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On 6 degrees of climate change

Editor's Note: (CNN columnist John D. Sutter is spending the rest of the year reporting on a tiny number -- 2 degrees -- that may have a huge effect on the future of the planet. He'd like your help. Subscribe to the "2 degrees" newsletter or follow him on Facebook, Twitter and Instagram. You can shape his coverage.)

(CNN) Climate change is all about degrees.

Six degrees Celsius of warming may not sound like much -- probably because "temperatures can swing by 6 degrees within an hour if a warm front passes, and it doesn't mean the end of the world," said Mark Lynas, author of a book called "Six Degrees: Our Future on a Hotter Planet."

But if we raise global average surface temperatures by just 6 degrees above pre-industrial levels, Lynas told me, we'll create "a scenario which is so extreme it's almost unimaginable."

"Most of the planetary surface would be functionally uninhabitable," he said. "Agriculture would cease to exist everywhere, apart for the polar and sub-polar regions, and perhaps the mid-latitudes for extremely heat-tolerant crops. It's difficult to see how crops could be grown elsewhere. There's a certain level above which plants just can't survive.

"There's a certain level where humans biologically can't survive outside as well ... The oceans would probably stratify, so the oceans would become oxygen-deficient, which would cause a mass extinction and a die off in the oceans, as well -- which would then release gases and affect land. So it's pretty much equivalent of a meteorite striking the planet, in terms of the overall impacts."

I chatted with Lynas, a science writer in the UK, about how to avoid a 6-degree world, the international goal of limiting global warming to 2 degrees -- and how to talk to kids about climate change.

The following is an edited transcript of our conversation.

Mark Lynas

For background: This interview came about because of you. Lynas's book was one of 12 chosen by readers for a book club as part of my "2 degrees" series on climate change. To follow along, and shape the coverage, sign up for the 2 degrees newsletter. Lynas has agreed to take your questions the week of June 8.

Sutter: I've gotten a lot of questions from readers about what 2 degrees of warming means. What does it mean for sea levels? What does it mean for hurricanes? Extinctions? Those sorts of things. Could I run through a couple of those and get your take on it, as someone who's read through just mountains of this research?

Lynas: Sure, sure. I'll do my best.

Sutter: OK. Could we start with extinctions? What will we see around the 2-degree mark?

In the marine environment, I think the most threatened ecosystems are coral reefs. (They're) threatened both by coral bleaching, due to rising temperatures and ocean acidification, plus the general degradation of everyday, general human activity. It's very tough to imagine that the world's coral reefs will continue to exist in their present day form in a 2-degree warmer world.

The other most-threatened environments probably are the mountain ecosystems -- where species will be left marooned in shrinking islands of habitat. As temperature rises, you can imagine biomes rising up the sides of the mountains, and species which are dependent on a certain level of temperature and humidity will get left with nowhere to go.

Sutter: What about hurricanes and severe weather?

There's really a lot of uncertainty about this. It's possible to imagine hurricanes will become less frequent but more intense, and possibly (form) over new areas.

Sutter: What about droughts? I get a lot of California questions.

Lynas: The overall global picture is kind of, 'unto them that have with be given more and unto them have not shall be taken away' -- if you want to get biblical. That maps out as the subtropics, which are already the drier parts of the globe, will become more water-deficient. The deep tropics will actually get more rainfall, as well as some of the mid-latitudes. But the subtropics -- which is the southwest of the U.S. -- would expect to see less rainfall, which indeed seems to be what's happening. That does call into question, really, the development model that large areas of the southwestern U.S. have adopted -- expecting a large amount of freshwater to be available to urban areas and agriculture, which are already in a pretty arid location.

So I do think it's going to be hard to adapt to that change.

Sutter: I've also gotten questions about the low-lying Pacific Island nations. At 2 degrees, what is their fate?

Lynas: I used to be adviser to president of the Maldives, who is, by the way, now in jail due to there having been a coup. But his challenge, and his main agenda, when he was president, was to bring to attention the fate of the small island states -- especially those that are coral atolls. For the Maldives, the entire country exists at a meter or less above sea level, and little more. It's difficult to imagine the survival of coral atoll nations at 2 degrees, it has to be said. Although the extinction process depends on the rate of sea level rise. It might take decades, it might take centuries, it's not clear at the moment. But I don't think they have a very long term future.

Sutter: Moving up the degree ladder, you describe a 6-degree world as a "sixth circle of hell." What do you mean by that -- and can you describe some of what we know about that world?

Lynas: It's a scenario which is so extreme it's almost unimaginable. Not many studies have addressed this because it's so far off the scale of what can be envisaged. I found myself looking back at the really serious traumatic events in the Earth's geological history, which have led to mass extinctions, such as the one at the end of the Permian period, 250 million years ago, which wiped out nearly 90% of life on Earth. Actually, a lot of the mass extinctions seem to have been associated with very rapid global warming events. Humans are releasing carbon more rapidly even than took place during mass extinctions. We haven't gotten there in terms of the overall amount, but we're certainly moving in that direction. So it's not a planet that I think any of us would want to live in, and it doesn't have to happen. While I think it's important to try to visualize what a 6-degree world would look like, it's also important to remember that we don't have to go there.

Sutter: What else do we know about a 6-degree world?

Lynas: Most of the planetary surface would be functionally uninhabitable. Agriculture would cease to exist everywhere, apart for the polar and subpolar regions, and perhaps the mid-latitudes for extremely heat-tolerant crops. It's difficult to see how crops could be grown elsewhere. There's a certain level above which plants just can't survive. There's a certain level where humans biologically can't survive outside, as well. We get close enough already in the Arabian Peninsula and some other parts of the world. Remember, 6 degrees is a global average. It would be probably twice that over land and somewhat less than that over the oceans. The oceans would probably stratify, so the oceans would become oxygen deficient, which would cause a mass extinction and a die off in the oceans, as well -- which would then release gases and affect land. So it's pretty much equivalent of a meteorite striking the planet, in terms of the overall impacts.

Sutter: I'm wondering why you took this approach -- looking at climate change by degrees?

Lynas: A lot of people want to know what warming we get with what emissions path, or what warming we might get by what date. That's pretty fundamentally uncertain because they depend on different factors which aren't very well quantified. I felt that looking at it degree by degree was much more robust. If the temperature rises by X amount then what will be Y impacts? There are three major sources of information about that. One is the observational changes we're already seeing in terms of impacts in temperature rise. The second is computer models showing different ecosystem changes or whatever. And the third is paleoclimate sources -- so looking at how the climate was different in earlier hotter periods in geologic time. So piecing those together and mapping them onto a degree by degree picture seemed to me to be a way to try to convey this in a visual and intuitive -- but also highly scientifically appropriate -- way.

Sutter: What do you think about the world's focus on the 2-degree mark? One activist said to me that 2 degrees is the only thing the international community agrees upon for climate change.

Lynas: I think it's important to have a target -- because it focuses policy and it focuses people's efforts. And it makes sense also to have a target based on the temperature. But it's not something we can meet, by definition. We don't have a simple thermostat where we can decide exactly how much carbon to emit and have an exact temperature result dependent on that. So, there's uncertainty, really, about what level of emissions will lead to what temperature outcomes, by when. However, I think that 2 degrees is really the absolute upper limit of what's tolerable in terms of ecosystems and, probably, adaptive capacities of human societies. A 2-degree world is a world without coral reefs, and with much less snow and ice and with fairly dramatic heatwaves -- and other impacts. So, I would like to see a global warming future in which warming actually is lower than that, personally.

Sutter: Do you think that's possible?

Lynas: I think it's possible. It's not very likely. If our current understanding of climate sensitivity is broadly correct then we're probably going to come in between 2 and 3 degrees, somewhere, by the end of the century. I guess the good news is the absolutely calamitous 5 and 6 degree outcomes are particularly unlikely, too, although still possible. And certainly, the risk of them happening is higher than the risk of an airplane crashing when we get onto it.

Sutter: Wait, so you're saying the risk of 5 or 6 degrees of warming -- a doomsday scenario -- is higher than an airplane crashing?

Lynas: Well, the likelihood of an airplane crashing is, I don't know, one in 1 million -- or something on that order of magnitude. Whereas the likelihood of coming within 5 or 6 degrees of warming is probably more than 1 in 100. It's the sort of risk that one would not tolerate at a personal level. But, perhaps because we can diffuse responsibility, we feel that it's tolerable for our species to take that gamble with the whole planet. Maybe it's because we just think there's nothing we can do about it. And we have an in-built optimism bias, myself included, where we like to think that things will just turn out all right, because they often tend to. And meantime we'll go on with our lives as normal. It's a big ask, I guess, to make society as a whole forgo the main energy source we all enjoy, which is fossil fuels, in order to forestall uncertain impacts decades into the future.

Sutter: One of the things that struck me from your book is that you were surprised people are depressed by climate change. Isn't this a pretty depressing subject?

It doesn't really matter whether you find it depressing or not, it's the scientific reality. We have to deal with it. A thing like climate change is known as a 'wicked problem.' It's seen differently by different people, according to their psychological, political and cultural biases. You can frame it as just a technology challenge: Let's get off of fossil fuels and let's get onto renewables and nuclear -- easy. Or you can frame it as a moral challenge: We're trespassing on the rights of future generations and how dare we do that. Or you can see it as a political challenge -- that somehow these big fossil fuel corporations are transgressing democracy and forcing us to stay hooked on oil and coal and gas. Different people, according to their politics, will see climate change fundamentally in this way. It's not a simple problem to understand.

Sutter: So how do you look at it? Do you ever find climate change overwhelming or depressing, personally?

Well, I'm a pragmatist. I think it's a solvable problem. I don't think we need to abandon capitalism or change our entire political system in order to tackle this challenge. Other people do, and I disagree with them on that. And we have debates late into the night. But I think with next-generation nuclear technologies, and particularly with the way solar power is developing so rapidly, and how rapidly it's coming down in cost, and how quickly the technology is improving, there are zero-carbon options now becoming much more widely available, which will bring down our emissions much more rapidly than people think -- or than people thought just a few years ago. I don't think there's any point being pessimistic about that. Pessimistic people don't achieve anything. It's important to do what's possible -- and to do it quickly.

Sutter: What do you make of the way the world's responding?

Lynas: We are now inhabiting a human-dominated planet. We are in a new epoch known as the Anthropocene. The Holocene is now considered to be over. And I don't think there's really been another species that has had that effect on the planet before -- maybe the first bacteria that emitted oxygen, or photosynthesizing microorganisms. But we really are into terra incognita looking forward. That gives our species a serious level of responsibility for planetary management that people just don't really appreciate at any kind of fundamental physiological or political level. We are in charge. It's up to us. We actually do have an overall effect on the earth's temp. It's not up to Mother Nature anymore to run the show.

Sutter: Do you have children?

Lynas: Yeah. The reason I was distracted just a minute ago was my kids just came back from school.

Sutter: Do you talk to them about this? What do you say?

Lynas: I talk to them a bit. They know what I do. Younger generations have grown up with this specter. It's a bit like how those of us who are older grew up with the Cold War and the threat of nuclear annihilation ... So they're not entirely in a different situation, I suppose, from previous generations. You could feel bad for future generations, but on the other hand I'm not sure their future is any worse than the future for somebody born in, say, 1900, who in all likelihood, in Europe, would have killed in one of the world wars. It's a manageable problem. We're beginning to get the grips of it. There are some positive signs already -- China and the U.S. agreeing to peak emissions, and things like that. So it's not a counsel of despair. And I think it's important to talk to kids at that level -- not to make them think that somehow they're fundamentally doomed.

That isn't the case and doesn't have to be the case.

Sutter: What needs to happen to ensure things do improve? What are the benchmarks you're looking to to say, 'OK, we're managing this problem. We're doing what's needed'?

Lynas: Well, I'm a ecomodernist, which is a new label a lot of environmental thinkers are beginning to attach to themselves -- because it's a bit different from more traditional environmentalism, which thought we were somehow doomed or we were fundamentally a destructive species. We can turn this around -- this and other problems as well, if we have a more pragmatic approach to politics, economist -- and especially technology. We need to have a price on carbon, so that emitting carbon dioxide isn't cheaper than other energy sources. We need to invest heavily in research and development in zero-carbon sources, including next-generation nuclear renewable energies, especially solar. And we need to deploy them on an ever wider scale, with increased financing. We also need to have a political agreement -- so there's a sense the whole world is moving in the right direction. All of those things are not just possible, but I think they're fundamentally achievable, and likely. But we need to keep the pressure up on politicians and on everyone else.

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