Poverty and violence in Central America are major factors driving migration to the United States. But there's another force that's often overlooked: climate change.

Retired Lt. Cmdr. Oliver Leighton Barrett is with the Center for Climate and Security. He says that in Guatemala, Honduras and El Salvador, crime and poor economic conditions have long led to instability.

"And when you combine that with protracted drought," he says, "it's just a stressor that makes everything worse."

Barrett says that with crops failing, many people have fled their homes.

"These folks are leaving not because they're opportunists," he says, "but because they are in survival mode. You have people that are legitimate refugees."

So Barrett supports allocating foreign aid to programs that help people in drought-ridden areas adapt to climate change.

"There are nonprofits that are operating in those countries that have great ideas in terms of teaching farmers to use the land better, to harvest water better, to use different variety of crops that are more resilient to drought conditions," he says. "Those are the kinds of programs I think are needed."

So he says the best way to reduce the number of climate change migrants is to help people thrive in their home countries.