Is climate change a matter of national security? In a warming world, sea-level rise, drought and soil degradation are putting basic human needs such as food and shelter at risk. In March, the U.S. Department of Defense called climate change a "threat multiplier," saying that competition for resources "will aggravate stressors abroad such as poverty, environmental degradation, political instability and social tensions—conditions that can enable terrorist activity and other forms of violence."
Connecting climate change to a global increase in violence is tricky, and attempts to make such a link receive a fair amount of criticism. A hotter planet doesn't automatically become a more conflict-ridden one. The 2000s, for instance, saw some of the highest global temperatures in recorded history—and some of the lowest rates of civil conflict since the 1970s.
But there are historical examples of civilizations that did not fare well when faced with drastic environmental change, and those examples may offer a window into the future—and even help prevent catastrophe. "We can never know with 100-percent certainty that the climate was the decisive factor [in a conflict]," says Solomon Hsiang, assistant professor of public policy at the University of California, Berkeley. "But there's a lot of cases where things look pretty conspicuous."
The Akkadian Empire
Around 2350 B.C., the Akkadian empire conquered and united the various city-states of Sumer in Mesopotamia. For almost two centuries, this powerful empire stretched from the Mediterranean Sea to what is now inner Iran, setting up vast stretches of agricultural land and trade routes. Then, around 2100 B.C., the empire collapsed, and the land remained unsettled for nearly 300 years.
Archaeologists attributed the empire's abrupt end to invasions and political strife. But in one region, formerly the center of the empire’s grain production, the soil also held an intriguing clue: a thin layer of volcanic ash covered by a thicker layer of wind-blown silts. That region, it seemed, suffered from a sudden shift to more arid conditions.
In 2000, an international group of scientists studied marine sediment cores taken from the Gulf of Oman, more than 1,000 miles from what would have been the heart of the Akkadian empire. From these cores, the scientists were able to create a holistic picture of climate in the region. They found distinct peaks of the minerals calcite and dolomite beginning around 2025 B.C. that lasted approximately 300 years. These minerals are transported to the ocean as dust from dry, arid regions, so their abundance suggests that the collapse of the Akkadian empire must have been caused, at least in part, by a rapid and unprecedented drying, which in turn led to mass migrations, overcrowded cities and eventually, internal violence within the empire.
The history of China is often told in dynastic cycles, where one family takes control of the country for hundreds of years until, for social or political reasons, they fall from power. Dynastic collapses were almost always followed by years of turmoil, which eventually led to the introduction of another ruling family.
But there's another crucial cycle that has ruled China for thousands of years: the monsoon. In 2008, researchers found a 2,000-year-old record of China's monsoon activity, in the form of a stalagmite that grew continuously between A.D. 190 and 2003. Stalagmites form as mineral-rich water drips onto the floor of a cave, so the chemical signatures in successive layers gave the researchers a glimpse of China's monsoon cycles over two millennia. When compared with China's historical record, periods of strong monsoon activity coincided with periods of prosperity—increased agricultural production, increased population and general peace. In contrast, periods of low monsoon activity lined up with periods of drought—and the turbulent declines of three major Chinese dynasties.
The Maya grew during a time when the Central American region was very wet, from A.D. 440 to 660. Writings and pictographs on the ruins of Maya cities tell the story of three stages of collapse, with the empire contracting abruptly as inhabitants moved from cities near the edge of the empire to cities closer to its center. Proposed reasons for the decline vacillate between the very plausible, such as foreign invasion, and the outlandish, including alien interference.
In 2003, a group of scientists looked at ocean sediment cores from the Cariaco basin, which lies off the northern coast of Venezuela. The concentration of titanium and iron swept to sea by rainfall in the various layers allowed the team to build a picture of the climate, and especially the rainfall amounts, during the Maya civilization's decline. The three contractions lined up pretty closely with three major drought events. One drought period in particular—from the years 600 to 1000—seems to have been especially marked by social conflict. Archaeologists have found stone monuments built between A.D. 750 and 775 that honor of 39 different rulers, evidence that those 25 years were filled with "rivalry, war and strategic alliances."
Europe During the Little Ice Age
The Little Ice Age was a period between about 1300 and 1870 when the Northern Hemisphere saw markedly colder winters, and glaciers expanded on mountains in various locations. The intervals of regional drying and cooling that happened during this time might have contributed to periods of increased violence. Looking at temperatures between 1400 and 1900, a group of scientists spearheaded by David Zhang at the University of Hong Kong found that "periods of relative peace and turbulence during those 500 years were a global phenomenon seemingly linked to temperature change."
The study looked at how cold, dry periods affected the land's ability to produce ample resources. A decrease in temperature and rainfall, the scientists noted, is likely to adversely affect agricultural production, which in turn can lead to famines, epidemics and armed conflicts. For instance, the Thirty Years’ War, the most violent conflict in Europe’s history, occurred during the coldest period of the Little Ice Age.
21st Century Climate Conflicts
Modern society often has the benefit of industrial agriculture, such as better irrigation or drought-resistant crop varieties. The hope is that these technologies will lessen the threat of conflict due to agricultural failings. "There is a lot of optimism with innovation in agriculture, and some of that is warranted, because we've done some amazing things," Hsiang says. "But some things we can't fix." Corn, for example, has become more sensitive to heat and drought—even with technological advances in planting and genetics.
The Center for Climate and Security argues that modern, human-induced climate change is already having violent repercussions, such as the recent Syrian uprising. "[The Bashar al-]Assad regime’s failure to effectively manage water and land resources, the drought, and the subsequent displacement of close to 1.5 million people contributed to social unrest," the center says in a 2012 report. Looking toward the future, U.S. security officials worry that Asia-Pacific, which could face the displacement of millions of people due to sea-level rise, might be the next location to face a breakdown in security due to climate change.
But it's not necessarily all violence on the horizon; some research has shown that countries are far more likely to cooperate over water shortages than go to war because of them, which is an encouraging precedent for a world on the brink of unprecedented change.