The topic didn't come up much at either the Republican or Democratic debates last month.

Moderators of Thursday's Republican presidential debate didn't ask candidates a single question on climate change.

Nor did the topic come up much at the Republican and Democratic debates last month.

President Barack Obama has linked climate change to national security, and a 2014 Pentagon report called it a "threat multiplier." A changing climate can limit the availability of natural resources, increase migratory pressures and lead to instability and conflict.

Sen. Ted Cruz (R-Texas), a Republican front-runner in Iowa, has questioned the science behind global warming, even though scientists say there is overwhelming evidence that the climate is warming. Other GOP candidates have said that the threat posed by climate change pales compared with other national security threats the United States faces.

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