In recent years, many foreign affairs experts have attempted to demonstrate the linkages between climate change and the social tensions that can lead to conflict. While critics may believe this is simply a fad in international affairs, history suggests otherwise. Over the last few millennia, climate change has been a factor in conflict and social collapse around the world. The changing climate has influenced how and where people migrate, affected group power relations, and provided new resources to societies while taking away others. Such circumstances cause large-scale alterations in lifestyles and illustrate pathways from climate change to conflict.
Because climate change can be a contentious subject, it's worth taking a moment to answer some basic questions and put forth a series of assumptions. First, what is climate change? The Intergovernmental Panel on Climate Change, the most authoritative source on the subject, assesses climate change by measuring changing temperatures and precipitation. Since trends in temperature often (but don't always) drive trends in precipitation, scientists consider temperature a more robust and stable measure of climate change. But in simplest terms, climate change is the long-term change in the patterns of these two meteorological characteristics. Second, does climate change affect the world the same everywhere? In fact, climate change is a heterogeneous phenomenon and produces different outcomes in different places. The subsequent case studies demonstrate that a changing climate can have acute regional effects such as near the equator or North Pole, the "hot" and "cold" zones, respectively. In both cases, "hot" and "cold" conflicts demonstrate how rising and falling temperatures have had different impacts on human survival and prosperity.
Finally, a fact: The relationship between climate and conflict isn't as simple as cause and effect. Instead, climate change events--such as temperature shifts of a few degrees or a precipitation change of a few inches--contribute to conflict gradually over the long term. Because the climate has been changing for millennia, it's possible to look at the past for examples. Do we see cases of climate-conflict interaction when rates of climate change have diverged? In fact, it's possible to assess historical events and records in order to construct pictures of how climate affects conflict. Historical case studies, therefore, allow us to identify three paths from climate change to conflict: sustained trends, intervening variables, and the need for conflict triggers.
Consider each of the paths. The first is that conflict has the potential to emerge after a sustained period of divergent climate patterns. While people can survive aberrant, short-term climate change by exploiting existing or stored resources, this strategy has temporal limits. On this particular path the issue isn't one of surviving an especially fierce rain or harsh winter, but the cumulative effects of many fierce rains and many harsh winters. Next, climate change alone won't cause conflict but, along with other factors, will contribute to and shape it. It's one variable among many others, such as cultural, economic, or demographic factors. Last, unless a society learns to adapt to sustained climate change, its wealth will decline and its social fabric will weaken with each passing year. But even if a society faces these environmental challenges, a trigger--such as an assassination, extreme natural event, or random act of group violence--is usually required to ignite violent conflict.
We can, nevertheless, draw lessons from natural (climate) and human (conflict) interaction that may be transferable to today's global climate challenge by considering three periods in human history: human pre-history (the time before written records), the medieval climate optimum, and the "little ice age."
During human pre-history, probably in today's Middle East region, humans found plentiful game and encountered the Neanderthals. Over time, humans pushed into Europe and forced the Neanderthals further north into the less hospitable parts of the continent, where game wasn't as abundant and temperatures were much colder. Although the Neanderthals had survived several ice ages over the course of hundreds of thousands of years, they couldn't survive both an ice age and the humans, who enjoyed advanced weaponry and social organization. Of course, theories about the end of the Neanderthals are controversial and unresolved. However, there is no question that climate change provoked the interaction of human societies and the Neanderthals and subsequently led to conflict.
The medieval climate optimum lasted from 500 to 1000. It brought about a period of sustained progress in Europe as warmer conditions allowed for longer growing seasons in the largely agricultural societies. However, conditions elsewhere were quite different. Between 700 and 900, rainfall in China was scarce due to weak summer monsoons that failed to develop over the Pacific Ocean. Gerald Haug and other researchers have concluded that famines caused by the failed monsoons resulted in peasant revolts and fueled the intrastate conflict that drastically weakened, and then led to the complete collapse of, the Tang Dynasty.
Across the globe in North America, the Mayans had settled in the lowlands around 8000 BC and began practicing large-scale farming as early as 2000 BC. By the beginning of the medieval climate optimum in AD 500, the population was nearly 14 million, making it one of the largest centers of civilization anywhere. But the thriving Mayan cities began to experience diminished long-term rainfall patterns. Dry conditions began in 760 and, after a 50-year wet period, drought again set in about 860. Another drought followed in 910. The boom-and-bust cycles of rainy and dry periods contributed to eras of both growth and decline. Technology, population sizes, and agricultural intensity overwhelmed the land. Yields declined with the dry conditions and these structural incongruities led to ongoing wars between Mayan city-states that eventually contributed to their collapse.
The warming in Central America that was disastrous for the Mayans was, on the other hand, fortunate for the Vikings. Warmer temperatures in the north meant their land was more hospitable to live on. Complex push-and-pull factors allowed the Vikings to expand their settlements from Scandinavia to Iceland, Greenland, and later Newfoundland. It was in Newfoundland that they encountered Native Americans. The Native Americans, too, perceived the warmer climate as a new opportunity and fought to control the increasingly abundant land. The Vikings and Native Americans would alternately trade and fight with each other throughout the Vikings' time in Newfoundland.
Enter the "little ice age"--a period marked by abnormally cooler temperatures. Scholars differ on the exact duration of this period; some researchers believe it started as early as 1000 in certain northern regions, whereas other historians, such as noted scholar Brian Fagan, believe it lasted from 1300 to 1850. Regardless, when the climate turned cold, the Viking colonies that had flourished in the warmth of the medieval climate optimum collapsed in Newfoundland (which they had abandoned because of ongoing conflict with the Native Americans). The western Greenland colony was the next to collapse, and around 1350, coinciding with the time of the Black Death, the eastern colony also began to decline. It survived only into the early 1500s.
During the same period, the Anasazi, a hunter-gatherer people who over centuries settled into a sedentary lifestyle, lived along the rivers of what is today the Southwest United States. With gradual improvements in technology and a beneficial climate, their population grew. But the little ice age brought a period of long-term drought, and Anasazi population growth exceeded its resource base. Timber, game, and other resources had to be imported from neighboring areas. The Anasazi had survived a long-term drought and many smaller ones in their long history. So why were they unable to cope with the little ice age drought? Like the Mayans, Anasazi city-states came into conflict as resources dwindled.
Because of its recency, scholars are able to theorize more completely about the little ice age. There were sharp extremes in temperature during the period. In fact, it had two temperature low points, one during the late 1400s and early 1500s, and another during the late 1700s and early 1800s. During the latter extreme cold period, a catastrophic geologic event occurred. In April 1815, Mount Tambora, on the Indonesian island of Sumbawa, threw a massive amount of volcanic dust into the atmosphere in one of the largest volcanic explosions in modern times. The volcanic dust travelled worldwide and blocked the sun's rays, lowering temperatures, especially in the northern hemisphere. The period of extreme cold, coupled with this sudden volcanic eruption, produced the "year without summer."
The year without summer illustrates two different stories--one in Europe and another in North America. In Europe, the cold forced people to migrate. But land across the continent was relatively densely settled, so conflict often came with population movement. On top of this structural problem, the Napoleonic wars culminated shortly before, sapping the vitality and stability of the economy. People were already suffering through instability, but the cold compounded social strife. There were no "pressure valves," such as open land, for populations under temperature-induced duress. The result was social upheaval, riots, and disease.
The cold was just as bad in eastern North America and Canada, where most summer crops were lost. But unlike Europe, there was ample land for climate migrants west of the United States. The building of the Erie Canal in 1817 gradually opened up a westward route for easy migration. Thus, where in Europe people lacked the pressure valve necessary to cope with difficult times, in North America the valve not only existed but its exploitation was encouraged.
These are just a few accounts that allow us to synthesize history into a set of discernible, often regionally specific, lessons on conflict manifestations for today. The impact of climate change is obviously differential in the "hot" and "cold" wars, where trends may reward one part of the world while punishing another--for example, in terms of economic subsistence. In particular, four lessons of conflict emerge from the aforementioned cases that may have application for today's challenges:
Today, we see the manifestations of climate change slowly emerging in melting glaciers and drying fields. We need to imagine how changes in climate will create possibilities for conflict, using these historical lessons as guides. It's important to note that reacting to the challenges of "hot" and "cold" wars will require different strategies. The past is a good guide, but new types and modes of conflict emerging from climate change are to be expected. In that respect, finding historical examples is easier than contemplating how climate change and conflict will create new models of interaction and present new challenges in the future. After all, the idea that climate change causes conflict is not revolutionary, but evolutionary.
Editor's note: This article has been modified to correct an error in the original version.