It's a hot day; sweat soaks your forehead, and you just want to get back to your air-conditioned home, so of course you're going to be angry if the car in front of you doesn't move when the traffic light turns green. Honk! Honk!
When the temperature rises, so does aggression -- and that can lead to large-scale consequences, considering that climate change is turning up the heat over the entire planet.
A new study in the journal Science shows that shifts in climate historically have been associated with violent conflicts, among both individuals and groups, and that current warming patterns could significantly increase the abundance of human conflict by midcentury.
Researchers' meta-analysis of 60 studies suggest that, consistent with links between conflict and climate shifts in the past, the risk of intergroup conflict around much of the planet would be amplified by 50 percent in 2050.
"It does change how we think about the value of avoiding climate change," said Solomon Hsiang, lead study author and researcher at the University of California, Berkeley. "It makes us think that avoiding climate change is actually something we should be willing to invest more in."
Future populations will hopefully also be able to figure out ways to adapt to these climate events, such that the predicted increase in violence will become less likely. Large-scale changes in technologies or political institutions may also alter the risk of violence in the future, Hsiang said.
A brief history of climate change and violence
Research on the topic of climate change and violence has been exploding onto the scene in recent years, study authors noted, with 78 percent of the studies included in this analysis having been published since 2009. Hsiang and colleagues incorporated research on civilizations dating back as far as 10,000 B.C. and across all major world regions.
Studies both in laboratory settings and of "natural" human situations have found a connection between heat and violence, the study said. Higher temperatures have been linked to both innocuous hostile behaviors, such as horn-honking while driving, and more serious behaviors such as domestic violence within households, assault and rape. Police officers are also more likely to use force at higher temperatures, studies have found.
Conflict is also associated with extreme rainfall, particularly in societies dependent on agriculture. Higher rates of personal violence are found in low-income settings, where agriculture income suffers from extremely wet or dry conditions.
The Mayan civilization appears to have collapsed during long periods of drought, Hsiang said. The same global climate event seems to have brought down the Tang dynasty in China -- in fact, according to Hsiang, most Chinese dynasties collapsed during dry spells.
The climate conditions that contributed to the decline of civilizations in both Central America and China look like an El Nino event, Hsiang said.
That directly relates to Hsiang's 2011 study in the journal Nature suggesting that the El Nino Southern Oscillation played a role in 21 percent of civil conflicts from 1950 to 2004. Evidence points to poorer countries of the tropics, such as Sudan and Rwanda, as being most affected in this way, whereas Australia did not have El Nino-related conflicts.
That's not to say that El Nino, a large movement of water in the Pacific Ocean that occurs about every four to five years, can be said to cause any given civil conflict. In fact, it's impossible to prove that any particular war or act of violence is the direct result of climate change. There are certainly other factors that cause and insight violence of all kinds.
But Hsiang's new study reinforces the idea that dramatic climate shifts increase the odds of violent conflict, as if loading a deck of cards in favor of this gruesome outcome.
Similarly, no individual hurricane or hot day in our time is definitively "caused" by human-induced climate change, but it does make extreme events such as last year's Superstorm Sandy more likely, said J. Marshall Shepherd, president of the American Meteorological Society and professor at the University of Georgia.
How much more violence, and why?
When Hsiang and colleagues put together evidence from 190 researchers, they determined that intergroup conflict increases 14 percent and interpersonal violence increases 4 percent for each standard deviation in climate variables. For instance, in terms of temperature, one standard deviation is about halfway between what you would expect for an average year and a really hot year.
"If your local level of violence increased by 10 percent to 15 percent, you would probably notice that," Hsiang said.
Poorer populations have been shown in other studies to be more vulnerable to shifts in climate generally, consistent with findings that such shifts raise the risk of violence among them in a more substantial way. It is possible, however, that the definitions of conflict outcomes or measurement errors could have influenced these results, the study authors noted.
As discomforting the analysis may be, Thomas Homer-Dixon, political scientist at the Balsillie School of International Affairs in Waterloo, Ontario, sees Hsiang's new study as strong overall. The conclusions are dramatic, he said, but the methods to compare various studies are sound.
What exactly explains these patterns concerning climate and violence? More research needs to be done on mechanisms, but scientists have some ideas based on available evidence.
Shifts in temperature and rainfall influence economic productivity and food prices, which may influence discontent and therefore riots. Climate changes can force the displacement of population, or urbanization, which may lead to clashes over resources.
There's also the mental and physical responses to rising temperatures, which need further scientific investigation. "For instance, climatic events may alter individuals' ability to reason and correctly interpret events, possibly leading to conflicts triggered by misunderstandings," the authors wrote.
Implications of climate change and violence