Likely Effects of Rapid Climate Change on Violence and Conflict
Summary and Keywords
Given the dire nature of many researchers’ predictions about the effects of global climate change (e.g., rising sea levels, droughts, more extreme weather), it comes as little surprise that less attention has been paid to the subtler, less direct outcomes of rapid climate change: psychological, sociological, political, and economic effects. In this chapter we explore one such outcome in particular: the effects of rapid climate change on aggression. We begin by exploring the potential for climate change to directly affect aggression in individuals, focusing on research showing the relationship between uncomfortably hot ambient temperature and aggression. Next, we review several lines of research illustrating ways that climate change can indirectly increase aggression in individuals. We then shift our focus from individuals to the effects of climate change on group-level aggression. We finish by addressing points of contention, including the challenge that the effects of climate change on aggression are too remote and too small to be considered relevant.
Direct Effects of Climate Change on Individuals
For decades, psychologists and sociologists have studied the relationship between heat (e.g., uncomfortably warm temperatures) and aggression (i.e., behavior intended to inflict harm upon others motivated to avoid that harm; for reviews on the subject, see Anderson & Anderson, 1998; Anderson et al., 2000; Anderson, 2001). Experiments testing this relationship typically involve randomly assigning participants to one of several conditions (e.g., “hot” vs. “comfortable”) and measuring their aggressive thoughts, feelings, and behavior in these settings. For ethical reasons, laboratory studies are unable to measure violent1 behavior such as that observed in crime or war. However, their results converge with “real-world” studies comparing violence in regions with differing climates or across time within the same region. After reviewing this real-world data, the section concludes by discussing some of the physiological, psychological, and sociological factors that underlie the relationship between temperature and aggression.
Experiments allow researchers to draw conclusions about the direction of a causal effect. For example, imagine that a researcher manipulates the temperature of a room and finds that participants randomly assigned to sit in an uncomfortably hot room are more aggressive than participants randomly assigned to sit in a room with a comfortable temperature. Because participants were randomly assigned to the rooms, and because the measure of participants’ aggression occurred after participants had spent time in the rooms, the researcher can conclude that differences in aggression between the two groups of participants were caused by the room conditions, and not the other way around (i.e., that more aggressive participants chose hotter rooms). Studies employing such designs have found that temperature, and even the concept of heat in general, increases aggressive thoughts, feelings, and behaviors.
In one set of studies, participants were exposed to images related to heat (e.g., fire) or to cold (e.g., ice). Participants exposed to the heat images were more likely to think aggressive thoughts and to judge neutral facial expressions as angry (Wilkowski et al., 2009). Similarly, participants primed with thoughts of heat (as opposed to cold or neutral thoughts) were more likely to have aggressive thoughts and to interpret another person’s ambiguous behavior (e.g., spilling a drink on you in a crowded bar) as an act of aggression (DeWall & Bushman, 2009). These studies suggest that the concept of “aggression” is closely tied to “heat” in the mind, and is more likely to be activated when the concept of heat is activated.
Other experiments illustrate the link between temperature and aggression more directly. In one set of experiments, participants randomly assigned to sit in a room that was uncomfortably hot were more likely to be hostile than participants sitting in a room with a comfortable temperature (Anderson et al., 2000). Participants in hotter rooms were also more likely to perceive other people as being more confrontational and hostile than those who observed the same behavior in a comfortable-temperature room. Hot room participants also behaved more aggressively towards another person when given an opportunity to do so.
Laboratory measures of aggressive behavior are not as extreme as the kind of violent behavior that concerns criminologists and policymakers (e.g., assault, murder), for obvious ethical reasons. Nonetheless, they demonstrate the basic psychological processes that underlie violent behavior. Furthermore, one field experiment showed that even extremely violent behavior is affected by temperature. Dutch police officers were randomly assigned to complete a training session in either comfortable conditions or uncomfortably hot conditions (Vrij et al., 1994). In the training session, officers responded to a burglary and were required to rapidly assess and respond to the situation, with the option of using lethal force. Officers in the uncomfortably hot condition were more likely to draw their weapon and shoot the target than officers in the comfortable temperature condition, demonstrating the potentially lethal consequences of temperature’s effect on aggression.
Studies Comparing Geographic Regions
Non-experimental studies have provided some converging evidence for the link between heat and aggression by comparing the rate of violence in warmer and cooler locations. From the experiments reviewed, one would predict higher per capita rates of violence in warmer climates than in cooler climates. In a test of this prediction, researchers found that hotter cities and hotter regions within the United States have higher rates of violent crime than cooler cities and cooler regions (Anderson, 1989; Anderson & Anderson, 1996). This relationship between temperature and violence held even after controlling for numerous alternative explanations, including differences in poverty, unemployment, age distribution, and other sociocultural differences. To be sure, these other factors can and often do independently affect violent behavior, but they are also likely amplifiers of the effects of climate on aggression (Van de Vliert, 2009).
Studies comparing violence across geographic regions are not limited to the United States. Researchers have modeled the relationship between temperature and violence globally, including one study looking at nearly 60 countries worldwide (Mares & Moffett, 2015). The authors concluded that temperature was associated with violence, particularly in regions plagued with existing conflict and instability. Applying their study’s implications to the issue of climate change, the authors predict that each degree Celsius increase in global temperature will increase homicide rates by 6%.
Studies Comparing Violence Rates over Time
Studies comparing warmer and cooler regions have a number of limitations, one of which is the numerous cultural, social, and economic variables that may explain regional differences in violence rates. And while it may be possible to statistically control for some of these differences, unmeasured alternative explanations always remain. To overcome this weakness, a third type of study examines relative rates of violence in a region over time. The scope of these studies varies considerably, ranging from a single city over a couple of weeks to an entire continent over hundreds of years. Across studies, however, a general trend seems to emerge: hotter periods of time are more violent, even after controlling for other time-related variables (e.g., whether children are in school that month; Anderson, 1989; Anderson & Anderson, 1984, Anderson & Anderson, 1998; Bushman et al., 2005a, 2005b).
Numerous studies have tested the relationship between temperature and violence in specific cities, including Chicago, Houston, and Dallas-Fort Worth (Anderson & Anderson, 1984; Harries & Stadler, 1988). The time periods examined in such studies have ranged from 3-hour blocks (e.g., Bushman et al., 2005a) to days (e.g., Anderson & Anderson, 1984) to months of the year (e.g., Anderson, 1989), to years (e.g., Anderson et al., 1997). For example, a study of police reports in the city of Minneapolis looked at 3-hour time blocks and found a significant effect of both time of day and heat on rates of physical assault (Bushman et al., 2005a, 2005b). A two-year study in the city of Vancouver found comparable results, with violent assaults on bus drivers occurring more frequently in hotter months (Yasayko, 2010). Similarly, instances of domestic violence in the city of Brisbane were more frequent on hotter days (Auliciems & DiBartolo, 1995).
Researchers have found similar effects using other measures of aggression and violence as well. These include different measures of violent criminal behavior (e.g., more riots on hotter days, Carlsmith & Anderson, 1979), but also include more creative measures. For example, researchers in Barcelona analyzed intake reports from psychiatric hospitals. They found that psychiatric patients admitted during a 15-day heatwave were more hostile than patients admitted outside the heatwave (Bulbena et al., 2009). In a quasi-experimental field study, drivers in the city of Phoenix engaged in more aggressive horn-honking (duration, number of honks) on hotter days than cooler ones. This was especially true of cars with windows rolled down (and presumably without air conditioning; Kenrick & MacFarlane, 1986). Other researchers have capitalized on the abundance of professional sport data to show similar effects, as they did in one study comparing the frequency with which Major League Baseball pitchers struck the opposing team’s batter. Across thousands of games, researchers found that on hotter days, pitchers were significantly more likely to strike batters, even after statistically controlling for variables related to the pitcher’s control (Reifman et al., 1991; Larrick et al., 2011).
These studies showcase more than just the creativity of researchers; they also illustrate the robustness of the relationship between temperature and aggression, an effect not limited to any one form of violence or any one location. What’s more, these effects have been shown to be robust over long periods of time—not just over weeks or months, but years. In a pair of studies, researchers looked at nearly 60 years of FBI Crime Reports and compared them to data from the National Oceanic and Atmospheric Administration. They found that the hotter annual temperatures yielded significantly higher violent crime rates and, in 96.4% of the years studied, violent crime rates were higher in the summer than the rest of the year (Anderson & DeLisi, 2011).
Taken together, these studies show that the relationship between heat and violence is a robust one, holding across different forms of violence, different periods of time, and different locations.
Mechanisms Underlying Temperature-Aggression Effects
Given the complexity of almost any human behavior, it makes sense to consider aggression as a product of numerous interacting factors: physiological predispositions, psychological processes, and sociocultural factors. While a fuller discussion of the different factors underlying aggressive behavior can be found elsewhere (e.g., Anderson & Bushman, 2002), we will briefly review some of the mechanisms thought to underlie the relationship between temperature and aggression.
Several biological factors are thought to underlie the link between temperature and aggression. Researchers have suggested, for example, that hot temperatures activate both the part of the brain responsible for thermoregulation and the part of the brain responsible for emotion regulation (Anderson, 1989; Boyanowsky, 1999, 2008; Boyanowsky et al., 1981). Others suggest that the human body produces extra adrenaline in response to extreme temperatures, which may in turn facilitate aggression (Simister & Cooper, 2005). Neither explanation is sufficient to fully explain the link between aggression and behavior, but studies such as these suggest some of the mechanisms underlying the temperature-aggression link may be “hard-wired,” a suggestion reinforced by the fact that this link is observed across sociocultural boundaries (e.g., Anderson, 1989; Boyanowsky, 1999, 2008; Boyanowsky et al., 1981).
There are also a number of psychological mechanisms thought to mediate the link between temperature and aggression. For example, research on embodied cognition suggests that our body’s response to environmental stimuli influences how we think, illustrated by the association between “hostility” and “heat” in cognitive studies (e.g., Wilkowski et al., 2009). Others suggest that hot temperatures produce discomfort, increasing irritability and hostile perceptions of others, both of which increase the likelihood of aggression (Anderson & Bushman, 2002; Anderson, 1989). Such findings cannot be fully explained through physiological mechanisms and suggest that both biological and psychological factors underlie the relationship between heat and aggression.
Finally, researchers recognize that sociocultural factors play a role in the relationship between heat and violence. While we will discuss the prevalence and relative importance of these factors in the last section of this chapter, it is worth briefly mentioning one theory, routine activity theory (RAT; Anderson & Anderson, 1984; Anderson & Anderson, 1998; Cohen & Felson, 1979), which puts forth a sociological explanation for the link between heat and aggression. Put succinctly, people are more willing to leave their homes in warmer temperatures, which has a two-fold effect on violent and criminal behavior: first, it increases the likelihood that people will interact with others (e.g., in public spaces), increasing the likelihood of interpersonal conflict; second, it decreases the likelihood of people being in their homes, which may encourage crimes of opportunity.
Taken together, research shows that individuals exposed to hot temperatures are more likely to be violent as a direct result of the heat. These effects have been found in laboratory and field settings, in regional comparisons, and in temporal comparisons within regions. When applied to climate change, the literature predicts that increases in global temperatures should also increase rates of violence. Researchers estimate that even modest increases in average annual temperature (e.g., 1.1°C) could result in 25,000 more serious and deadly assaults per year in the United States alone (Anderson & DeLisi, 2011; IPCC, 2007).
However, human behavior is complex and multi-determined. It is unlikely that rapid climate change effects on aggression and violence will be limited to the immediate and direct effect of heat on individuals. In this next section, we review a growing body of research showing that climate change is also likely to have a multitude of subtler, indirect effects on individuals’ violent behavior, effects that may be even larger in terms of the amount and types of violence engendered.
Indirect Effects of Climate Change on Individuals
In the previous section we outlined evidence suggesting that increasing global temperatures could directly increase violent behavior in individuals. Rapid global climate change will likely affect other variables that, in turn, also increase aggressive behavior in individuals. Indeed, research suggests that such effects have already occurred.
Some of the factors discussed in this section are developmental factors that increase the likelihood of a child developing into a violence-prone adult—that is, an adult who relies heavily on violence to resolve conflicts, attain desired resources, and satisfy desires in a short-sighted way (Gottfredson & Hirschi, 1990; Moffitt, 1993). Such developmental effects arise because rapid climate change increases exposure of children (and fetuses) to risk factors that are known to lead to violence-prone adults. These include growing up in poverty, poor prenatal and childhood nutrition, dysfunctional families, exposure to war and conflict, low education, and unstable living conditions (DeLisi, 2005). Although there are many such indirect pathways to increased violence, we outline four that clearly link rapid climate change to the development of violence-prone adults: food insecurity, economic deprivation, susceptibility to terrorism, and preferential ingroup treatment.
Food Insecurity and Violence
Food security means having a reliable source of affordable, nutritious food. In the United States, 1 in 8 households struggles with food insecurity (Anderson & DeLisi, 2011). Food insecurity brings with it a multitude of problems, and the research reviewed below shows that, among these problems, compromised access to food increases aggression and antisocial behavior.
In one longitudinal study, researchers studied a cohort of Mauritanian children over the course of their childhood, looking at, among other variables, nutritional intake and the presence of behavioral disorders. Children who suffered from malnutrition at the age of 3 were more likely than children who were adequately fed to be aggressive and hyperactive at age 8 (Liu et al., 2004). Three years later, the same malnourished children were more likely to misbehave in school than their peers; eleven years later, they were more likely to show symptoms of conduct disorder. The researchers were able to rule out a number of alternate explanations (e.g., that aggressive and hyperactive children were less likely to have access to food in the future), and were able to isolate the independent contribution of malnutrition in early years (as opposed to malnutrition in adulthood) on aggressive behavior. The outcome variables studied (childhood aggression, hyperactivity, school problems, conduct disorder) are, themselves, risk factors for adulthood antisocial and violent behavior.
Converging evidence for these findings comes from an unfortunate natural experiment that affected 100,000 Dutch men born before and after World War II. From October 1944 to May 1945, residents living in the western Netherlands were subjected to a German blockade that resulted in significant food insecurity. Researchers used birth records of men born during this period to compare those who were exposed to significant malnourishment during the first and second trimesters of pregnancy to those conceived after the blockade had been lifted, who had far better access to food. In the decades that followed, the men who experienced maternal malnourishment during pregnancy were 2.5 times more likely than men who had not to develop antisocial personality disorder, a condition characterized by frequent violence and antisocial behavior (Neugebauer et al., 1999). Other researchers have similarly found a relationship between malnourishment and antisocial aggression (e.g., Huston & Bentley, 2009), and have suggested that one mechanism driving these effects is the mother’s release of cortisol (a stress hormone) during pregnancy (Chen et al., 2010).
Given that climate change is projected to significantly compromise agricultural production and increase food insecurity for hundreds of millions of people (IPCC, 2007), and given the role that food insecurity plays in the development of violent and antisocial behavior, it is reasonable to expect that climate change’s impact on food accessibility is a risk factor for violence at the individual level. Moreover, this effect is theoretically distinct from the effect that food shortages also have as a catalyst for intergroup conflict, described later in this chapter.
Climate change is projected to have a detrimental impact on economies worldwide, including reduced crop yields, less grazing land, and the loss of homes and jobs due to wildfires and flooding (IPCC, 2007). Although the economic impact will likely be felt by most people, the most vulnerable populations are likely to be disproportionately affected, including increased poverty and income disparity (Cullen & Agnew, 2011). And although poverty and income disparity are problems in and of themselves, they can also foster decreased life satisfaction, increased resentment, and dissent, all of which are risk factors for retributive aggression (Doherty & Clayton, 2011). These types of economic effects are most pronounced in countries and regions that are less stable politically and which have difficulty feeding, clothing, and housing their citizens.
As noted earlier, one consequence of rapid global warming is an increase in the intensity of extreme weather events, such as hurricanes, droughts, and flooding. When Butler and Gates (2012) studied income disparity in East African cattle herders, they discovered that resource asymmetries caused by extreme and adverse weather contribute to conflict in the region. In particular, they found that poorer pastoralists resorted to aggression and banditry, both for survival and as retribution for perceived grievances and inequalities. This conflict, in turn, is thought to fuel retaliatory attacks, both for vengeance and as a means of dissuading other herders from attacking them in the future. Perceived inequality is also thought to increase the motivation to exact violent revenge, whether the inequality is based on class, regional disparity, or age (Archibald & Richards, 2002; Cramer, 2003; Hage, 2003; Keen, 2000; Reno, 1997; Stewart, 2000). Of course, actual poverty is not a necessary condition for violence to occur—researchers have shown that significant income disparity is sufficient to yield violent behavior (Barnett & Adger, 2007), particularly if the relative deprivation occurs rapidly (e.g., in the wake of a natural disaster) or if it contributes to uncertainty about one’s own future (Goodhand, 2003; Nafziger & Auvinen, 2002; Ohlsson, 2000).
In extreme cases of perceived inequality, poverty, or in situations with considerable, rapid-onset uncertainty about one’s own future, individuals are more easily recruited into terrorism, a particularly extreme form of violent behavior. The motivations of terrorists involve a complex interplay of social and environmental conditions (e.g., Kruglanski et al., 2009) and are still a relatively young area of study. Nevertheless, researchers have begun to understand what motivates people to pursue this extremely dangerous and violent path. Many of the factors thought to motivate terrorism are byproducts of climate change.
For example, one factor that motivates people to pursue terrorism is frustration over the sudden loss of one’s livelihood, particularly when the loss can be attributed to the behavior of others, or when people perceive the loss as disproportionately affecting them, their families, and their larger ingroup (e.g., others are prospering while they suffer; the decisions made by others contributed to their plight). These factors are particularly impactful when there is no foreseeable opportunity to regain one’s lost livelihood (Goodhand, 2003; Homer-Dixon, 1994; Ohlsson, 2000; Reno, 1997). Such factors have motivated recruits to join militias in Sierra Leone (Archibald & Richards, 2002; Keen, 2000), Palestine (Hage, 2003), and Managua (Maclure & Sotelo, 2004) as a means of re-acquiring power, status, social mobility, protection from violence, and economic autonomy (Goodhand, 2003; Hage, 2003; Keen, 2000; Maclure & Sotelo, 2004; Mwanasali, 2000; Stewart & Fitzgerald, 2000; Weinstein, 2004). Droughts, which are projected to increase in frequency as a result of climate change, bring about many of the conditions that foment terrorism (e.g., threatened livelihood, perceived inability to sustain oneself) and can lead to increased violence in an already violent and vulnerable region. Illustrating this point, researchers estimate that a one standard deviation increase in drought intensity and duration increases the likelihood of conflict in a region by 62% (Maystadt & Ecker, 2014).
The economic effects of climate change are projected to have a number of detrimental effects. While poverty and starvation are some of the more direct and immediate outcomes, research is beginning to show how climate may have remote effects that include terrorism and militia violence, civil war, and interstate war, illustrating the complexity of climate change effects on humanity and the multitudinous pathways to violent behavior.
Preferential Ingroup Treatment
While terrorism represents a particularly extreme and indirect outcome of climate change, researchers also believe that climate change will lead to more moderate forms of “defensive” hostility toward others, particularly if they belong to different racial, ethnic, or religious groups. To illustrate: one group of researchers found that asking people to think about the threat of climate change caused them to behave more aggressively toward members of other groups (i.e., outgroups), whom they believe may threaten their own safety and the stability of their society (Fritsche et al., 2012). Threatened people showed greater preference for authoritarian leadership styles and are more likely to approve of policies that promote outgroup derogation, intolerance, and outright aggression toward outgroup members. Far from an unusual finding, nearly two decades of research have shown that reminding people of mortality and death has similar effects on ingroup preference and outgroup hostility (e.g., Greenberg et al., 1990). Converging evidence has also been found in studies of people living in particularly demanding climates, where such regions have been argued to constitute a threat to both individual survival and the survival of their genetic progeny, leading to cultures that favor ingroup members (e.g., nepotism, compatriotism; Van de Vliert, 2011) and, by extension, are likely to foster hostility toward outgroup members (Hogg, 2003).
Climate change is expected to create harsher, more threatening climates (e.g., more frequent and extreme storms, droughts, floods, reduced crop, and livestock yields; IPCC, 2007). Such climates threaten both livelihoods and lives, but, as the reviewed research suggests, it will also likely increase violence, particularly toward outgroup members. Because ongoing rapid climate change in the next century is projected to displace hundreds of millions of people as a result of lost homes and insufficient resources, it is increasingly likely that people will be forced to interact with outgroup member refugees. This already-volatile situation, combined with the other factors described in this chapter, may well lead to eruptions of violence.
In the preceding sections, we have outlined a number of direct and indirect pathways through which climate change may increase the risk of violence in individuals—directly through increased temperature, or indirectly through genetic predispositions, food insecurity, economic deprivation, and defensiveness against outgroups. It should be apparent by now, however, that many of these pathways, especially the indirect ones, are inextricably intertwined with population-level effects. We evaluate these population-level effects in the next section, considering how climate change may affect not just individual aggression, but aggression within and between groups.
Effects of Climate Change on Groups
In this section we review several ways that climate change will increase violence and conflict amid and between groups of people. Most of the examples presented involve populations whose livelihood is threatened by deleterious climate change and more frequent and extreme weather patterns. Such changes can lead to increased economic and political instability and ecomigration—migration of groups ranging from small herding kinship groups to whole nations—as a result of ecological disasters. Ecomigration further contributes to violent conflict risk through increased competition for dwindling resources, tensions between disparate groups suddenly occupying the same region, distrust regarding each group’s motives, and other socioeconomic issues (Reuveny, 2007). Not all ecomigration is the result of rapid climate change, of course (e.g., volcanoes, earthquakes), but ecomigration as the direct result of rapid climate change (e.g., increased frequency and intensity of heat waves, droughts, flooding) is quite common (Piguet, Pécoud, & de Guchteneire, 2011).
To demonstrate the impact that climate can have on intergroup conflict, we first present evidence from illustrative case studies in which rapid climate change contributed, in part, to conflict in the region. While many of these examples take place in regions that are particularly vulnerable or prone to conflict, several show that even financially secure and resilient countries are at risk. As the maxim goes: any society is just nine meals away from anarchy. We conclude this section by discussing some of the mechanisms underlying or which amplify climate change effects on intragroup and intergroup violence.
Case Studies and Time-Period Effects
Many of the case studies presented here result from rapid-onset environmental disasters. To be sure, not all natural disasters are caused by climate change (e.g., earthquakes, volcanoes). However, many environmental disasters are directly influenced by climate change (e.g., flooding due to glacial melting, droughts due to shifting precipitation patterns, increased hurricane frequency and intensity; IPCC, 2007). Moreover, we recognize that each specific case study also involves an idiosyncratic interplay of political and socioeconomic factors. However, to argue that climate change is a risk factor for violence, it is not necessary for climate change to be the largest or even the most immediate contributing factor to the violence in a region. In many of these cases, an environmental disaster was a flashpoint which ultimately culminated in interpersonal violence due to lost infrastructure, fear and uncertainty, perceived scarcity or competition, or massive relocation—all factors that weaken social control and counteract the mechanisms societies use to resolve disputes peacefully (Nardulli et al., 2015).
The African continent is one of the most-studied global regions when it comes to climate change and its effect on conflict. Numerous qualitative case studies and large-scale quantitative studies have attempted to better understand the onset of civil unrest, war, and violent conflict in this region. For example, significant changes in rainfall patterns and frequent droughts in Kenya, Sudan, and southern Ethiopia threatened the livelihood of pastoralists in these particularly arid regions (Boko et al., 2007), sparking violent conflict as herders were forced to share dwindling water sources and pastures (Leff, 2009). In Uganda, droughts in cattle-producing regions led to food costs soaring by more than 200% (IFRCRCS, 2006) and forced more than 1.5 million to move due to violent internal strife. Worsening the situation, armed herdsmen from neighboring Sudan also moved into Uganda in search of viable pastures and water (IRIN, 2006). Together, these examples illustrate the tensions that can arise, both within groups and between groups, when environmental conditions destroy vital resources.
Researchers have also looked beyond the conflicts of individual African countries, showing that climate change increases violence across the continent as a whole. For example, one study suggested that for every 1°C increase in average temperature, civil war frequency across Africa are expected to increase by 5%, even after accounting for social and economic factors (Burke et al., 2009). Other researchers have modeled violent conflict in East Africa from 1990 to 2009, (O’Loughlin et al., 2012, 2014). This research incorporated a number of geographical variables, and found that warmer temperatures predict increased violence across the continent. Models of civil war in sub-Saharan Africa suggest that droughts threaten personal income and livelihood, both of which contribute to the prevalence of civil war (Devitt & Tol, 2012). In the largest study of this type in the region, Hendrix and Salehyan (2012) looked at small- and large-scale conflicts across 47 African countries. They found that extreme fluctuations in rainfall (droughts and floods) led to political conflict, protests, riots, strikes, intra-governmental violence, coups, violent repression, and anti-government violence. When looked at as a whole, the research suggests that rapid climate change (and the extreme weather it produces) plays a major role, even if not the largest or most direct, in violent conflicts in Africa.
The effect of climate change on conflict is not limited to the African continent, however. For example, the country of Bangladesh has experienced significant ecomigration due to its rapidly-growing population, unsustainable farming practices, and environmental disasters: more than 25 million have been affected by droughts, 270 million by floods, and 41 million by severe storms. The result has been a mass migration of 12 to 17 million Bangladeshis into neighboring India since the 1950s, a migration which has led to significant conflict. In one poignant example, a 5-hour rampage in 1983 led to 1,700 Bengali migrants being killed in India, having been accused of stealing farmland (Homer-Dixon et al., 1993). This incident was just one of numerous group conflicts that arose as a result of the disrupted land and economic distribution and the balance of power between religious and ethnic groups in the region (Homer-Dixon, 1994). Ecomigration has played a similar role in the Syrian Civil War, after a multi-year drought turned 60% of the country’s land into desert and killed entire herds of cattle. The result was a mass migration of desperate and angry farmers to urban centers, where they lashed out against their government for its poor management and unsustainable environmental and agricultural policies (Kelley et al., 2015; NPR, 2013). In both cases, rapid climate changes led to ecomigration which, in turn, contributed to conflict—both between ethnic groups and within the citizens of a single country. And, as noted earlier, these conditions also create fertile breeding grounds for terrorism.
In other regions, the problem is not water scarcity, but land scarcity: island countries such as Tuvalu and Kiribati face possible inundation and loss of nationhood due to rising sea levels (Barnett & Adger, 2001; Nurse & Sem, 2001; Perry, 2012; Rahman, 1999; Watson, 2000). It has been suggested that by 2080 as much as 70% of the world’s current coastal wetlands could be lost due to rising sea levels (Nicholls et al., 1999), forcing tens or hundreds of millions of people to relocate to other regions.
Although the relationship between climate change and conflict is often mediated by ecomigration, this is not always the case. For example, water scarcity amplified religious and political tensions in the Arab-Israeli wars due to disputed control over the Jordan River basin, which is shared by Israel, Jordan, Lebanon, and Syria (Gleick, 1993; see Postel & Wolf, 2001, for additional examples of other important water conflicts).
So far, our examples have involved regions suffering from significant political or economic turmoil, and which are already predisposed to violent conflict. Wealthier, more stable nations are more resilient in the face of climate change effects, but even in such nations there are examples of ecological disasters contributing to conflict. One example is the U.S. Dust Bowl in the 1930s, where poor farming practices, a prolonged drought, and strong winds caused 2.5 million Americans to lose their livelihoods and leave the Great Plains to adjacent states (Reuveny, 2008). The sudden influx of new residents from other states was often met with hostility, as state residents and even the police attempted to block their entry and encouraged them to leave through beatings and home burnings. In a more contemporary example, Hurricane Katrina hit the southeastern United States in 2005 and displaced more than a million people, who took refuge in nearby states. In subsequent months, homicide rates increased in cities that had taken refugees (e.g., Houston) and polls revealed growing tensions between residents and refugees (Anderson & DeLisi, 2011; Reuveny, 2008). Moderating factors (e.g., federal aid) prevented these incidents from becoming full-blown armed conflicts and a civil war, but the impact of these natural disasters nevertheless had a visible impact on violence in these regions, demonstrating that no country is immune to climate-driven effects on aggression.
Researchers have also found evidence for climate change’s impact on group aggression by looking at changes within particular regions over time. For example, the Little Ice Age—a period of cooling from 1300–1850, ushered in a number of significant cultural changes, including shorter growing seasons, changing agricultural practices, and civil war, as disruption in food production led to shortages, famines, and unrest, particularly in agrarian societies lacking the resources to cope with these crises (Fagan, 2000). Rapid climate shifts in the past millennium are said to have contributed, in part, to wars across the Northern Hemisphere and in China (Zhang et al., 2007a, 2007b). Looking at shorter, predictable changes in inter-annual climate in the past half-century, researchers suggest that as many as 21% of all civil conflicts since 1950 were influenced, in part, by climate conditions (Hsiang et al., 2011).
To summarize, whether looking within specific regions or across regions, at one point in time or across centuries, converging evidence suggests that a region’s climate contributes, in large and small ways, to social, economic, and political outcomes, including violent conflict. A comprehensive meta-analysis of such studies predicted that a 1 standard deviation increase in global temperatures or extreme rainfall could increase the frequency of interpersonal violence by 4% and intergroup conflict by 14% (Hsiang et al., 2013). Although rapid climate change and ensuing ecological disasters may not be the sole or largest factor in determining whether conflict will occur in a region, it is a statistically significant contributor. And, as Hsiang, Burke, and Miguel point out, by 2050 temperature and rainfall variables are expected to change by a factor of 2–4 standard deviations, a figure which underscores the practical significance of climate effects.
Moderators and Mechanisms Underlying Group-Level Climate Change Effects
In addition to understanding whether climate change affects conflict and violence, scientists have begun to explore whether it is possible to predict when, where, and for whom these effects are likely to be strongest, and to explore some of the mechanisms driving climate change effects on aggression. In this section, we briefly outline examples of both.
While it seems that most places are susceptible to the effects of global climate change on violence, the impact of these effects is more likely to be felt by some groups than others. In particular, a UN report suggests that women and children may be particularly vulnerable to these effects (Engelman, 2009). While not studied within the context of global warming, there is evidence that, in the aftermath of severe floods, food shortages, and war, when social norms break down, there is an increase in rape, assault, and homicide (Anderson & DeLisi, 2011). For example, in the aftermath of a 2009 cyclone that devastated the Sundarbans—an island region bordering India and Bangladesh—many people lost their homes and their jobs. As a result, women and children were forced to move to urban centers to find work and support their families. Many were pressured into marriage with wealthy strangers as a way to escape the dire situation, a solution which led to many women and children being sold to sex traffickers (Erbentraut, 2015). While natural disasters impact everyone involved, it would be worth pursuing research on particularly vulnerable groups within these populations to better understand the full extent of climate change effects and to understand both the factors that contribute to conflict and violence as well as the impact it has on its victims.
Additionally, researchers recognize the importance of understanding the mechanisms underlying violent conflicts that arise due to climate change. For example, while climate change may cause droughts and other resource shortages, knowing the variables which comprise the causal chain may help policymakers to anticipate and minimize the damage from climate change effects. For example, one of the important variables in determining whether climate-change-induced resource scarcity will lead to violent conflict is whether a region is already dealing with violence and insecurity (Adano et al., 2012). Existing violence reduces people’s resilience by reducing efficient resource use, market stability, and access to education—all factors that would normally make a region resilient to natural disasters. What’s more, Barnett (2003) argues that as a region is required to spend more on their military to address existing violent situations, they are less able to spend money on infrastructure and programs that would otherwise contribute to a region’s resilience—including social support programs for farmers, welfare programs, irrigation systems, reforestation, dams, and other power sources (see also Barnett & Adger, 2007; Levy & Sidel, 2014). Illustrating this point, annual fuel costs alone for Australia’s military in 1997 were more than three times its funding for renewable energy research and development in 1998 (Parer, 1998). Money spent on the military is money not being spent on social and economic programs that confer resilience. And with recent estimates suggesting that some countries may need to spend as much as 5%–10% of their GDP to deal with the impact of climate change (e.g., rising sea levels), and estimates suggesting that as many as 100 million people may be plunged into poverty by 2030 as a result of global climate change, allocating funds to the military may hamper nations’ readiness to respond to the effects of climate change (Hallegatte et al., 2016; IPCC, 2007).
It is also worth noting that climate change has both a direct and an indirect cost to the military itself as well. In a 2015 article, Goodell writes that the United States military faces multi-billion dollar expenses as a direct result of climate change, including the need to upgrade, replace, or relocate naval and air force bases located along vulnerable coastlines or on islands threatened by rising sea levels. Climate change also indirectly increases military expenditures by increasing the need to prepare for new and more complex deployments (e.g., responding to emergencies such as Hurricane Sandy, climate-related conflicts in regions such as Syria; Department of Defense, 2015), and by increasing the cost of supporting domestic installations (e.g., upgrading port facilities in response to rising sea levels; Department of Defense, 2014). As an illustrative example of increased military costs due to climate change: receding Arctic ice cover requires deployment of new naval vessels to address growing tensions between Canada, the United States, Russia, and China over newly-exposed natural resources and shipping lanes (Department of Defense, 2015). And, as these direct and indirect costs accumulate, a greater proportion of nations’ GDP needs to be allocated to the military and, in turn, taken from programs that would otherwise contribute to the nation’s resilience against national disasters.
In this final section, we address criticisms of the position put forth in this chapter that global climate change contributes to increased global conflict and violence. The three most common criticisms can be summarized as follows: a) climate is change is not the biggest, or even a practically significant cause of conflict or violence; b) abundance, not scarcity, causes conflict; and c) climate change is neither necessary nor sufficient for violence to occur.
“Climate change has a trivial effect or no effect”
The first criticism argues that climate change is not the sole cause of violence and conflict, nor is it even a large enough effect to warrant practical consideration. These critics often agree that rapid climate change can contribute to violence through resource scarcity and ecomigration. They argue, however, that other factors (e.g., preexisting poverty, income disparity, presence of weapons, cultural tensions, institutional insecurity, political climate, corruption) are better predictors of conflict (e.g., Barnett, 2003; Baechler, 1999; Benjaminsen et al., 2012; Furlong et al., 2006; O’Loughlin et al., 2012; O’Loughlin et al., 2014; Raleigh & Urdal, 2007; Salehyan, 2008; Slettebak, 2012; Theisen, 2012). Similarly, others argue that whether people go to war or seek out peaceful resolutions (be they pastoral cattle herders or countries) is more strongly determined by rational considerations of the cost of war and the value of potential gains from the conflict (Gartzke, 2012), the presence of diplomatic agreements (e.g., Bernauer & Siegfried, 2012), or the availability of technological solutions (e.g., Feitelson et al., 2012). As such, they argue, researchers should be focusing their efforts on ameliorative strategies such as cooperation and adaptation (Harrod & Martin, 2014; Salehyan, 2008).
In responding to this criticism, it is worth noting that we largely agree with this position. In most of the evidence presented throughout the chapter, the effects of temperature and climate on aggression are relatively small compared to other situational factors. We are not proposing that rapid climate change is the only contributing factor to human aggression, or even the most important one. Rather, our position is that models of climate change or of conflict are incomplete if they do not include the direct and indirect effects that climate change has on violence and intergroup conflict. This position is in line with a report from the Center for Security Studies and swisspeace, which suggested that environmental factors are inextricably intertwined with political, economic, and cultural factors as causes of conflict (Mason et al., 2008). Furthermore, acknowledging the existence of protective and moderating factors is not the same thing as saying that climate change effects are unimportant. Illustrating this point, Tir and Stinnett (2012) argued that water scarcity can increase the risk of a militarized conflict, but suggested that institutionalized agreements represent factors which may circumvent or avoid conflict. This conclusion acknowledges variables that counteract the effects of climate change on violence while still acknowledging that climate change, in and of itself, constitutes both a direct and an indirect risk factor. This point is made even more apparent in reports suggesting that many of the “bigger” factors are, themselves, caused by or amplified by climate change (CNA, 2007; Department of Defense, 2014), as exemplified by Raleigh and Kniveton (2012), whose research suggests that extreme rainfall variation increases the frequency of violent conflict in East Africa. To summarize, it is unlikely that future conflicts will be attributed solely to climate change; this does not mean, however, that climate change is not among the distal causes of such conflicts.
“Abundance causes conflict, not scarcity”
A second criticism leveled against the proposed relationship between climate change and conflict states that in some studies, an abundance—not scarcity—of resources causes conflicts; moreover, these authors claim that, in times of scarcity, people are more willing to cooperate than in times of abundance (e.g., Hendrix, 2010; Hendrix & Glaser, 2007). In responding to this position, it should be noted that climate change is expected to bring about both scarcity and abundance of resources, depending on the location. As such, as has been conceded by some holding these positions (e.g., Hendrix & Salehyan, 2012), conflicts fueled primarily by an abundance of resources do not necessarily detract from the argument that climate change, scarcity, and ecomigration are important risk factors for violence. To illustrate, Goodell (2015) argues that with the recession of ice from the Arctic, significant mineral and petroleum reserves have become available, creating tension between Russia, Canada, the United States, and China. Climate change can contribute to the sort of abundances that, in and of themselves, contribute to conflicts. Furthermore, other data support the claim that both scarcity and abundance can contribute to conflicts (Raleigh & Kniveton, 2012), and that the relationship between resource availability and conflict is likely not a simple one-or-the-other relationship.
“Rapid climate change is neither a necessary nor sufficient cause of conflict”
A final criticism of the position put forth in this chapter characterizes the position as an overly simplistic and deterministic neo-Malthusian argument: that the effects of climate change on conflict are physical laws that fail to recognize or account for the agency and decision-making capacity of political leaders and individual people (e.g., Salehyan, 2008; Verhoeven, 2011). Such an argument reduces the present position to an indefensible straw-man argument, given that most evidence shows that temperature and precipitation levels are neither necessary nor sufficient to fully predict violent conflict in a region. For example, Reuveny (2007) looked at 38 contemporary examples of environmental migration and found that 19 of them resulted in violent conflict while 19 of them did not. These data alone “prove” that climate-driven migration is neither necessary nor sufficient to explain conflict. However, few, if any researchers who acknowledge the role that climate may play in violence and conflict would take such a simplistic and deterministic position. Indeed, scientists working in any complex system have long abandoned the “necessary and sufficient” criteria for judging whether a particular risk factor (e.g., smoking tobacco) plays a causal role in a particular outcome (e.g., lung cancer). Instead, they argue that climate is one of many risk factors that contribute to violence and conflict, shifting the debate to one about the magnitude, mechanisms, and moderators of climate change effects as compared to social, political, and economic effects (e.g., Zhang et al., 2007a). Indeed, we agree with Solow (2013), who called for researchers to take a more nuanced position that recognizes both the moderating role of social, economic, and political factors while simultaneously acknowledging the risk that environmental and physical factors may pose in sparking violence.
There is good and bad news to be taken away from the current chapter. The bad news is that countries already particularly vulnerable to conflict and aggression, or which are already experiencing significant land degradation, water scarcity, or high population density, are at the greatest risk of experiencing an increase in conflict and violence due to global climate change (Hallegatte et al., 2016; Mares & Moffett, 2015; O’Loughlin et al., 2014; Raleigh et al., 2014; Raleigh & Urdal, 2007; Van de Vliert, 2013). Furthermore, even though some countries are better-shielded from famine by protective economic or political factors, even developed countries are likely to see increases in the proportion of children exposed to risk factors for violence, and may find themselves struggling to defend their way of life against worsening climate-driven economic conditions (Van de Vliert, 2013). Even within wealthy, highly developed countries, the socially disadvantaged are likely to experience the detrimental effects of climate change, as illustrated by a study of crime data in St. Louis, Missouri, which found that unusually hot temperatures disproportionately increased violent crime in disadvantaged neighborhoods (Mares, 2013). Moreover, increased poverty, civil dissolution, and wars in developing countries can have a spreading impact on developed countries, either from an increase in the global need for resources, the involvement of developed countries in wars worldwide, and the breeding of terrorist groups in have-not countries fueled by resentment toward primarily Western countries (e.g., Doherty & Clayton, 2011). And while some may argue that technological breakthroughs will ameliorate some of these effects, existing technology, including air conditioning in cars and buildings, water desalination techniques, and better irrigation systems often consume power themselves, which only further contributes to greenhouse gases and the problem of climate change.
But there is good news as well: while the situation seems dire, action can be taken by individuals, groups, and governments to prevent these outcomes. Such actions include reducing greenhouse gas emissions, which reduces the speed and magnitude of climate change, and the use of better population control (given that most of the increase in population in the next decade is expected to take place in developing countries, with huge increases in greenhouse gas emissions as a result). Dietz, Gardner, Gilligan, Stern, and Vandenbergh (2009) have shown how relatively small-scale household behaviors (e.g., carpooling, changing air filters, getting appliances and vehicles tuned up, and setting back thermostats by a few degrees) can lead to significant population-level effects on carbon emission reduction. Other studies (e.g., Zaval et al., 2015) have shown that encouraging people to think long-term about leaving behind a positive legacy can motivate them to engage in pro-environmental behavior. Engelman (2009) has suggested that investing in voluntary family planning and girls’ education may also reduce greenhouse gas emissions, at least as much as spending that same money on nuclear or wind energy. Other research suggests that framing climate change as a global issue, rather than as a source of localized disasters, fosters peaceful coexistence and reconciliation (Pyszczynski et al., 2012).
In short, there is a growing body of research suggesting a number of promising solutions to the problem of climate change and the increased risk of violent conflict associated with it. If governments begin preparing now to feed, shelter, educate, and move at-risk populations to regions in which they can maintain their livelihoods and cultures, we could reduce both the development of violence-prone individuals and the civil unrest, ecomigration, and war associated with climate change. It will not be cheap, and will involve significant cooperation, not just between citizens within a country, but at the international level as well. Given the potentially disastrous consequences of inaction, however, the costs seem well-justified.
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(1.) Violence is defined in social psychology as extreme aggression intended to cause serious injury or death.