Being president of the United States is a job that can be about life and death. Donald Trump learned that first-hand recently as the first American serviceman died under his watch. This is a reality of being president, as is the security threat posed by a changing planet.

When President Donald Trump became the commander-in-chief of the U.S. Armed Forces on Jan. 20, he accepted the responsibility to defend the country against all enemies, foreign and domestic. Climate change is high on that list.

As President Trump and Secretary of State Rex Tillerson will soon learn, it’s high atop of the list of every head of state, from Europe to Asia, and in critical clubs like the G20 and NATO.

Indeed, it’s also high on the priority list of many within their own party. Just last week, veteran Republicans with years of collective experience in international diplomacy, including former Secretaries of State James Baker and George P. Shultz, publicly called for a carbon tax that would give an incentive to U.S. companies to find the most efficient ways to reduce their carbon emissions.

Even if it might be convenient, there are no alternative facts when it comes to climate change. The science is clear and irrefutable — climate change is a clear and present danger to the United States. We know already that it is among the key forces displacing populations, leading to massive migrations that are straining stability and fuelling terrorism.