Residents collect drinking water from a municipal tanker in Kolkata, India, on July 8. India's National Institution for Transforming India has warned that many states in the country will gradually run out of water because of climate change and population growth. (Piyal Adhikary/EPA-EFE)

During last month’s U.S. Democratic presidential candidate debates, former congressman Beto O’Rourke, Sens. Elizabeth Warren and Cory Booker, and former housing and urban development secretary Julián Castro all identified climate change as a major geopolitical threat facing the United States. Some even gave it equal billing with China and the prospect of nuclear war.

The United Nations warned that there’s now a climate change crisis every week — after cautioning that a “climate apartheid” would divide the world between those who have the means to adapt to higher temperatures and those who don’t. And experts identify Europe’s extreme heat wave as a taste of what is likely to happen more often because of climate change.

The human consequences of climate change include more food insecurity, reduced crop yields, increased migration and dramatic damage to coastal communities — including large cities such as Miami, New Orleans and New York. Less clear is how climate change will affect conflict between individuals, armed groups and national armies.

Consensus on this relationship has been difficult — until now. But our work as part of a team led by Katharine Mach has resulted in a clear message: Climate and climate change have had modest effects on past conflicts, but these effects are expected to get much larger in the future.

Five years ago, there was little discussion about climate change and conflict.

The last Inter-Governmental Panel on Climate Change (IPCC) 5th Assessment in 2014 was relatively muted about whether climate change would contribute to a rise in conflict. One of the reasons was the relative infancy of scholarship linking climate change to conflict.

Recently, research on the topic has surged — but different disciplines take different approaches, and there’s a lack of information about regions beyond sub-Saharan Africa and Asia. Experts don’t agree on how to study the effects of climate change on conflict. Some argue that increased temperatures and erratic rainfall have a strong effect on violence at all levels, from the individual to between national armies. Other researchers challenge this view and stress how non-climate factors raise conflict risk.

To reconcile these differences, an expert elicitation process began in 2017. Here’s what this means: Eleven experts discussed these topics both independently, with the “elicitors,” and then together for several days — until the group reached consensus about what they agreed upon — and what they didn’t.

This approach is quite different from the standard mode of scientific discourse, where authors hash out their differences in peer-reviewed journals. Here, the elicitor approaches the subject by trying to mine these different positions for consensus — building on shared ground in an inclusive, face-to-face deliberative process while still acknowledging ongoing areas of disagreement.

These experts — including the two authors of this article — came from the disciplines of political science, geography, environmental studies and economics. Their previous research came to different conclusions about climate-conflict links. The task: Reach a consensus on the evidence of violence related to climate change over the past century and what would be likely to occur in the future under scenarios of 2- and 4-degree Celsius increases in the global temperatures.

What did they find?

The expert consensus, just published in Nature, is that climate variability has contributed modestly to the risk of conflict to date and is less important than other contributing factors. The figure below maps contributing factors from most to least important, and indicates a lot of uncertainty about climate change effects vs. a much greater certainty for the major factors that cause conflict — such as weak economies with high unemployment and poverty, poor governance and institutions and inequalities between groups within countries. The dark blue bars indicate how important the experts thought the factor was (longer = more important); the light blue bars indicate how certain they were about the importance of the factor (longer = less certain).

But if the globe does not reduce emissions, the risk of climate-induced violence is fivefold under a business-as-usual scenario (4-degree C increase), up from the 5 percent average effect over the past century.


Factors that drive conflict risk and their relationship to climate in experiences to date. Reproduced with permission of Nature: copyright by the authors.

Why will climate change raise the risk of conflict?

The factors behind any conflict are a mix of political, economic, social and environmental elements that intersect in locally sensitive ways. This suggests it’s a mistake to designate any individual conflict, such as the Syrian civil war, as a climate war. Journalists and policymakers are often keen to link complex outcomes like civil wars to straightforward causes — such as drought. But the story is almost always much more complex.

Instead, the idea that climate change has a “threat multiplier” effect, adding to already-stressed societies, is gaining traction. It’s difficult to parse out the individual effect of any one factor on violence since its influence will vary from country to country. But cumulatively, we can speak of probabilities: Think of climate change as “loading the dice,” making conflict more likely to occur in subtle ways across a host of different country contexts. In that sense, the future looks bleak unless the world changes its current trajectory.

Here’s why this will be important:

The Trump administration announced its intent to withdraw from the 2015 Paris climate agreement and rejects the scientific consensus on the need to curb global temperature increases, silencing its own government policy advisers and removing climate change references from public reports. While majorities in the United States — and in the U.K. — believe a crisis looms, there are few signs our global community is willing to curb emissions because of the costs and the inconveniences to rich societies and the perfectly natural desire of developing countries to enjoy the fruits and comforts of carbon-intensive development.

As our expert elicitation project makes clear, climate change does pose security threats. Security threats from climate change are just some of the reasons to care about our global environment. Ignoring the consequences of climate change could well be catastrophic for poorer countries. But there is growing evidence to suggest that the rebound effects from seemingly far-off lands are likely to be more consequential than we think.

Editor’s note: This article has been amended to clarify the Trump administration’s intent to withdraw from the 2015 Paris climate agreement.

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John O’Loughlin, college professor of distinction at the University of Colorado at Boulder, is a political geographer with research interests in the human outcomes of climate change in sub-Saharan Africa and in the geopolitical orientations of people in post-Soviet states.

Cullen Hendrix, professor and director of the Sié Chéou-Kang Center for International Security and Diplomacy at the Korbel School of International Studies, University of Denver, is a political scientist with research interests in the environment, political violence, and the security implications of climate change.