When asked during the Democratic debate Tuesday night to name the biggest national security threat facing the United States, Sen. Bernie Sanders did not bat an eye before answering: "The global crisis of climate change."

"The scientific community is telling us: if we do not address the global crisis of climate change, transform our energy system away from fossil fuels to sustainable energy, the planet that we’re going to be leaving our kids and our grandchildren may well not be inhabitable," Sanders said. "That is a major crisis."

Highlighting their more establishment—and hawkish—perspectives, his Democratic rivals responded with more 'on the ground' threats: current frontrunner and former Secretary of State Hillary Clinton pointed to nuclear proliferation; former Rhode Island Gov. Lincoln Chafee, named the turmoil in the Middle East; former Maryland Gov. Martin O'Malley said a nuclear Iran; and former Virginia Sen. Jim Webb mentioned China, cyber warfare, and the Middle East.

However, the Independent senator from Vermont's response most closely echoed the concern of a majority of people worldwide, particularly those in the global south, where communities are most at risk from the impacts of global warming. U.S. military leaders, as well as President Barack Obama, have also identified global warming as an "immediate risk to our national security"—which viewers Monday evening were quick to note:

Greatest national security threat? #Climate change. Just ask some Generals. #DemDebate — David Turnbull (@david_turnbull) October 14, 2015

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Overall, environmentalists and progressives were thrilled to see the threat of climate change given due attention.

I think @BernieSanders is exactly right--biggest threat to natl sec. is climate change. And he's been leading the charge for a long time now — Bill McKibben (@billmckibben) October 14, 2015

Thank you @BernieSanders for giving the big picture view: Earth's climate crisis is the biggest threat. #DebatewithBernie #DemDebate — NationalNursesUnited (@NationalNurses) October 14, 2015

Greatest global threat? Climate change says Bernie. Crowd at @Neumos erupts in cheers, including us! #DemDebate #DebateWithBernie — Vote Sawant (@VoteSawant) October 14, 2015

Meanwhile, those on the other side of the political spectrum had their own take on the matter.