The Democratic debate in Iowa began with a moment of silence for the victims of the Paris terror attacks before pivoting to a discussion on how to address terrorism.

Bernie Sanders, who vowed to "rid our planet" of ISIS in his opening statement, also said at a previous debate that the greatest threat to national security is climate change. A day after the terrorist attacks, did he, asked moderator John Dickerson, still believe that?

"Absolutely. In fact, climate change is directly related to the growth of terrorism," Sanders said on Nov. 14. "If we do not get our act together and listen to what the scientists say, you’re gonna see countries all over the world — this is what the CIA says — they’re going to be struggling over limited amounts of water, limited amounts of land to grow their crops, and you’re going to see all kinds of international conflict."

A day later on CBS’ Face the Nation, Sanders doubled down on his statement, elaborating, "When people migrate into cities and they don't have jobs, there's going to be a lot more instability, a lot more unemployment, and people will be subject to the types of propaganda that al-Qaida and ISIS are using right now."

We were curious about the link between climate change and terrorism that Sanders highlighted.

While there is a body of literature backing his broader point that climate change contributes to the growth of terrorism, Sanders is overstating the "direct" connection.

A complicated relationship

The Sanders camp referred us to statements from President Barack Obama and leaders of the defense community, as well as a Defense Department report that suggests indirect links between climate change and terrorism:

• Obama in a May 2015 speech: "Understand, climate change did not cause the conflicts we see around the world. Yet what we also know is that severe drought helped to create the instability in Nigeria that was exploited by the terrorist group Boko Haram."

• The Defense Department in a 2014 report: "In our defense strategy, we refer to climate change as a ‘threat multiplier’ because it has the potential to exacerbate many of the challenges we are dealing with today — from infectious disease to terrorism. We are already beginning to see some of these impacts."

• Adm. Samuel J. Locklear, then-commander of the U.S. Pacific Command, said in 2013 that the significant upheaval related to the warming planet "is probably the most likely thing that is going to happen … that will cripple the security environment."

We found a few other federal and academic reports naming climate change a threat to national security, but that’s not the same thing as "directly related to the growth of terrorism."

That’s not to say that climate change isn’t contributing to existing problems. Democratic presidential candidate Martin O’Malley used more accurate language when he said, "One of the things that preceded the failure of the nation-state of Syria and the rise of ISIS was the effect of climate change and the mega-drought that affected that region, wiped out farmers, drove people to cities, created a humanitarian crisis." We rated his comments Mostly True.

Sanders was more sweeping in his comments, though.

"I wouldn’t have said it as strongly, but (climate change) is an accelerant to conflict and helps produce instability," said Daveed Gartenstein-Ross, a senior fellow at the national security think tank the Foundation for Defense of Democracies. "Within the national security sphere, more and more see climate change as a real problem. They would agree with the broader point."

Climate change, however, is not "hermetically sealed from other risks," writes Francesco Femia and Caitlin Werrell of the Center for Climate and Security in a blog post. "The impacts of climate change interact with other factors to make existing security risks worse."

That’s why experts and the Pentagon prefer the term "threat multiplier." Here’s how: Climate change can lead to food and water scarcity, which may in turn increase poverty, the spread of disease, and mass migration — in short, instability. In places with already weakened governments, this breeds the conditions for terrorism to thrive.

Factors that contribute to terrorism

Of course, there are myriad conditions that motivate terrorism.

Ideology and political interests are much more direct factors, according to Michael Doran, who served on the National Security Council and in the Defense Department under President George W. Bush.

The latest Global Terrorism Index report lists political, nationalist and separatist movements as well as weak political systems and a lack of political legitimacy as main drivers. It found that countries with high levels of terrorism share three "statistically significant factors":

• Hostility between different ethnic, religious and linguistic groups;

• State-sponsored violence such as extrajudicial killings, political terrors and high levels of group grievances;

• Violence from organized conflict, demonstrations and crime, and perceptions of criminality.

According to the Global Terrorism report, 66 percent of deaths from terrorist attacks in 2013 were caused by four extremist groups: ISIS, Boko Haram, the Taliban and al-Qaida. Their commonality is, obviously, religious extremism.

Our ruling

Sanders said, "Climate change is directly related to the growth of terrorism."

We couldn’t find any evidence of a "direct" relationship between climate change and terrorism, though many reports have noted an indirect link. There are, of course, many other factors that contribute to terrorism, including religious and ethnic tensions and political repression.

We rate Sanders' claim Mostly False.