气候变化、移民与人类的龛位，与蒂姆-科勒和马腾-谢弗合著

ALEX WOODSON: Welcome to Global Ethics Weekly. I'm Alex Woodson from Carnegie Council in New York City.

This week's podcast is with Drs. Tim Kohler and Marten Scheffer. Tim is regents professor, archaeology and evolutionary anthropology at Washington State University. Marten is a theoretical biologist and a professor at Wageningen University & Research in the Netherlands. Both are also external professors at Santa Fe Institute.

Tim and Marten are co-authors of the report Future of the human climate niche, recently published by Proceedings of the National Academy of Sciences of the United States of America. There was a lot of media attention on this report when it first came out and with good reason. To quote from the summary, the report demonstrates that "depending on scenarios of population growth and warming, over the coming 50 years, 1 to 3 billion people are projected to be left outside the climate conditions that have served humanity well over the past 6,000 years."

We talked about what was surprising in their research, what it means from a historical perspective when huge populations move, and how the pandemic has affected their thinking.

For now, calling in from Washington State and the Netherlands, here's my talk with Tim Kohler and Marten Scheffer.

We're speaking about the report Future of the human climate niche, which both of you are co-authors on.

Marten, we'll start with you. Can you give us a brief summary of the report? What do you want the public to know about your research?

MARTEN SCHEFFER: I'll just tell how it started. It started off when I was in Mérida at a tropical ecology conference, and it was really hot, and I wondered, Is there a limit to what is good for humans? I come from closer to the poles, so to say.

I started looking for literature. I couldn't find so much. And then, later I was working with Chi Xu, our Chinese co-author. We had been working on finding the niche for, for instance, the tropical rainforest climate niche, the savanna, and we just thought out of curiosity, Let's see if we can do the same for humans.

It turned out that actually a kind of bell shape came out. Most of the humans also tended to be restricted in where they would live to a limited range of not only rainfall but also very clearly temperature. We found that kind of strange because we knew that people are living everywhere—on the pole and in the desert.

We started wondering, Would it be a causal thing? So the first question that came to us was: How would it have been in the past? Of course neither of us knew anything about that, so that's why we contacted Tim Kohler—Tim and I had been working previously—and neither did any of us know too much about the climate, so we contacted Tim Lenton for the climate things, and Jens-Christian Svenning for expertise on human habitation patterns. That became a kind of "dream team" with whom we worked actually for years to figure this out.

First, we found, with Tim's help, that in the past humans turned out to be restricted to the same climate niche, to the same narrow temperature zone—not restricted entirely, but mostly concentrated there. That suggested that it was somehow the natural place for humans to be.

Still you don't know anything about causality of course, but it happened that we found exactly the same temperature optimum that another study had found just looking at economic variation from year to year with temperature, in economic production. So we started feeling comfortable that that niche that we found was something really fundamental.

And then we wondered how it would be with climate change, obviously, and then we found a really striking result, that this niche is going to move geographically more over the coming 50 years than it has moved over the past 6,000 years.

Then we came to the estimates that that would seriously affect about 2-to-3 billion people who we speculated might want to move if the climate does indeed warm so much.

That is in a nutshell the story.

ALEX WOODSON: Tim, do you want to add anything to that before we go on to another question?

TIM KOHLER: The thing that I was particularly interested in seeing is something that I didn't frankly expect to see, and that is that there has been so much stability, at least over the last 6,000 years, in human temperature preferences.

I teach an undergraduate course in world prehistory and every year I get up in front of the undergraduates and say, "Look, our marvelous language, our cumulative culture, our technology, our boats, and everything have enabled us to colonize every place on the face of the Earth."

That's all very true, but it did not dawn on me until we started to run the numbers that despite the fact that we can live everywhere, we have certain fairly narrow temperature preferences. This comes out quite clearly once you begin to look closely at what happened in prehistory.

It was, frankly, a big surprise for me, and I think probably for a lot of other archaeologists as well. This is not a question that we have looked at much in archeology. I can only think of one other paper that has looked at this at all, and that was from a very narrow geographic area.

So we are breaking some new ground and I hope we will continue to look at these questions with more rigor and over broader periods of time.

ALEX WOODSON: Tim, I'll stay with you. After looking through this research, you said you were surprised by what you found. Do you have any thoughts as to why humans decided to stay in these climate zones? There can be obvious answers to that of course, but what have you found when you looked at that question?

TIM KOHLER: First, I can't help but raise with you the nature of the world 6,000 years ago. We're talking about the middle of our present climate period, the Holocene. The mid-Holocene was actually—compared to the average over the entire last 12,000 years, which is more or less the Holocene—slightly warmer than the average, and probably actually fairly similar to what we are experiencing today.

In some respects, it's a choice. Going back to 6,000 years and looking at the temperature niche at that point might actually skew things towards the warmer side. But it turns out, of course, that things are going to get much warmer very fast if the climate models are even approximately correct, as we think they are.

I'd also like to put these temperature bands into some kind of living perspective for you and for our listeners. If you were in Europe, the cooler temperature limit for what we're calling the preferred temperature niche is roughly what Vienna and Brussels experience today and the warmer side would be roughly what Madrid and Rome experience today. In North America the band comes between, roughly speaking, Seattle or Vancouver and a place like Nashville or Charlotte. So between those are what we're considering the "sweet spots" for the human temperature niche.

Of course people live outside those areas, but what we found is that there's a strong preference for that particular zone. Once again, that's a little bit surprising because at 6,000 years ago quite a few of the world's people—almost certainly the majority of the world's people—were hunters and gatherers and foragers and fishers, they weren't agriculturalists, and so they weren't necessarily tied down to specific places. They could go where they wanted. It was a world of low population density. If things weren't good, they moved someplace else.

To find that, despite that they were predominantly within that relatively narrow temperature niche, puts for me as an archeologist quite a bit more urgency on the problem as to what will happen in the future when the location of that temperature niche on the globe starts to change.

MARTEN SCHEFFER: Perhaps I can expand a little bit on that study that points to the same temperature sweet spot that we find. It's a study that looks at within countries in warmer years or colder years does their economic output become better or worse. They didn't just look at the agricultural output, but also at patents and all kinds of output, and for all of that it points to the same sweet spot. So, again, there seems to be something there.

Of course there are all kinds of studies about the effect of heat stress or cold stress on people, but those are very limited studies—you put people in a room, you see if they can concentrate of whatever—and you can't really extrapolate from that to the dynamics of whole societies.

What we've taken is a view from the opposite end—not bothering about any mechanisms or details, just look at the big pattern, and that big pattern turns out to be strikingly steady through time.

ALEX WOODSON: Marten, I want to continue with you for this next question. Is there an actual limit to how hot it can be for humans to live productive lives? I believe 0.8 percent of the planet already has temperatures that the study was projecting for decades in the past, so I assume that there are some people in that 0.8 percent of the planet. How do people live in this extreme heat, and maybe looking into the future—we'll get to people leaving, people going to different parts of the world, more hospitable parts—can humans adapt to this extreme heat in the decades to come?

MARTEN SCHEFFER: Of course there are many aspects of temperature. We just looked at the mean annual temperature. We looked at many more things, but in the end most of the variance could be still explained by the mean annual temperature.

But of course we know that heat waves are very dangerous especially when they are also humid because then you can't lose your heat anymore through perspiration.

It is of course possible to live anywhere if you have the means. Whatever temperature it gets, if you have a good air-conditioned and well-insulated house, you fly in your fruit and so forth, you're fine. The problem is that the expansion of that very hot area will cover a region where now about 3 billion people live and those are mostly really poor people that have to go outside to work, who can't fly in their fruit, don't have working air conditioning.

So what you can handle depends a lot. If you have to work on the land, you can handle a lot less than when you're Skyping on your computer in an air-conditioned room.

ALEX WOODSON: Just to push this point a little further, I think about huge cities in this area that might become uninhabitable, places like Singapore or Lagos, where millions of people live. Do you envision cities like that just not existing anymore or is there something else that can happen to improve the situation there?

MARTEN SCHEFFER: We don't know. What's interesting is that right now India is a place where a lot of people live—and always a lot of people lived there, I think—and it's very hot. So you would say, "Well, that's an anomaly, and how do they cope?" What we know is that India seems to be a bit on the edge because in India whenever it gets a bit hotter than normal, there is real trouble there. So it does seem that it creates lots of trouble.

ALEX WOODSON: Yes, and we're seeing now there's a huge storm hitting India as we speak, and that's another consequence of warming temperatures as well.

MARTEN SCHEFFER: It's another consequence. We will get probably flooding in Bangladesh. You see that poverty makes it very hard to be resilient against such natural attacks so to say.

ALEX WOODSON: Yes.

Tim, I'll turn to you now. What can archeologists tell us about what humans do when the temperature rises? We're dealing with something now that we've never seen in human civilization, but the temperature has changed, the climate has changed, over the past 6,000 years. What have you seen happen to humans with the temperature change?

TIM KOHLER: One thing is that in fact humans have experienced very rapid temperature increases in the past. But the main episode of that that we could hold up as an analog would be the end of the Pleistocene, the rather sudden onset of the Holocene.

But of course it's not very analogous in a number of ways. If we were to look at North America, for example, at that time, it had only been colonized within the last 3,000 years or so by the ancestors of the present Native Americans, so population levels were extremely low. That means as people's food supplies shifted, people could adapt very readily.

We do know that Pleistocene megafauna died off and part of that, but not all of that, is due to climate change. There was predation as well putting pressure on these animal populations. Those things combined lethally for a number of different species.

But when population levels are extremely low people move very readily and mobility is the most ancient human response to changing temperature and precipitation regimes, to subsistence risk in general. It's something we've always done very well.

The problem of course now is that we live in a fully packed-up world so people can't just get up and leave. For one thing, most of us at least have quite a bit of investment in where we are. If we're farmers, we've improved the land, we put in irrigation systems. If we are homeowners, we've fixed up our house and so forth. So migration now is very difficult for people.

And, of course, given population levels all over the world, nobody wants to see vast numbers of people move into their country, especially if they're uninvited.

So it raises enormous issues. I'm positive that we can avoid the future that we talk about in this article, but it's going to take some work to make sure we can avoid it. We can perform adaptation, we can develop crops that mature under more severe conditions, we can change population growth rates with determined effort, and we can mitigate change too—that is, we can keep greenhouse gases from getting into the atmosphere in the first place so that we don't exacerbate our present problems.

But all this requires a will to work in those directions. It requires changing our social norms, changing what we admire, perhaps trying to think about a world that is rather different than the world we live in right now, and think about how to achieve that. These are very difficult questions. We're not the first to ponder those questions and we don't—at least I don't—have any terribly clear answers.

But I do know that if 3 billion people try to get up and move it's going to create enormous difficulties, and I hope that we can avoid that scenario.

ALEX WOODSON: Marten, I'll come back to you for some thoughts on how we can avoid this scenario too.

I want to speak about Syria for a minute because the report mentioned the civil war in Syria possibly being connected to a drought in that region and just maybe being a harbinger of things to come for when people have to leave because of drought, because of extreme heat. I was just wondering if you could expand on that a little bit.

MARTEN SCHEFFER: There is a whole literature about the relationship between climate and big conflicts and migration. It is always difficult to pin down one particular event to one cause; it is always multi-causal, so it always has to do also with the political situation.

In the case of Syria, for instance, it's experiencing a very long drought, probably more severe than it has experienced in centuries. Now, maybe that wouldn't have been a problem if they had managed their groundwater better.

What happened now is that it became impossible to live in the countryside even with subsistence farming. Lots of people went to the cities to look for work, but there wasn't any work, so there was increasing tension. Conflict started and escalated, and the result we know is a large stream of refugees.

It's never just the climate, but in many cases the climate is pushing the system to a point where things happen. People maybe in the end will not move because they find it too hot but because of a combination of violence, of loss of ways of living. So it's always a complex story, but climate is likely a big driver in the background.

TIM KOHLER: Let me just add to that. In these cases, as Marten said, it's awfully difficult to establish causality. If you're a social scientist and you want to pin down causality, what you want is a large number of independent cases that are ideally randomly assigned across the globe. We can't achieve anything approaching that, even remotely approaching that, for studies of the connection of climate change and either violence or migration.

So we have to take our cases where can get them, and they are all flawed in one respect or another, but they do seem to point in a common direction. That is, if you have a large population of people who are subsistence agriculturalists, if you have relatively weak state organizations or other institutions, and then if you have climate change on top of all that, then the outcome of either migration or violence is much more likely than it would have been in the absence of those conditions.

ALEX WOODSON: Just thinking about the nations, the regions that would be affected by this, a lot of those nations have those characteristics. That's kind of a scary thought.

One more thing about migration that I think is an important point. [In one of the articles about the report, Tim] made the point that people don't want to move, people don't like to leave, people like to stay where they were born, and for the most part people do stay where they were born. I think that's an important point to make when thinking about 3 billion people having to possibly move.

TIM KOHLER: What we see in history and prehistory is that migration is highly non-linear—that is, there are certain thresholds: people will stay in some place as long as they possibly can, and then at some point it just gets to the point where they can no longer make their living there and then they move. When people move, it's often quite a few people making the same decision to move at the same time. Rather than just a few people trying to move all the time, there is this nonlinear threshold sort of effect. It's quite marked in migration studies.

MARTEN SCHEFFER: In general, behavior and choices are contagious of course; especially when you don't know what to do, you are going to follow what others do, and we get this set of massive changes.

Actually, now with Tim we are working on data of an area that Tim has studied a lot. I don't know if you find it appropriate to make the comparison to what is it—the greatest vanishing act in American prehistory?

TIM KOHLER: What Marten is referring to is a paper that we're working on having to do with the depopulation of the Northern Southwest by the people who archaeologists and others used to call the Anasazi and now are called pre-Hispanic Pueblo folks, the ancestors of people who now continue to live among the Northern Rio Grande in New Mexico in places like Zuni and places like Hopi.

What we see there is that in fact under—well, this is a rather complicated story. The famous depopulation of the late 12th century was almost certainly precipitated by a number of different factors.

One of those is changing climates that were unfavorable with respect to both temperature and precipitation for these people to raise their corn—and they were massively dependent on corn—and there were very few other resources available on that landscape, so if they ran into trouble with corn agriculture, they had very little way of leaving.

But, at the same time, their lives were complicated by the fact that other people in the Northern Southwest had also been forced to leave their homes and move to the places that seemed to be the most stable and most promising. That meant that the most stable and most promising places—you could think of the area contained now by Mesa Verde National Park as being an example of such an area—reached such high population levels that even though they could continue to raise some corn, nevertheless they couldn't support the number of people who were there.

Then you have violence that makes it even more difficult for people to make a living on the landscape in addition to all these other problems.

And possibly there are the beginnings of a migration from the North into the South, into this area, of more nomadic people, the ancestors of the present-day Navajo and Apache, and that would have been perhaps the final straw that would have precipitated this massive migration of probably on the order of 30,000 people. That's a big movement of population.

ALEX WOODSON: Fascinating. I'll be looking for that paper. It definitely ties into everything that we're talking about.

Marten, I want to give you a chance to speak about what Tim was talking about, ways to avoid this scenario. I'll connect that to a question that I had as well—and maybe, Tim, you can jump in on this after Marten speaks—whether there is anything about this report, about this research, that made you hopeful for the future.

MARTEN SCHEFFER: Basically, we're not saying how many people migrate, we are showing that for each degree of warming about a billion people get in serious trouble and may need to move.

Under the RCP 8.5 scenario, given the current climate sensitivity used in models, we would get a warming that would bring 3.5 billion people in 50 years in trouble, about a third of the world's population, such a serious problem that they may need to move.

Of course we have ways of avoiding that scenario and we are working hard on that. The most important thing we should do is to prevent that dangerous climate change.

Even if we can reduce the forcing from atmospheric greenhouse gases, it may still be that the Earth could warm quite a bit because there are two uncertainties. One is we don't know how our carbon future will look. The other uncertainty is that we actually still don't know how sensitive the climate is to the carbon, and the new models suggest that actually the climate might be more sensitive than we thought, so, even if we do our best, still we might see quite a bit of warming.

We used to be talking about two things we needed to do to prevent trouble. One is reduce warming. The other is local adaptation of people in the ways that they live and they grow crops.

I would say in the light of our results it makes sense to think of at least three things we need to seriously consider. Those are the two things I already mentioned—make sure that we have less warming and make sure you adapt as well as possible—but also anticipate people having to move. It's better to anticipate that and to think about that and to cooperate globally on the best ways of accommodating that.

Of course global cooperation is really important here. It's important for fighting climate change. It will also be important for accommodating the need for people to move. I think that's an important message.

The good news is each degree we reduce climate warming saves 1 billion people a lot of trouble. The other good news is I think it's better to know what's ahead of us so we can actually plan for that and cooperate.

There is some kind of really good news in the background, and that is while some places of the globe will become worse to live on, other places on the Earth will become better to live on. It's just that it moves in space, and that's mainly a social challenge to deal with that in the best way.

TIM KOHLER: I would just like to emphasize how important this look-ahead ability is. If we were in China 4,000 or 5,000 years ago, the analogous people to scientists today were people who would take a sheep or pig scapula and heat it over a fire and interpret the pattern of cracks to tell the emperor what's going to happen in the future.

When you compare that with the ability to look ahead with climate models, we have an enormous advantage over ancient civilizations. We ought to be able to do a lot better than they did because of this look-ahead capability.

The question is: How seriously will people use these capabilities of looking ahead? Will they continue to just point out the flaws in the models—and of course no model is perfect, but I think the climate models are a great deal better than the patterns of cracks on a scapula—or will they take them seriously and try to head off what could be a massive worldwide catastrophe if we don't plan for it or if we simply avoid it?

ALEX WOODSON: Just one more question addressed to both of you. This is not something that we have touched on before. We're all dealing with this COVID-19 pandemic now in Washington, in the Netherlands, and in New York City where I am. Obviously, this research was done before the pandemic. It came out now, but I know that you've been working on this for years.

Something like this pandemic happening wasn't at the top of my mind personally before it happened and the way that we're living now is not something that I envisioned. I wonder if you have a different perspective. Has the pandemic changed the way you think about this research at all?

MARTEN SCHEFFER: One thing you could say about this is that the scientists actually knew that it was very likely that sooner or later a pandemic like this was coming and that we should prepare for it. People didn't really pay much attention to it until it happened.

So it kind of tells you that scientists actually are pretty smart and can look ahead and tell you what kind of things are coming. It also tells you that people don't really want to believe that until they see it.

Another thing that I found surprising is how swiftly over almost the whole globe people were able to respond to this in quite smart ways despite huge economic cost. I think that's a sign of hope because it tells you that actually we are capable of doing quite a bit and we don't care about the economy so much anymore if we really think crucial and important things are happening. I think that might also show people that yes we can actually do something about dangerous climate change too, and maybe it's not too bad to do that. That was almost a no-go area before.

I also hear that—I don't remember how many—one-third or half of the Christian Americans think that this is a kind of a sign from God that we have been treating the Earth badly and now it's time to do something about it. This kind of view might also motivate a lot of people.

So yes, the COVID-19 pandemic has created eye-openers into human behavior and human responses, the way they listen or don't listen to scientists. The things they were able to do—there have been a lot of eye-openers. Of course there are a lot of questions on how things will proceed, but definitely this is a huge experiment on a global scale when it comes to human behavior.

TIM KOHLER: I would also like to underscore the impressive adaptation that the world has exhibited in response to these conditions. I have been quite amazed at how effective it has been.

I'd like to point out, though, in addition to what Marten said, that there is a kind of subtle connection between the COVID-19 crisis and the climate change crisis that we talk about in this paper. That is that both of them are exacerbated—and to a great extent caused—by very high population levels. The COVID-19 per capita death rate has been much, much higher in major metropolitan areas like New York City than it has been in little towns like where I live in Eastern Washington, which has had no fatalities whatsoever.

So it's a big problem in dense areas and it's brought on and propagated by very dense populations, and also by the fact that we expose ourselves to new pathogens as we, in our great and ever-expanding numbers, move into biomes that we haven't lived in in great numbers in the past before.

And, of course, having all these people in the world is making climate change more severe by putting additional greenhouse gases into the atmosphere, especially since our populations continue to grow, although, thankfully, not quite so fast as they have been for the last couple hundred years. So there is that connection in terms of population size that's exacerbating both of these issues.

ALEX WOODSON: Tim and Marten, thank you so much. This has given us a lot to think about. I appreciate it.

MARTEN SCHEFFER: You're welcome.

TIM KOHLER: Thank you very much. Nice to talk with you.