Climate bones of contention: How climate variability influences territorial,maritime,and river interstate conflicts

Abstract

Many scholars examine the relationship between climate variability and intrastate conflict onset. While empirical findings in this literature are mixed, we know less about how climate changes increase the risks for conflicts between countries. This article studies climate variability using the issue approach to world politics. We examine whether climate variability influences the onset and militarization of interstate diplomatic conflicts and whether these effects are similar across issues that involve sovereignty claims for land (territory) or water (maritime, river). We focus on two theoretical mechanisms: scarcity (abundance) and uncertainty. We measure these concepts empirically through climate deviation (e.g. droughts/floods, heat waves/cold spells) and climate volatility (greater short-term variance in precipitation/temperature). Analyses of issue claims in the Western Hemisphere and Europe (1901–2001) show that greater deviations and volatility in climate conditions increase risks for new diplomatic conflicts and militarization of ongoing issues and that climate change acts as a trigger for revisionist states.

Keywords

climate conflict maritime river territory

A 2014 report from the Intergovernmental Panel on Climate Change (IPCC) identifies a variety of climate changes that will continue in the next century, such as increases in land and ocean surface temperatures and weather and climate related disasters.¹ Climate change will affect the quantity and variability of available freshwater, agricultural productivity, the frequency of natural disasters, coastal erosion, the seasonality of water run-off, land degradation, and countries’ territorial and water borders (Hendrix & Glaser, 2007; Busby, 2008; Bauer, 2011; Bernauer & Siegfried, 2012).² These environmental shocks pose many risks to human and state security by increasing water scarcity, creating more environmental migrants and refugees, harming economic productivity, and raising risks for intrastate and interstate conflicts. While many scholarly studies examine the relationship between natural disasters, climate changes, and intrastate violence (Burke et al., 2009; Buhaug, 2010), we know less about the connections between climate variability and interstate conflicts over land and water resources. Most interstate conflict research on this topic focuses on how climate change affects militarized interstate disputes in general (Stalley, 2003; Gartzke, 2012) or how shared water resources influence the risks for interstate conflict (Toset, Gleditsch & Hegre, 2000; Hensel, Mitchell & Sowers, 2006; Brochmann & Hensel, 2009; Brochmann & Gleditsch, 2012; Devlin & Hendrix, 2014).

However, climate variability can be connected more explicitly to diplomatic issue claims involving territorial, maritime, and river areas given that the effects of climate shocks influence the salience and uncertainty of resource control for contested issues (e.g. water quantity/quality, fisheries stocks, agricultural viability of land). Issue scholars show that diplomatic issues higher in salience are more likely to experience militarized disputes and interstate wars (Hensel et al., 2008), although the effects of climate variability have been mostly ignored in this literature. This study considers whether climate variability in weather patterns influences interstate conflict using data from the Issue Correlates of War (ICOW) Project (Hensel et al., 2008; Hensel & Mitchell, 2017).³

We examine whether climate variability influences the onset and militarization of interstate diplomatic conflicts and whether those effects are similar across issues that involve sovereignty claims for land (territory) or water (maritime, river).⁴ We focus on two theoretical mechanisms – scarcity (abundance) and uncertainty – and we measure these concepts empirically through climate deviation (e.g. droughts/floods, heat waves/cold spells) and climate volatility (greater short-term variance in precipitation/temperature). We start with a set of dyads that could experience diplomatic conflicts (politically relevant dyads) and determine if deviations and volatility in climate variables increase the propensity for new diplomatic conflicts over land or water issues. For issue claims that occur, we also determine if precipitation and temperature deviations alter states’ foreign policy decisions to militarize claims.

The topic is highly relevant because climate variability has increased over time. Figures 1a and 1b present systemic changes in climate conditions between 1901 and 2016.⁵ We see that average temperatures and precipitation levels have increased, especially since 1960, and that countries are experiencing larger deviations from their long-run means in more recent decades, especially for temperature. While scholars have shown that long-run increases in global temperatures may be associated with fewer interstate conflicts, especially in the pre-industrial period (Zhang et al., 2007; Tol & Wagner, 2010; Gartzke, 2012), we have less evidence about how climate variability influences conflict dynamics at a local, dyadic level. We illustrate our theoretical logic with diplomatic conflicts between Bolivia and Chile over the Mauri and Lauca rivers, showing that diplomatic and militarized interactions increased when these states experienced greater deviations from long-term precipitation means.

More generally, our analyses of issue claims in the Western Hemisphere and Europe show that greater deviations and volatility in climate conditions increase risks for new diplomatic conflicts and that the risks of conflict are greatest for revisionist states. Our study makes several contributions to the existing literature on climate variability and conflict. First, we go beyond previous interstate conflict studies that focus on higher levels of military engagement by also considering how diplomatic interactions at lower levels of hostility are influenced by climate variability. Second, our connection of climate issues and the issue approach to world politics allows us to think about scarcity and uncertainty through a new theoretical lens. Third, we show how precipitation and temperature deviations influence diplomatic issues collectively or separately. While the river literature considers climate conditions, our results suggest climate variability influences territorial and maritime issues also. Given that territorial issues are the most escalatory of all diplomatic issues, this is a potential concern for states’ future security. Finally, by looking at challenger and target states in diplomatic exchanges more carefully, our study helps us understand whether climate variables are factors that push states into revisionist positions and whether challengers are more strongly affected by climate changes than targets.

Literature review

Conflict scholars examine how climate change and climate variability influence the risks for interstate and intrastate wars.⁶ Global warming over the past few centuries has been associated with the decline of interstate warfare, reflecting the economic efficiencies that globalization creates relative to territorial conquest (Zhang et al., 2007; Tol & Wagner, 2010; Gartzke, 2012). On the other hand, long-term changes in population and natural resources can spark interstate conflict as predicted by lateral pressure theorists (Choucri & North, 1975). Dyadic studies show enhanced interstate conflict

Figure 1a.

Aggregated precipitation and temperature, 1901–2016 Figure 1b. Aggregated standardized precipitation and temperature, 1901–2016

risks with greater population density (Stalley, 2003) and higher water scarcity (Hensel, Mitchell & Sowers, 2006; Brochmann & Hensel, 2009; Dinar, 2009; Devlin & Hendrix, 2014). Social unrest (e.g. food riots) is negatively associated with long-term increases in average temperature, although the effects change over time through adaptation (De Juan & Wegenast, 2020).

Other work focuses instead on how short-term climate variability influences the likelihood of conflict. Short-term climate triggers (e.g. droughts, floods, heatwaves) are measured with deviations from panel specific precipitation or temperature means. Some studies find that climate variability increases intrastate conflict risks (Miguel, Satyanath & Sergenti, 2004; Burke, Hsiang & Miguel, 2015; Burke et al., 2009; O’Loughlin et al., 2012; Landis, 2014; van Weezel, 2019), while others find little to no relationship between short-term climate changes and armed conflict (Hendrix & Glaser, 2007; Buhaug, 2010; Theisen, Holtermann & Buhaug, 2011; Koubi et al., 2012; Klomp & Bulte, 2013; Couttenier & Soubeyran, 2014; Wischnath & Buhaug, 2014). Curvilinear patterns also emerge with conflict occurring more often with floods or droughts than average rainfall (Hendrix & Salehyan, 2012). Analyses of interstate conflicts suggest that militarized disputes are more likely for dyads experiencing increased volatility in precipitation or temperature (Devlin & Hendrix, 2014). The disparate findings in this field depend greatly on methodological choices, temporal spans, and geographic units of analysis (Sakaguchi, Varughese & Auld, 2017; Koubi, 2019).

Several causal mechanisms are posited in the climate–conflict literature, but we focus on three related to our theory: agricultural productivity, seasonality, and water/resource scarcity.⁷ First, climate variability can be a threat multiplier for violence by producing agricultural shocks that alter individuals’ relative deprivation and opportunity costs for fighting (Theisen, 2017: 211). Short-term shocks in rainfall or temperatures significantly reduce agricultural output and farmers’ incomes, making rebel recruitment easier (Miguel, Satyanath & Sergenti, 2004). A sudden decline in agricultural production also diminishes the government’s ability to provide public goods (e.g. crop insurance) due to reduced tax revenues, increasing relative deprivation in society (Devlin & Hendrix, 2014). Countries more dependent on agricultural production face greater conflict risks in response to climate shocks (von Uexkull et al., 2016; Vesco et al., 2021). More broadly, climate variability generates uncertainty for land property rights (Gartzke, 2012) and increases new claims to territory as environmental changes turn productive farmland into deserts and harm fishing and farming through intensified salinization of water supplies. Threats to agricultural production generate resource scarcities that encourage states to look outside their territories for additional land and resources.

Second, climate variability influences seasonality conditions which can negatively impact water flows and agricultural growing seasons (Bernauer & Siegfried, 2012). Shifts in growing seasons and crop production influence the ability of insurgent groups to recruit fighters (Landis, 2014). Climate change also increases the rate of glacial and snowpack melting, which reduces water supplies during summer seasons and increases tensions in transnational river basins. Seasonality changes generate more uncertainty about future water supplies for agriculture and fishing and create more property rights conflicts.

Finally, climate volatility influences the supply of important natural resources such as freshwater and raises uncertainty about future resource stocks (Homer-Dixon, 1991; Hendrix & Glaser, 2007; Raleigh & Urdal, 2007). Militarized conflict is more likely in river basins as water scarcity increases (Brochmann & Hensel, 2009), partly because climate variability makes it difficult for states to comply with the terms of river treaties (De Stefano et al., 2012). The empirical and theoretical connections between climate change, climate variability, and conflict are well developed in the intrastate conflict literature, but we know less about the conditions under which climate factors make interstate conflict more likely. Our theory fills this gap by building upon the issue approach to world politics and connecting this general theory to climate variability through the mechanisms of scarcity (abundance) and uncertainty.

The issue approach and climate variability

International relations scholars often assume that conflict begins for a reason such as contestation of shared land borders, competition over important resources like oil, or the removal of other states’ leaders. Yet many interstate warfare theories fail to consider how the issues at stake influence the onset and escalation of violence. The issue approach to world politics fills this gap by arguing that foreign policy is issue directed, that cooperative and conflictual foreign policy tools are substitutable means for issue-related ends, and that actors’ preferences over such foreign policy options are driven by the salience that states attach to issues (Mansbach & Vasquez, 1981; Hensel, 2001; Hensel et al., 2008). Issue scholars classify the tangible and intangible aspects of issue salience in diplomatic conflicts⁸ and use this information to understand the onset of diplomatic claims and escalation of claims to militarized conflict. Issues that have high levels of intangible salience, such as territorial disputes involving sacred sites or historical homelands, are more likely to be settled through war than issues that involve mostly tangible stakes, such as EEZ fishing rights or water quantity rights in cross-border rivers (Hensel & Mitchell, 2005). However, higher values for both dimensions of issue salience increase militarized conflict risks.⁹

The issue approach starts by identifying dyads that could experience various types of diplomatic issue conflicts. For example, states that share contiguous land borders or cross-border rivers have more opportunities to start new diplomatic conflicts over territorial and river issues. Bolivia and Chile share a direct land border and multiple rivers (e.g. Mauri, Lauca, and Silala), thus they have potential for diplomatic conflicts over these issues. States with opportunities for issue claims either start new diplomatic conflicts or maintain the issue status quo. States seeking to change the status quo (revisionists) are called challengers, while states defending the status quo are called targets. Chile challenged Bolivia’s sovereignty claims over the Atacama Desert starting in 1848, while Bolivia sought access to the sea and challenged Chile’s territorial control of the area from 1884 on. Once an issue claim is underway, states can try to achieve their issue goals with military force, by negotiating peacefully, or doing nothing. Chile went to war with Bolivia (and Peru) in the War of the Pacific (1879–84) and won control of the contested territory. A maritime claim also arose after the war, with Bolivia contesting Chile’s control of the port of Antofagasta and cutting off its access to the sea. The issue approach hypothesizes that militarized force is more likely if the diplomatic issues are highly salient, if states have a history of previous militarization, and if there are many failed peaceful negotiations to resolve the claim (Hensel, 2001; Hensel et al., 2008). The issue approach’s focus on diplomatic claims allows for a better understanding of theoretical factors that encourage or deter militarized settlement. In the next section, we explain how climate variability alters these diplomatic interactions.

General effects of climate change on diplomatic conflicts and militarization of issue claims

To understand how climate variability influences interstate conflict, we focus on two broad effects: (1) scarcity (abundance), or increased competition for resources, and (2) uncertainty, or increased ambiguity about the stock or value of future resources. Climate variability alters the issue status quo by creating uncertainty about borders and resource rights, which can prompt new diplomatic conflicts or complicate existing claims. Resource scarcity changes the salience levels of existing diplomatic claims, which increases the potential for issue militarized conflict. Challenger (revisionist) states are more strongly affected by climate changes than target states because they typically do not have sovereign control over contested resources (e.g. downstream states in river basins).

First, climate variability can intensify competition for resources by increasing resource scarcity (e.g. droughts) or creating problematic resource abundance (e.g. flooding). The quantity and quality of freshwater can be changed by higher temperatures, creating greater rates of evaporation or increased water flows from mountain glacial melting (Bauer, 2011; Bernauer & Siegfried, 2012). Higher than average temperatures create more rainfall and snowmelt in the winter and contribute to winter flooding and summer droughts. Regions like the Middle East experience more diplomatic and militarized conflict over river basins relative to other regions due to greater water demands and fewer water supplies, especially as populations have grown (Hensel, Mitchell & Sowers, 2006). Climate variability can reduce the strategic and economic value of territory, maritime areas, and cross-border rivers. For example, areas that were once sustainable for agriculture become unusable due to desertification or intense droughts or flooding. Drought and desertification in Northern Kenya increased violence between pastoral communities within Kenya and with groups from neighboring states like Ethiopia, Sudan, and Somalia as actors compete for scarce land and water to sustain livestock.¹⁰

Potential revisionist actors are more affected by climate changes. Downstream states (e.g. Syria, Egypt, Jordan) experience greater water scarcity on average than upstream states and their environmental security is more

Figure 2.

Precipitation and river claim onset for Bolivia and Chile, 1910–40

negatively impacted when climate deviations occur. States experiencing a loss of productive fishing grounds in their coastal areas have incentives to take other states’ resources. Thus, we may see more diplomatic conflicts between countries and the use of military force to defend issue goals if climate changes increase resource scarcity (abundance) for some states. Climate shocks can motivate states to contest the ownership of areas that are not experiencing this reduction in strategic value. If part of a state’s territory becomes a desert, for example, it could see benefits in territorial conquest of neighboring areas that are more suitable for agriculture. Warmer temperatures and droughts exacerbate the problems of desertification by degrading the quality and quantity of soil and making it more difficult for dry land to return to productive farmland.

For an example of how climate variability influences the onset of diplomatic conflict through the mechanism of scarcity, consider the plot of Bolivia and Chile’s precipitation deviation levels from 1910 to 1940 in Figure 2.¹¹ Bolivia initiated a diplomatic conflict against Chile in June 1921, protesting Chile’s decision to divert water from the Mauri river through a contract with a local company that would use the water for irrigation.¹² Eighteen years later, Bolivia protested Chile’s decision to divert water from the Lauca river into the Rio Azapa valley. Prior to both claims, Bolivia experienced declining precipitation levels in water supplies. These short-term climate deviations increased Bolivia’s overall water scarcity, encouraging the country to press its water quantity claims against Chile. In 1962, Bolivia cut off diplomatic relations with Chile and asked the Organization of American States to intervene, arguing that diverting water was an act of aggression (Tomasek, 1967). The value of issue salience increased over time in the Lauca claim for Bolivia, as it came to see the water scarcity issue as more important to its national security. Based on the issue approach, rising issue salience increases the probability of militarization. This case illustrates the expectations of our first hypothesis.

Hypothesis 1 (Scarcity (Abundance)): As temperature or precipitation deviations increase, interstate issue claims are more likely to be initiated and to be militarized.

Second, climate change can increase interstate conflict risks by increasing uncertainty about future resource stocks, especially when climate changes are highly volatile.

Climate change will likely affect levels of precipitation, with some countries growing more arid and others wetter. However, climate change will also result in increasing climatic variability: more frequent dry spells and flooding, more erratic rainfall patterns, and larger year-to-year variability in precipitation levels. (Devlin & Hendrix, 2014: 28)

A country could experience several years of general drought (deviation), but also experience more volatility in those years with greater winter floods and summer droughts compared to other drought-stricken states. Larger deviations from long term climate averages (e.g. a severe drought) and increased variance of short-term weather patterns (e.g. much higher and much lower average temperatures in the same year) create uncertainty in interstate relationships by altering the feasibility of status quo agreements and raising the stakes of contested issues. Agreements governing shared resources are more difficult to strike and maintain in times of climate uncertainty (Devlin & Hendrix, 2014: 30) and few existing treaties include stipulations for variability management for droughts or floods (De Stefano et al., 2012). Greater volatility in climate conditions also increases the salience of contested resources, as states secure control of resources to minimize future drought situations. ‘Rapid change can lead to uncertainty about property rights or the disposition of resources, which in turn can lead to conflict’ (Gartzke, 2012: 180). We expect that diplomatic and militarized conflict is more likely when states experience greater volatility in mean temperatures/precipitation.

For an example of how climate variability influenced the onset of diplomatic conflict through the mechanism of uncertainty, consider Figure 2 and the plot of Bolivia’s and Chile’s precipitation deviation levels from 1981 to 2001 in Figure 3. Bolivia experienced large variance in its precipitation deviations prior to initiating a river claim against Chile in 1939 (Lauca River). Chile also experienced greater variance in precipitation deviations prior to initiating a river claim against Bolivia in December 1999.¹³ Bolivia began to accept bids for water rights in the Silala river, which Chile claimed was an international river (and thus not under Bolivia’s control). This followed Bolivia’s implementation of tighter border controls and a new military outpost along the river in response to its perception that Chile was trying to divert water from the Silala. As both countries began to

Figure 3.

Precipitation and river claim onset for Bolivia and Chile, 1981–2001

experience greater volatility in water supplies, they pressed diplomatic claims over shared river waters more intensely. Our general expectation is that we should observe a U-shaped relationship between climate volatility and interstate conflict given that large short-term shifts in precipitation or temperature can generate uncertainty.

Hypothesis 2 (Uncertainty): Higher volatility in temperature and precipitation deviations increases the likelihood for issue claim initiation and militarization.

Issue specific expectations for climate change

While we expect climate changes to influence all diplomatic interactions through the scarcity and uncertainty mechanisms, we also recognize that mechanisms connecting climate variability and conflict may vary across diplomatic issues. In this section, we describe how climate variability influences territorial, maritime, and river issues distinctly.

First, sovereignty claims over territory can be influenced by climate change. Resource stocks and agricultural productivity of land can change because of global warming (e.g. desertification), which generates revisionist behavior with respect to neighboring states’ territories. Busby (2008: 477) notes that climate change creates direct homeland threats by altering the territorial border or waters of a country and by increasing transnational threats. Gartzke (2012) argues that rapid climate change increases uncertainty about property rights, which could lead declining powers to be more territorially revisionist. Guo’s (2007) survey of more than 200 disputed areas since World War II finds support for this type of hostile lateral pressure, as resource scarcity at home prompts many states to make claims to neighboring states’ territories. Precipitation changes can alter the course of rivers that form the border between two countries, prompting the need for renegotiation of the boundary.

Second, river claims are also more likely to occur in the face of climate changes, something that pundits warned about decades ago when discussing the potential for water wars (Cooley, 1984). Most militarized disputes that have occurred over river claims have taken place in the most water scarce region of the world, the Middle East (Hensel, Mitchell & Sowers, 2006). Increasing global temperatures affect water evaporation and rainfall patterns, altering water quantity and water quantity and quality in river basins (Bauer, 2011). Many major river basins in the world are facing 25% to 50% reductions in average water flows by mid-century, creating serious water scarcity problems in the future (Eckstein, 2009). States recognize these potential risks for future water scarcity when designing variability management clauses in river treaties (De Stefano et al., 2012). Uncertainty in future water supplies increases the salience of river basins and can lead to revisionist behavior if future river flow variability goes beyond the bounds of existing models (Dinar el al., 2015). A country like Egypt, for example, which is projected to face major water shortages in cities like Cairo, may convert its behavior of verbal threats of force towards states on the Nile (e.g. Ethiopia) to militarized actions as uncertainty about its future water supplies grows. The river literature identifies a curvilinear relationship between water scarcity and cooperation (Dinar, 2009), which implies that climate induced river conflict is most likely at very high or very low precipitation values. This fits with our second hypothesis about greater volatility making diplomatic and militarized conflict more likely.

Finally, maritime claims are also influenced by climate change and variability. Warmer oceans increase scarcities in many fisheries’ stocks by changing migration patterns, increasing fish mortality rates, and changing water acidity levels (Sumaila et al., 2011). The issue approach shows that migratory fish stocks are the leading cause of militarized disputes for maritime claims (Nemeth et al., 2014), thus climate changes exacerbate these risks. Increasing global temperatures also make some areas of the ocean more accessible (e.g. Arctic, Northern Sea Route), which could create new diplomatic conflicts between states. Rising sea levels and increased coastal erosion will also have detrimental effects on existing baselines for maritime boundaries. This occurs because maritime baselines are ‘ambulatory’ such that ‘if the baseline moves, the boundary moves’ (Caron, 2009: 9). Maritime claims around islands are much larger than claims around rocks and rising sea levels will turn many islands into rocks.¹⁴ Some countries (e.g. Japan) spend a great deal of money to fortify ‘rocks’ (e.g. Okinotorishima reef) with steel and concrete to help prevent further erosion and solidify EEZ claims around their ‘islands’. Climate change creates uncertainties for maritime boundaries and could lead states to question the ‘fairness of past delimitation agreements with neighboring states’ (Caron, 2009: 13). Some island states face existential threats as sea levels rise. Climate variability should have general effects on interstate conflict patterns, but the mechanisms by which it produces diplomatic conflicts may vary across issues.

Research design

We test our hypotheses by employing models of issue claim onset and militarization in the Western Hemisphere and Europe from 1901–2001 with ICOW datasets.¹⁵ We utilize politically relevant directed dyad-years as the unit of analysis for the claim onset model because these pairs of states have high opportunities to have diplomatic conflicts.¹⁶ Politically relevant dyads include contiguous states (through a land border or a water border up to 400 miles) or contain at least one major power.¹⁷ We employ directed dyads in order to give both dyadic partners the potential to initiate a claim as well as be targeted in a claim. We use the challenger vs. target distinction as coded by ICOW, but we allow each state in the opportunity group to be a potential challenger and a potential target. For the Western Hemisphere and Europe, these coding rules create 68,708 total observations for the issue onset model. To examine the conditions under which issue claims militarize, we utilize ICOW issue claim dyad-years as the unit of analysis. This captures every year that a diplomatic claim is ongoing (e.g. 1939–78 for the Bolivia–Chile Lauca river claim). For the militarization model, we have 6,152 total issue claim dyad-year observations.

Our dependent variables are issue claim onset and militarized settlement attempts over issue claims as coded by ICOW. First, Issue claim onset captures whether any new diplomatic issue claims (territory, river, or maritime) occur in a politically relevant directed dyad-year. Issue claim onset is coded 1 if at least one issue claim was initiated by a potential challenger in a year and 0 otherwise. There are 168 issue claim onsets (0.24%). Second, Militarization captures any militarized settlement attempts over a claimed issue between a pair of states. Militarization is coded 1 if at least one militarized dispute occurred over the issue in a year and 0 otherwise (Hensel et al., 2008). There are 198 MIDs (3.2%).

To capture climate variability, we utilize information collected by the Climate Research Unit (CRU) at the University of East Anglia (2019) and derived from more than 4,000 weather stations around the world.¹⁸ First, to test our scarcity (abundance) hypothesis, we follow existing literature (Hendrix & Salehyan, 2012) to capture countries’ standardized deviations in precipitation and temperature from their long-run averages. We subtract

Table I.

Example of climate change variables, USA and Canada

State	Year	Precipitation (mm)	Standardized precipitation	Temperature (° Celsius)	Standardized temperature
USA	1920	737.1	0.402	7.9	–1.560
USA	1921	687.5	–0.698	9.3	1.733
USA	1922	717.6	–0.031	8.5	–0.149
USA	1923	744.1	0.558	8.5	–0.149
USA	1924	654.4	–1.432	7.8	–1.796
–	–	–	–	–	–
CAN	1920	494.8	–1.308	–5.8	–0.685
CAN	1921	501.1	–0.980	–5.5	–0.255
CAN	1922	480.1	–2.074	–6.1	–1.116
CAN	1923	494.1	–1.345	–5.8	–0.685
CAN	1924	490.6	–1.527	–5.7	–0.542

each yearly observation from a country’s long-run mean (1901–2001) and then divide by the standard deviation of the country’s time series. Positive deviations suggest the country is experiencing above average precipitation (floods) or temperature (heat waves), whereas negative deviations suggest the country is experiencing below average precipitation (droughts) or temperatures (cold spells). For example, consider the first few values for these measures for the United States and Canada in Table I; the complete time series are plotted in Figures 4a and 4b. In 1920, precipitation in the United States was above average (0.402 deviations), but temperature was below average (–1.560 deviations). Compare this to Canada in 1920, which had below average precipitation and temperatures (–1.308 and –0.685, respectively). As we see in Figures 4a/4b, the use of deviations helps to standardize climate effects across countries that have different means (e.g. Canada’s average temperature is much lower than the USA’s average temperature). Our theory suggests that larger positive or negative deviations in climate variables are associated with more interstate conflict, thus we include squared terms for the standardized climate variables.

To test our uncertainty hypothesis, we add volatility variables to our models following Jones, Mattiacci & Braumoeller’s (2017) approach. The volatility measure is a standardized variable that compares variance within a single year (using monthly data) to the variance for a country over all years, with larger values implying much more localized variance in precipitation and temperature compared to historical data. For example, prior to initiating the 1999 river claim against Bolivia, Chile experienced more monthly variation in rainfall compared to previous years. This generated more uncertainty for Chile about its future water quantity and increased the country’s incentives to protect its transnational water supplies through diplomacy.

The dependent variables are binary, therefore we use logistic regression with robust standard errors. We include other control variables used in previous issue approach research (Hensel et al., 2008). First, we control for relative capabilities between the two dyad members by measuring the challenger’s total share of dyadic CINC capabilities (military, economic, and demographic).¹⁹ Second, we include a variable for democratic dyad if both countries score six or higher on the aggregated Polity IV scale.²⁰ Third, we include the logged distance between the dyad members’ capital cities. Finally, we control for the temporal dependence in the dependent variables using years since last issue onset (Diplomatic peace years) and years since last militarization (Militarized peace years).²¹ See Tables A1 and A2 in the Online appendix for summary statistics and correlations.

Empirical findings

Testing the scarcity (abundance) hypothesis

We begin by evaluating evidence for the scarcity/abundance hypothesis, which posits that as temperature or precipitation deviations increase, interstate issue claims are more likely to be initiated (Table II) and to be militarized (Table III). First, we look at Table II, Model 4 where we include all types of ICOW issues in the same model. Several climate deviation parameters for the challenger state are significant, while all climate parameters for the target state are insignificant, showing that revisionist states seeking to change the issue status quo are

Figure 4a.

Precipitation in the USA and Canada Figure 4b. Temperature in the USA and Canada

influenced more by climate variability than target states who are defending their existing claims to territorial, maritime, and river areas. Challenger temperature is negative and statistically significant, while challenger precipitation is insignificant. However, the squared term for the challenger state’s precipitation deviation is positive and significant. To see the relationship more clearly, we plot the marginal effects for the potential challenger’s climate variables in Figures 5 and 6.

The results show support for the scarcity (abundance) hypothesis, but only for precipitation deviations. Figure 5 shows that when challenger states are experiencing droughts (negative deviations) or floods (positive deviations), they are more likely to initiate diplomatic conflicts. Scarcity or abundance of water can push states to make claims against neighboring states to defend or expand water resources or seek more agriculturally productive land. The effect is substantively strong: going from a ‘normal year’ (0 deviation) to a flood year (+2.5 deviation) is associated with a 73% increase in the probability of claim initiation (0.0015 compared to 0.0026). The effect is even higher going from a normal year (0) to a drought year (–2.5

Table II.

Effect of climate variability on issue claim onset (PR dyads), 1901–2001

	Standardized precipitation and temperature				Volatility
	Model 1: Territory	Model 2: River	Model 3: Maritime	Model 4: All	Model 5: Territory	Model 6: River	Model 7: Maritime	Model 8: All
Potential challenger
Precipitation	−0.031	−0.224	0.078	−0.039
	(0.130)	(0.199)	(0.109)	(0.084)
Precipitation squared	0.205**	0.038	0.124*	0.100*
	(0.079)	(0.111)	(0.054)	(0.048)
Temperature	−0.170	−0.194	−0.045	−0.205†
	(0.187)	(0.292)	(0.190)	(0.112)
Temperature squared	−0.051	0.158	−0.521**	−0.119
	(0.108)	(0.140)	(0.185)	(0.083)
Precipitation volatility					0.007	−0.251	0.081	−0.049
					(0.143)	(0.220)	(0.115)	(0.086)
Temperature volatility					0.168	0.214	0.301*	0.233*
					(0.121)	(0.233)	(0.131)	(0.093)
Potential target
Precipitation	0.045	0.580*	−0.236†	−0.059
	(0.188)	(0.286)	(0.131)	(0.097)
Precipitation squared	−0.296	−0.368†	−0.006	−0.100
	(0.220)	(0.222)	(0.069)	(0.070)
Temperature	−0.163	0.001	−0.007	0.011
	(0.197)	(0.265)	(0.149)	(0.104)
Temperature squared	−0.098	0.125	0.013	0.044
	(0.136)	(0.160)	(0.094)	(0.073)
Precipitation volatility					0.173	0.335	0.081	0.124
					(0.128)	(0.204)	(0.121)	(0.085)
Temperature volatility					0.177	−0.075	−0.051	0.000
					(0.125)	(0.252)	(0.122)	(0.089)
Controls
Relative capability	−0.635†	−2.185**	0.808**	−0.086	−0.588†	−2.233**	0.804**	−0.101
	(0.357)	(0.704)	(0.308)	(0.218)	(0.341)	(0.665)	(0.301)	(0.209)
Democratic dyad	0.087	1.166**	0.289	0.464**	0.006	1.130**	0.246	0.426*
	(0.287)	(0.380)	(0.250)	(0.165)	(0.291)	(0.369)	(0.250)	(0.166)
Distance	−0.277***	−0.503***	−0.109***	−0.217***	−0.273***	−0.505***	−0.109***	−0.216***
	(0.037)	(0.090)	(0.029)	(0.021)	(0.038)	(0.092)	(0.030)	(0.021)
Diplomatic peace year	−0.023**	−0.013	−0.007	−0.013***	−0.026**	−0.013†	−0.009*	−0.015***
	(0.007)	(0.008)	(0.004)	(0.003)	(0.008)	(0.007)	(0.004)	(0.003)
Constant	−4.808***	−5.596***	−6.176***	−4.606***	−4.886***	−5.427***	−6.349***	−4.591***
	(0.305)	(0.525)	(0.321)	(0.209)	(0.242)	(0.395)	(0.297)	(0.183)
Observations	64034	64034	64034	64034	64034	64034	64034	64034

*** p < 0.001, ** p < 0.01, * p < 0.05, † p < 0.1.

deviation): a 100% increase in the probability of initiation (0.0015 compared to 0.003). These empirical patterns are driven primarily by conflicts involving territory (Model 1) and maritime (Model 3) issues.

Counter to our expectations, higher temperature deviations appear to decrease the probability of a state initiating a diplomatic conflict (Table II, Model 4). This suggests that states experiencing heatwaves are less likely to initiate diplomatic conflict, a finding that is consistent with other studies showing less interstate conflict in times of global warming (Gartzke, 2012). Figure 6 shows the substantive effect of temperature deviations for potential challengers. Going from a normal year (0 deviation) to a heatwave year (+2.5 deviations) is associated with a 300% decrease in the probability of a challenge (0.002 to 0.0005), while going from a normal year to a colder year (–2.5 deviations) is associated with a 33% decrease in the probability of a challenge.

Table III.

Effect of climate variability on militarization (claim dyad years), 1901–2001

	Standardized precipitation and temperature				Volatility
	Model 1: Territory	Model 2: River	Model 3: Maritime	Model 4: All	Model 5: Territory	Model 6: River	Model 7: Maritime	Model 8: All
Challenger
Precipitation	0.424**	0.193	0.087	0.262**
	(0.126)	(0.936)	(0.119)	(0.082)
Precipitation squared	0.107†	0.155	0.063	0.087*
	(0.058)	(0.401)	(0.084)	(0.044)
Temperature	0.239	−0.972	0.049	0.127
	(0.148)	(0.766)	(0.185)	(0.113)
Temperature squared	−0.006	0.423**	−0.019	−0.002
	(0.077)	(0.140)	(0.086)	(0.052)
Precipitation volatility					0.058	0.524	0.056	0.058
					(0.109)	(0.355)	(0.116)	(0.078)
Temperature volatility					0.146	1.235**	0.140	0.177*
					(0.117)	(0.412)	(0.136)	(0.089)
Target
Precipitation	−0.286*	−1.093	−0.020	−0.177*
	(0.127)	(0.787)	(0.121)	(0.082)
Precipitation squared	−0.008	0.008	−0.036	−0.013
	(0.068)	(0.180)	(0.078)	(0.050)
Temperature	−0.076	1.342	0.248	0.078
	(0.146)	(0.818)	(0.188)	(0.104)
Temperature squared	0.018	0.119	0.003	0.041
	(0.075)	(0.255)	(0.087)	(0.048)
Precipitation volatility					0.054	−0.023	0.016	0.034
					(0.103)	(0.360)	(0.130)	(0.077)
Temperature volatility					0.148	−0.100	0.035	0.088
					(0.109)	(0.404)	(0.167)	(0.093)
Controls
Relative capability	0.716**	3.734*	0.104	0.534**	0.727**	2.343†	0.060	0.534**
	(0.251)	(1.697)	(0.339)	(0.187)	(0.237)	(1.246)	(0.326)	(0.182)
Democratic dyad	−0.394	−1.632	0.042	−0.379*	−0.361	−0.044	0.152	−0.252
	(0.297)	(1.786)	(0.246)	(0.179)	(0.294)	(1.278)	(0.250)	(0.178)
Distance	−0.131***		0.007	−0.076***	−0.120**		0.007	−0.072**
	(0.036)		(0.033)	(0.022)	(0.035)		(0.033)	(0.021)
Mid peace year	−0.056***	−0.012	−0.041**	−0.048***	−0.058***	0.011	−0.040**	−0.048***
	(0.013)	(0.045)	(0.013)	(0.009)	(0.014)	(0.024)	(0.013)	(0.009)
Constant	−2.570***	−7.552***	−3.164***	−2.804***	−2.437***	−6.418***	−3.127***	−2.712***
	(0.234)	(1.058)	(0.359)	(0.183)	(0.211)	(0.956)	(0.306)	(0.161)
Observations	3085	420	2611	6134	3085	420	2611	6134

*** p < 0.001, ** p < 0.01, * p < 0.05, † p < 0.1.

Next (Table III, Model 4 and Figures 7 and 8) we evaluate how the climate deviation variables relate to militarization of ongoing issue claims. First, we find that the challenger’s precipitation deviations (and squared deviations) are positively and significantly related to militarized disputes. Figure 7 shows that the challenger’s abundance of precipitation increases conflict risks more than scarcity of rainfall. The probability of an issue militarizing when the challenger is experiencing normal levels of precipitation is 0.018 compared to 0.057 when experiencing floods (217% increase) and 0.016 when experiencing droughts (13% decrease). These effects are driven mostly by territorial conflicts (Table III, Model 1) and suggest that excessive flooding creates incentives for leaders to capture contested territory with force. For example, Venezuela experienced significant rainfall amounts and greater precipitation volatility in 1998 in the year before initiating a militarized conflict against Guyana over

Figure 5.

Challenger state’s precipitation on probability of issue claim onset

Figure 6.

Challenger state’s temperature on probability of issue claim onset

contested resources in the Essequibo claim. Guyana had been seeking greater control of water (hydroelectric dams) and oil resources in the area, prompting a militarized response from Venezuela (the challenger).²² On the other hand, militarized conflicts are more likely when the target state experiences large negative rainfall deviations or droughts (left side of Figure 8). Climate change variables do not significantly influence militarization of maritime claims (Table III, Model 3), which is interesting because over 25% of maritime claims have at least one militarized dispute (Hensel & Mitchell, 2017). This may reflect the fact that rising sea levels will disrupt maritime

Figure 7.

Challenger’s precipitation on probability of issue militarization

Figure 8.

Target’s precipitation on probability of issue militarization

boundary baselines, but that changes on this front have been accumulating only slowly to date.

Overall, our tests for Hypothesis 1 reveal that climate variability influences diplomatic conflicts and issue militarization, but mostly for revisionist states and for countries experiencing larger variability in rainfall. Contrary to our expectations, higher deviations in annual temperatures are associated with lower risks for new geopolitical conflicts. Yet this may reflect the general decline in territorial claims over time, with territorial claim cases dominating our analyses since we code onset only in the first year of an issue claim.

Testing uncertainty hypothesis

The uncertainty hypothesis anticipates that greater short-term volatility in weather patterns increases the risks for

Figure 9.

Challenger’s temperature volatility on probability of issue claim onset

diplomatic and militarized conflict. We add the volatility measure to our baseline models (Models 5–8) for issue claim onset (Table II) and militarization (Table III).²³ The results support Hypothesis 2, but only for temperature volatility. Greater monthly variance in temperatures significantly increases the likelihood of issue claim onset (all issues and maritime issues) and issue militarization (all issues and river issues). The risks for diplomatic conflict are three times higher for states with high (vs. average) temperature volatility as we see in Figure 9. Militarized conflict is 50% higher for states experiencing the highest volatility in monthly temperatures compared to states with average volatility (Figure 10). As we observed with our scarcity (abundance) tests, the effects are significant for challengers but not targets. While we see strong effects for challenger precipitation (squared) deviations on interstate conflict, these results show that temperature volatility for the challenger state also increases the risks for diplomatic and militarized conflict. More generally, climate variability acts as a conflict trigger for states seeking to change ownership of territorial or water resources.

Discussion

Our climate variables are created using a fixed period to define sample means. To capture the changing trends in precipitation and temperature over time, we estimate models with climate variables generated with a 30-year moving average or a recursive rolling average (Online appendix, Tables A9a,b, A10a,b). We continue to find that challenger precipitation deviations and temperature volatility increase risks for issue claim onset and militarization. However, the challenger precipitation variables have weaker effects in some models, in part because we lose many observations with this modeling strategy (e.g. up to 44% of the sample).

While politically relevant directed dyads create a comparable opportunity set across diplomatic issues, it is possible that these criteria include dyads with few opportunities for issue claims which could weaken our results. For example, river claim onset models (Brochmann & Hensel, 2009) focus on states in shared river basins, while maritime claim occurrence models examine coastal states in the same region (Daniels & Mitchell, 2017). We replicate our findings using previously published territorial, river, and maritime claim onset models. Territorial claim onset is estimated for states sharing contiguous land borders in all regions (Online appendix, Table A5a). We find similar results; the squared term for challenger precipitation deviations is positive and significant. In the militarization model (Online appendix, Table A5b), squared deviations are also positive and significant for challenger precipitation and target temperature. Our uncertainty results are stronger when looking at all territorial claims globally and including only countries that share contiguous borders in the onset model.

Figure 10.

Challenger’s temperature volatility on probability of issue militarization

For river claim onsets, we replicate Brochmann & Hensel (2009) in Tables A6a (claim onset), A6b (negotiation onset), and A6c (militarization) (see Online appendix). These models allow us to match climate conditions to upstream and downstream status in a river basin. We get stronger support for the scarcity (abundance) hypothesis in this restricted river basin dyad analysis, as higher levels of rainfall in the upstream state significantly reduce the chances for new diplomatic conflicts between riparian states. We also find that increases in temperature deviations in the upstream state increase the potential for diplomatic river clashes.

Maritime claim occurrence (Online appendix, Table A7a) and militarization (A7b) is modeled using the Daniels & Mitchell (2017) dataset. We find that higher levels of precipitation deviation (and the squared term) are positively related to the onset of maritime claims, while higher temperatures tend to decrease maritime competition. Militarized disputes are more likely as precipitation increases, but less likely as both states experience increases in average temperatures. Overall, when we use a more restricted opportunity set for issue onset, we see that climate variables have stronger effects in these models compared to our results.

Finally, we recognize the geographical and temporal limitations of our analyses. In the Online appendix (Tables A3a,b, A4a,b), we present models for our two regions separately. Our combined regional findings are very similar in the Americas, but somewhat weaker in Europe (especially for volatility). This demonstrates that there is variability in states’ vulnerability to weather shocks across regions, with states in the Americas facing higher vulnerability on average. Yet our replication of territorial claims globally shows that our results are stronger when expanded to all geographic contexts. In terms of the project’s temporal limitations, climate variability has accelerated since the end of the ICOW dataset (2001) as we see in Figures 1a and 1b. Countries are experiencing greater climate deviations and volatility on average in the last 20 years; thus, our results imply that the interstate conflict risks we identify may be increasing.

Conclusion

This project considers how greater exposure to extreme weather events and volatility in temperatures and precipitation influence the likelihood of interstate conflicts through the theoretical mechanisms of scarcity and uncertainty. Our study moves beyond the literature’s focus on intrastate violence and our empirical results help us understand the connections between extreme weather events and international conflict.

First, we show that precipitation deviations and volatility strongly influence the behavior of revisionist states seeking to change diplomatic issue status quos, such as challenging existing territorial borders or riparian water rights. This suggests that agreements designed to resolve competitive land and water issues must consider ways to compensate states more negatively impacted by climate changes, such as downstream states in a river basin experiencing droughts.

Second, we find that precipitation shocks/volatility have more consistent interstate conflict-inducing effects than temperature shocks/volatility. Given the trends in our climate variables since the end of our ICOW data towards greater volatility (2002–16, Figure 1), diplomatic issues involving water and marine/riparian resources may increase in frequency in the future. Our curvilinear results for precipitation deviations and interstate conflict also confirm existing findings connecting scarcity and conflict in river basins.

Third, we find that territorial disputes are strongly affected by climate deviations and volatility. Issue claims discussed in the article involving Bolivia–Chile and Venezuela–Uruguay help us understand why we see these patterns. Many dyads like Bolivia and Chile contest their land borders before expanding their rivalry to other diplomatic issues. If climate deviations trigger land disputes, they are very likely to generate multiple points of contention over river and maritime boundaries as well, which increases the risks for militarized disputes.

Fourth, some of our empirical results are not significant or consistent with our theory, but we use a very conservative estimation strategy for our models, employing all politically relevant dyads. Results in the Online appendix show that when we use a narrower set of case selection rules for the opportunity set (e.g. states with contiguous land borders, states with shared river basins, or states in the same coastal areas), our results linking climate deviations and interstate conflict are much stronger.

We could do more to analyze how issue salience changes in response to climate shocks. We identified several climate consequences that are likely to provoke interstate disputes (e.g. decline in agricultural productivity, desertification, decreasing water supplies, lost fisheries’ stocks) and we could explore these intervening mechanisms more fully. We can connect climate variability to other diplomatic issues such as identity claims that capture interstate disagreements over the treatment of ethnic kin. Neighboring states might provide military support to insurgents across the border if the government fails to respond adequately to disasters that affect shared ethnic kin (e.g. 1970 Bhola cyclone). Understanding how climate changes influence interstate conflict is imperative.

Footnotes

Replication data

The dataset and do-files for the empirical analysis in this article, along with the Online appendix, can be found at http://www.prio.org/jpr/datasets and .

Acknowledgements

We are grateful to the National Science Foundation, Department of Defense (Minerva), and USAID for funding to support the collection of ICOW data.

ORCID iD

Bomi Lee

Sara McLaughlin Mitchell

Notes

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