JOHN DANKOSKY: This is Science Friday. I’m John Dankosky.

SHAHLA FARZAN: And I’m Shahla Farzan. John, as climate change brings hotter temperatures and shifting patterns of precipitation, Science Friday has talked a lot about how that affects our physical structures, agriculture, and even physical health. But what about our behavior and mental health?

JOHN DANKOSKY: Yeah, I assume here you’re not talking about just trying to drive less or anxiously checking your weather apps all the time.

SHAHLA FARZAN: Yeah, in this case, we might actually be responding to climate change in ways that we’re not even aware of, like individual-level violence or society-level conflict. Dr. Marshall Burke researches this question at Stanford University– specifically, the question of how hotter temperatures and more erratic rainfall might be tipping us toward more violent behavior. He’s also found that this is very much a historic pattern, and we may see it worsen as our entire climate regime shifts. Marshall, welcome to the show.

MARSHALL BURKE: Thanks so much for having me. Great to be here.

SHAHLA FARZAN: So you’ve done research looking pretty far back into history for a possible connection between major climate shifts and conflicts over the last 12,000 years. How strong of a connection did you find there?

MARSHALL BURKE: What we see when we look far back in history is we certainly see societies that have thrived when the climate has changed, but we also see examples of the opposite. We see iconic societies throughout the world that have really struggled in the face of either slow-moving climate change or rapid-onset climate change. And we see many examples in the historical record where societies have really fallen apart and sort of disappeared off the map when faced with dramatic climate shocks.

Some classic examples that we’ve seen in the collapse of the Mayan empire in the Yucatan, the collapse of Angkor Wat in Southeast Asia– both followed protracted periods of drought, so many, many years where things were historically dry. And what archaeologists and anthropologists have shown in these settings is that these societies were trying to do their best. So Angkor Wat is very well studied, and they had a very extensive network of canals that would bring water in from miles and miles away to the city. And what they showed is that in these protracted droughts, you see these canals silting in, unable to get water into the city. You see the hydrologists in these cities actually responding, trying to move the canals, but really just being unable to keep up with the rapid climate change that they experienced.

SHAHLA FARZAN: The research that you’re describing here involves working with a lot of historical and paleoclimate data from things like sediment cores and tree rings. How can you be sure that this connection between climate and conflict actually exists? How do we know it’s not explained by some other factor that we’re not looking at?

MARSHALL BURKE: Yeah, that’s a great question. And as researchers, this is the fundamental question that we’re always worried about. Are we actually looking at the effects of climate, or are we looking at the effects of something else that was going on? So these very deep historical looks, looking at ancient societies, often it is a data challenge. Scientists are putting together all the data sources they can from various climate proxies, as you said, you know, tree rings, cores, various things, sediments, and trying to reconstruct what the climate was, again, as best they can.

Similarly, on the societal side, we don’t have perfect records of what was going on in these societies. They have to be reconstructed from the various data sources that we can put together. So there’s absolutely uncertainty as to whether climate was the only cause here or even the main cause of some of the collapse. And I think that’s true in more recent times, as well, even where we have a lot more data. There, I think we’re able to isolate the role of climate a little more specifically.

That said, climate never acts alone, right? It often amplifies other things that are going on in these societies. The defense establishment in the US calls climate a threat multiplier, right? A force that multiplies other threats that might already exist. And so I think that’s what we see in the deep historical data as well.

SHAHLA FARZAN: So maybe not necessarily that the climate is the sole factor that’s driving these conflicts or that’s pushing these conflicts forward but that it’s a contributing factor.

MARSHALL BURKE: Yeah, that’s right. A thumb on the scale.

SHAHLA FARZAN: So let’s move towards the present day. You’ve also found connections between climate extremes and more recent conflicts between groups. What kinds of contemporary conflicts can we trace back to the climate?

MARSHALL BURKE: Using more recent data, we can actually take a pretty granular look at many different types of conflict. So we can look at individual-level conflict, things like homicide or violent assault, individuals harming one another, or we can look at group-level conflict, so we can look at when groups fight each other. So think of communal violence or civil conflict, even up to the large-scale civil wars that unfortunately still happen in parts of the world.

And from a research perspective, what’s nice here is we actually have very good data in some parts of the world on where these events occur, when they occur, how serious they were. And we also have very good data on what’s going on in the climate system. Was it dry or wet? Was it hot or cold? And so we have a lot of data to really be able to line these things up and try to understand, was it climate that caused these events, or was it something else?

And here again, we see consistent evidence that changes in climate– and in particular, we mean more extreme rainfall, typically dry, sometimes wet, but higher temperatures is where we see the strongest signal. So higher temperatures can induce many different types of conflict. Individual-level conflict– again, clear links to more violent assault, more homicides in places we can measure it– and clear increases in civil conflict. Hotter temperatures appear to increase the risk of civil conflict in many parts of the world.

SHAHLA FARZAN: Is it possible for us, in this research, to kind of put a finger on a specific reason here or factor that’s driving this? Like, for instance, food scarcity that’s driven by climate extremes. Like, some reason why these climatic events would increase conflict even now in modern day.

MARSHALL BURKE: So this is a question about mechanism. What is the mechanism that links changes in climate to conflict? And there’s likely multiple mechanisms at play. So one that has been studied and I think for which there is growing evidence is the role that climate plays in shaping economic conditions and then, when economic conditions change, how that might change people’s incentives to start or join a rebellion.

So when rainfall is more extreme or when temperatures increase, this can worsen economic conditions. So imagine you’re a farmer. Your agricultural yields drop when it gets very high or when there’s no rainfall. And this can happen to millions of people in a given country in a given time.

Now, certainly not all of these people, or very, very few of them, would even think of joining a conflict, an existing conflict. But a few might, right? A few might be driven to that extreme just because they have no other option. And we see in the data that you don’t need that many people to join a conflict for one of these to occur. And so that appears to be one of the mechanisms, climate worsening economic conditions and that changing people’s incentives to join or start conflicts.

Economic conditions are certainly not the only mechanism, and they do not explain what we see in the individual-level data. So what we see there is, you get a hot day. On that day, you see increases in violent conflict. You see increases in homicide. You see increases in domestic violence. And that’s unlikely explained by changes in economic conditions. Our incomes just don’t change that quickly with temperature.

It’s much more likely explained by a physiological, a human physiological response to hot temperatures. I think this is intuitive on some level. How do we feel when it’s really hot? If you get really hot, you’re wearing a jacket, you feel grumpy, right? You might feel irritable.

And what psychologists have shown in the lab for decades now is that you can induce aggression in humans if you put them in a room and you heat up the room. You can make them irritable and act more aggressively. And indeed, that’s what it looks like in the data. What we see is that a small number of people get irritated enough that they are more likely to carry out violence.

SHAHLA FARZAN: We’ve been talking a lot about the negative effects of climate change on human behavior. But is there anything good that we can pull from the data here?

MARSHALL BURKE: I think so. Again, we should not be climate determinists here. We shouldn’t think that climate is destiny. We have many examples in the past, communal-level examples where people come together, share resources during climate shocks. This has been documented by anthropologists, by economists in many different settings around the world, all the way up to societal-level examples where, again, societies have responded to and survived, been resilient to, really negative climate events.

And so absolutely we shouldn’t– this is not just a gloom and doom story. The climate problem is caused by human choices and human behaviors. The solutions will be also due to human choices and human behaviors. And those are under our control. And we can absolutely choose to work together. And we’ve seen examples of that in the past.

SHAHLA FARZAN: Yeah, along a similar vein there, I mean, now that we know that there is this connection between climate and human conflict, can we use that knowledge in some way to do something to help alleviate that heightened risk? Are there any solutions here?

MARSHALL BURKE: That’s absolutely my goal as a researcher. So in studying this, we hope to, number one, understand the relationships, and then, number two, use that understanding to help guide interventions that will make us more resilient as the climate changes. So one thing we see in the group level, group conflict setting, is that certain government programs– in particular, social safety nets– appear to reduce or even break the link between climate and conflict.

So there’s a very nice study in India by Thiemo Fetzer– not done in my group– that looks at the rollout of a large Indian social safety net program and shows that once people were able to access this social safety net program, basically, it guaranteed them work and a wage when there was no rain or when agricultural productivity failed, so they could get a job and they were backstopped against really bad outcomes when the climate worsened.

What that did was completely break the link between rainfall and communal violence in this Indian setting. So this is, right now, our best piece of evidence that social safety nets and broader social support programs can really help build climate resilience and reduce the likelihood that violence breaks out when climate takes a turn for the worse.

SHAHLA FARZAN: That’s interesting. So there is some evidence, then, that there are these solution-based programs that can help erase that connection between climate and conflict, then.

MARSHALL BURKE: Absolutely. And there’s other examples, too. So we could think of insurance programs. It doesn’t have to be government programs. Drought-tolerant crops. Again, all the ways we can think of to help bolster people’s incomes when the climate worsens.

SHAHLA FARZAN: I think we’ll have to leave it there for now. Thank you so much for joining me, Dr. Burke. This has been such an interesting conversation.

MARSHALL BURKE: Thanks so much for having me.

SHAHLA FARZAN: Dr. Marshall Burke is an associate professor of earth system sciences at Stanford University in Stanford, California. I’m Shahla Farzan.

JOHN DANKOSKY: And I’m John Dankosky. And this is Science Friday from WNYC Studios.

Shahla, we’ve been talking about how climate change may encourage us toward more intergroup conflict or even interpersonal violence–

SHAHLA FARZAN: –which researchers are still trying to isolate exactly why either of those things happens.

JOHN DANKOSKY: I want to take a look at a new piece of research that looks at something a little bit different– mental health and the intergenerational toll of a single stressful event like a hurricane.

SHAHLA FARZAN: Like Superstorm Sandy, which happened 10 years ago next month on the East Coast and which some researchers have found was made worse by climate change.

JOHN DANKOSKY: Dr. Yoko Nomura researches stressful events in pregnancy and how they may touch the mental health of children after they’re born. And in her research of children whose mothers were pregnant during that very stressful, catastrophic event, she found some pretty dramatic results. Dr. Nomura, welcome to the show.

YOKO NOMURA: Thank you for having me.

JOHN DANKOSKY: I want to talk about this study. And you looked at the children of women who were pregnant when Superstorm Sandy hit the Northeast. So you looked at the children after they were born, and these children ended up having very high rates of diagnoses of psychiatric disorders. Maybe you can talk about the types of problems that these kids were showing.

YOKO NOMURA: So we specifically did structure the interview for psychiatric diagnosis. And we focused on the disorders which are prevalent in young children, young age. So they are specifically anxiety disorder, phobia, depression, and behavioral problems such as ADHD, conduct disorder, oppositional defiant disorders. My sample had a high rate of disorders because I have children who are fetus during Superstorm Sandy or children who are already born, which is our control subject.

So both of them, even if there is a control versus exposure group, they are really at high risk. So their rate of disorder are higher than the general population. Even so, children who are exposed to Superstorm Sandy in utero have about two to three times higher rate of disorders. Specifically, a five-fold increased risk for anxiety disorders and about 16-fold increased risk of depression and about four-fold increased risk of disruptive behavior disorders.

JOHN DANKOSKY: So help us, if you would, understand the connection between the stress experience of a mother during pregnancy and what happens in a child’s brain. I mean, what would cause a child to develop anxiety or depression as a result of that stress?

YOKO NOMURA: The short answer is we don’t know. We know the association. We know if you are exposed to Superstorm Sandy or any sort of disaster or stressful condition in utero, leads to an elevated risk of psychiatric disorders, which are related to emotional regulation. This study doesn’t really investigate the causes underlying a mechanism of that increased risk.

What I do know is placenta is a key. Children who are exposed to Superstorm Sandy in utero is connected to their mother through a placenta. Mother’s experience, mother’s nutrients, mother’s oxygen, mother’s everything is passed on to the fetus. And among those is a stress hormone. So mothers who are exposed to traumatic stressor produces stress hormone, and that stress hormone is going to be passed on to the fetus.

JOHN DANKOSKY: So if we know that these events are potentially setting up kids who are born afterward to have extra mental health challenges, what do you think, Doctor, the solution is?

YOKO NOMURA: People tend to think disaster is short lived. When it happens, it happens, and people are able to recover from that, without really knowing there is the long-term consequences of that trauma during pregnancy. So what I am advocating for now is to focus on strengthening the community, investing in community health. Having an idea, having a backup plan, having a safety net, by itself, is a stress modulator. You are going to feel safer because of the fact that something is there for you in case something happens. And it’s not fair for us to just put everything on pregnant women or our teachers or our health care providers.

JOHN DANKOSKY: Yoko Nomura is a neuroscientist at Queens College in New York City. She studies child development. Thank you so much for this research, and thank you for your time on the show today. I really appreciate it.

YOKO NOMURA: Thank you very much for having me.

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