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 governments could be so preoccupied with managing the effects that they have little time or energy left to engage on broader foreign policy issues.


 * Fueled by unusually warm Arabian Sea waters, two separate tropical cyclones hit Yemen in 2015 in the span of just 10 days, including the first hurricane-strength storm to hit the country in recorded history. Already suffering a humanitarian crisis from war and water shortages, Yemen was unable to provide adequate relief for its citizenry. Heavy rains have since fueled the breeding of an unusually large population of desert locusts that threaten to devastate Yemen’s agriculture, and efforts to eradicate the locusts have been stymied both by the difficult security situation and by fear of killing bees, a crucial pollinator for the region’s honey and crop production.

Heightened Social and Political Tensions
Decreases in water and disputes over access to arable land will increase the risk of conflict between people who share river basins, aquifers, or land areas. Although historically water disputes between states have led to more water-sharing agreements than violent conflicts, according to academic studies, we judge that this trend could change over the next 20 years as water scarcity and variability intensify, possibly leading to disputes that pose greater challenges to security. Although environmental stress is rarely the sole cause, disputes between groups within countries over land and water resources are increasingly common as triggers for social violence and internal conflict, particularly when social and political tensions already exist.


 * In 2014, citizens in a village on the outskirts of Mexico City, already water-stressed by drought, battled antiriot police during a protest over the diversion of spring water to a new development nearby. More than 100 police were injured, many seriously.
 * In 2014, farmers and herdsmen in Nigeria clashed over access to grazing land and dwindling well water. Then President Goodluck Jonathan ordered military operations to reduce the violence.
 * In 2012, mass protests and violence erupted over water shortages in Nouakchott, Mauritania. More than 70,000 refugees had migrated to Mauritania by July that year because of deteriorating conditions in neighboring Mali, putting additional pressure on Mauritania’s water and soil resources, already strained by drought and desertification.

Even if climate-induced environmental stresses do not lead to conflict, they are likely to contribute to migrations that exacerbate social and political tensions, some of which could overwhelm host governments and populations. Sudden extreme weather—such as from floods, droughts, and severe tropical storms—almost certainly will increase the number of displaced people, particularly in regions that are unaccustomed to or unprepared for such events. Rising sea levels and unexpectedly large storm surges could threaten small island states and low-lying coastal regions—including many megacities—with flooding and saltwater contamination of freshwater. Over 20 years, the net effects of climate change on the patterns of global human movement and statelessness could be dramatic, perhaps unprecedented. If unanticipated, they could overwhelm government infrastructure and resources, and threaten the social fabric of communities. 7