Why extreme climate scenarios no longer seem so unlikely

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• Amna Nawaz:

  The United Nations has called climate change the defining issue of our time.

  A U.N. panel estimated the planet's temperature will likely rise by at least by 2.7 degrees Fahrenheit between 2030 and 2052. That could lead to more extreme weather and harsher conditions.

  But those estimates may be on the more conservative side. There are new estimates this week from French scientists suggesting the temperature rise between now and 2100 could be much higher.

  Business and economics correspondent Paul Solman looks at the risks from some of the worst-case scenarios.

  His story is part of our Making Sense series and part of our contribution to Covering Climate Now, a global collaboration of more than 300 news outlets to enhance coverage of the climate story.

• Martin Weitzman:

  So, look, you can start to see erosion along here.

• Paul Solman:

  Economist Martin Weitzman four-plus years ago on the homestead he built on the North Shore of Massachusetts.

• Martin Weitzman:

  The water has risen a couple of inches, at least, in the time, in the 40 years I have been living here.

• Paul Solman:

  A usually understated math- and data-driven theorist, Weitzman, who died in the past few weeks, began as a climate catastrophe skeptic.

• Martin Weitzman:

  I was wondering, how could it be possible that mere human beings could change the climate in a serious way?

• Paul Solman:

  But geologic samples of carbon dioxide going back millennia began to scare him.

• Martin Weitzman:

  We were way outside the historical range for at least 800,000 years, and we're climbing very strongly.

  At the current trajectory that we're on, we're going to blow right past the doubling of carbon dioxide.

• Paul Solman:

  And what might that mean for homes like Weitzman's? His former student, Gernot Wagner.

• Gernot Wagner:

  Last time concentrations of CO2 were as high as they are today, we did, in fact, have sea levels up to 20 meters, 66 feet higher than today. Well, 66 feet, and this house is gone.

• Paul Solman:

  What really shocked Weitzman, though, and led to his book with Wagner, "Climate Shock," was probability math.

• Martin Weitzman:

  What scared me was how much probability there was out there in extreme events. And it's worrisome how little we know and how high the probability is of some catastrophic temperature.

• Paul Solman:

  How much hotter?

• Gernot Wagner:

  Six degrees centigrade, 11 degrees Fahrenheit. That's a planet, like, nobody would recognize.

• Paul Solman:

  Wagner is now a professor at NYU.

• Gernot Wagner:

  Marty's insight on this was, he looked back to sort of the consensus science, hundreds of climate scientists analyzing the most likely developments.

• Paul Solman:

  Now, this is the famous bell curve, estimated future temperatures increasing from left to right, the likelihood of any given temperature indicated by how high the line is at that point.

  Here's the hump of the curve. That's where it's likely to fall. Temperature will be between here and here. But, then, if you keep going…

• Gernot Wagner:

  There is this tail. And there is a small probability of being all the way out there. We can't exclude that.

• Paul Solman:

  Weitzman's rigorous estimate of what's called the tail risk is what transformed environmental economics.

• Martin Weitzman:

  There's a valid, about almost a 10 percent chance of an increase of 4.5 degrees Centigrade.

• Paul Solman:

  But 10 percent, I mean, that is just one chance in 10. I have been at the racetrack long enough to know how rarely a 10-1 shot comes in.

• Martin Weitzman:

  It's not that rare. You buy fire insurance for probabilities that are much lower than 10 percent. You buy car insurance for probabilities that are much lower than one in 10 over a lifetime.

  So this is well within the range of things that we like to insure against.

• Paul Solman:

  Soon after our time with Wagner and Weitzman, though, things seemed to be improving.

• Gernot Wagner:

  In that we sort of had a handle of, right, basically growing the global economy, while not growing CO2 emissions.

• Paul Solman:

  Because we wouldn't be using as much coal, for example. Alternative energy was replacing fossil fuels.

• Gernot Wagner:

  For example, yes. Well, turns out that happened about three years ago or so ago. Last year, we again had more emissions than the year prior. This year, we have more emissions than the year before.

  So, yes, temperatures are rising, sea levels are rising, but what is frightening, frankly, is, what puts the shock into climate shock, if you will, is year after year, we add more CO2, and that increase in the increase is increasing.

• Paul Solman:

  One reason the rate is increasing? The hotter it gets, the more energy we use to keep cool.

• Catherine Wolfram:

  If the world used as much electricity for air conditioning as the U.S. currently does, then we would use as much electricity for air conditioning as we do for everything right now.

• Paul Solman:

  Just look at Southern Asia, says economist Catherine Wolfram.

• Catherine Wolfram:

  You look at, like, the hottest places in the U.S., Miami, it's not as hot as the coolest places in India. And so the potential growth in air conditioning demand in India is enormous.

• Paul Solman:

  Now, both Wolfram and Wagner stress that air conditioning is a blessing, especially given global warming.

• Gernot Wagner:

  Extreme heat simply kills.

• Paul Solman:

  But to the extent that a country like China or India becomes wealthier, and people start putting in air conditioners…

• Gernot Wagner:

  Which, for the most part, is a good thing, right?

• Paul Solman:

  Right.

• Gernot Wagner:

  Fewer people dying because of heat waves.

• Paul Solman:

  Yes, but that then is going to contribute to the acceleration that we're talking about.

• Gernot Wagner:

  If — right, if the energy produced is through fossil energy, yes.

• Paul Solman:

  Was Marty more worried than when we walked along his property in Gloucester?

• Gernot Wagner:

  Yes and no.

  More worried because there's increasingly more science that tells us things may well be worse than we had previously thought. Less worried, hopeful, because there's a lot happening too, kids on the streets, millions of them around the world advocating for the right policies.

• Paul Solman:

  The insurance policies Weitzman and his student were urging in back in 2015.

• Gernot Wagner:

  Economics 101, right, price up, demand down. Don't ask economists what they think which new technologies will come into play. All we are doing is setting the right price and getting out of the way.

• Martin Weitzman:

  That's right. Let 1,000 flowers bloom. We can't know today what that future technology is going to bring. So let's put a price on carbon that incentivizes carbon-free technologies, and the winner will take all.

• Paul Solman:

  And, says Wagner today, more countries and states in the U.S. have begun doing what Weitzman urged.

• Gernot Wagner:

  India has a coal tax. China is experimenting with emissions trading. States here in this country are experimenting with the kinds of policies that we ought to have at the federal level. We don't yet, but the emphasis on yet.

  There's only so long we can pretend nothing is happening and walking in the wrong direction.

• Martin Weitzman:

  So here is a wooden walkway that has been lifted up by high tides that will come over it.

• Paul Solman:

  You mean this was flat before?

• Martin Weitzman:

  Yes, yes. When it was constructed, it was flat.

  Careful.

• Paul Solman:

  Business and economics correspondent Paul Solman, stepping ever more carefully in Massachusetts and New York.