a diet. He tells the patient: "If you keep gaining weight, your chances of having bad things happen, like a heart attack or diabetes, go through the roof."

SCHMIDT: Does your doctor know exactly that you're going to get Type 2 diabetes, if you continue to put on two or three pounds every year for the next 10 years? No, he doesn't. He is uncertain. And yet, is that bad advice? No, it's very smart advice. And so the climate scientists, we're in that same position.

GARDNER: And in this case the advice climate scientists are giving us is we're gluttons for fossil fuels, we need to go on a major diet, right?

EATON: Right. And tomorrow we'll talk about what that diet might look like, and why it might be harder than we think.

About the author

Sam Eaton is an independent radio and television journalist. His reporting on complex environmental issues from climate change to population growth has taken him all over the United States and the world.

Gavin Schmidt, climate scientist at NASA's Goddard Institute for Space Studies.

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