From time to time, people still ask me why I was so focused on global climate change as Secretary of State.

It's not simply because climate change is a threat to the environment. Nor is my agenda as a lifelong environmentalist.

It's because -- by fueling extreme weather events, undermining our military readiness, exacerbating conflicts around the world -- climate change is a threat to the security of the United States and to the security and stability of countries everywhere. And guess what? No country can solve it alone.

In other words, climate change is a matter of national security -- and solving it is a job that demands diplomacy, the tool of the trade when it comes to foreign policy.

"...climate change is a threat to the security of the United States and to the security and stability of countries everywhere ... No country can solve it alone ... climate change is a matter of national security -- and solving it is a job that demands diplomacy, the tool of the trade when it comes to foreign policy."

When we talk about climate change, we are talking about how climate's impact on people -- people everywhere -- from severe droughts and rapid sea level rise. We're talking about how it impacts whole cities due to unpredictable and uncontrollable extreme weather events. We're talking about its impact on entire countries -- how it causes fundamental shocks to the global agricultural system.

These impacts bring with them any number of social and political consequences. In response, we have to strengthen our national security readiness in order to deal with the possible destruction of vital infrastructure and the mass movement of refugees, particularly in parts of the world that already provide fertile ground for violent extremism and terror. So, make no mistake, and I know I've said it before -- climate change is not just about Bambi; it's about all of us in very personal and important ways.

"...we have to strengthen our national security readiness in order to deal with the possible destruction of vital infrastructure and the mass movement of refugees, particularly in parts of the world that already provide fertile ground for violent extremism and terror."

Don't just take my word for it. A group of 16 retired three-and four-star flag and general officers from the Army, Navy, Air Force, and Marine Corps, who make up the Center for Naval Analysis released a report on the accelerating national security impacts of climate change. Attached to the report was a letter, signed by all 16 officers, imploring readers to look beyond the polarizing public discourse and understand the severity of what, I quote, "so many believe to be a salient national security concern for our nation." As they put it, "Time and tide wait for no one."

The physical security implications are straightforward. The summer after I became Secretary of State, tragedy hit the Philippines. I visited Tacloban weeks after Typhoon Haiyan. No words can do justice to the level of destruction that I saw: an entire community leveled; water up to the second floor of the airport's traffic control tower; cars, homes, and lives turned upside-down. And as we flew in, we saw trees scattered like toothpicks along the mountainside. And most devastating of all, the storm destroyed more than a million homes and killed more than 6,000 men, women, and children.

"A group of 16 retired three-and four-star flag and general officers from the Army, Navy, Air Force, and Marine Corps, who make up the Center for Naval Analysis released a report on the accelerating national security impacts of climate change."

And when extreme weather leads to natural disasters and humanitarian suffering, our military responds and they do so bravely and with great skill, but it means taking our troops away from work on other important missions.

Which leads me to another major factor and another major reason that climate change is a security threat: It has a direct impact on military readiness.

Norfolk Naval Station is the biggest naval installation -- not just in the United States, but in the world. The land it is built on is literally sinking. The Virginia Institute of Marine Science has projected that if the current trends hold, the sea in Norfolk could rise by five and a half feet or more by the end of this very century. That's within the life expectancy of a newborn child today. Think about what that could mean for that base and the military community around it -- and for the 28 other military facilities in the Hampton Roads area. Think about what that could mean for the U.S. Navy fleet -- 20% of which is home-ported nearby.

"What's causing most of this sea level rise is the one-two punch that is mandated by the laws of science -- not politics -- science: As ocean water warms, it expands. But that's not all: As the atmosphere warms, ice all over the world melts."

What's causing most of this sea level rise is the one-two punch that is mandated by the laws of science -- not politics -- science: As ocean water warms, it expands. But that's not all: As the atmosphere warms, ice all over the world melts. Now, we're seeing this dramatically in the Arctic -- from the glaciers of Alaska, to the massive Ice Sheet of Greenland.

If our military vehicles are unable to move because they're up to their axles in water and all the roads leading into and out of a base are flooded, that affects military readiness.

Similarly, if the high risk of wildfires prevents our troops somewhere from training with live ammunition, that affects readiness.

If the permafrost our Alaska bases are built on begins to thaw out, as it is in some places, and then becomes less stable, that affects military readiness.

"If our military vehicles are unable to move because they're up to their axles in water and all the roads leading into and out of a base are flooded, that affects military readiness."

And the direct impacts on our military's ability to defend our nation are not the end of the peril that climate change could pose to our national security; they're just the beginning. This is a point my friend and former Senate colleague from Virginia, Senator John Warner, made eloquently and repeatedly. He was chairman of the Senate Armed Services Committee and secretary of the Navy. And in testimony before the Senate Foreign Relations Committee, Senator Warner said, "Climate change is a 'threat multiplier' making worse the problems that already exist.”

And we're already seeing that happen. In Nigeria, climate change didn't lead to the rise of the terrorist group Boko Haram. But the severe drought that that country suffered -- and the government's inability to cope with it -- helped create the political and economic volatility that the militants exploited to seize villages, butcher teachers, and kidnap hundreds of innocent school girls.

"Scholars suggest that access to air conditioning could well mean the difference between life and death in such hot spots as Kuwait, Qatar, the UAE."

Similarly, it's not a coincidence that immediately prior to the civil war in Syria, the country experienced its worst drought on record. As many as 1.5 million people migrated from Syria's farms to its cities, intensifying the political unrest that was just beginning to roil and boil in the region.

Climate change wasn't the primary reason for the crisis in Syria -- not even top five. The war was launched by a brutal dictator who began attacking, torturing, and barrel-bombing his own people. But the drought that devastated communities across the country exacerbated instability on the ground and made a bad situation worse and forced people to migrate, so you had a mixing in a very sectarian place, where, at a sectarian time of definition, where people were exploiting that sectarianism, that made a ready pool of recruits for violence.

A study was published indicating, quote, "the combination of high temperatures and humidity could, within just a century, result in extreme conditions around the Persian Gulf that are intolerable to human beings, if climate change continues unabated." Scholars suggest that access to air conditioning could well mean the difference between life and death in such hot spots as Kuwait, Qatar, the UAE.

And the prospect of a hotter, drier climate throughout the Middle East and parts of Asia will place even more strain on the most precious and essential resource of all -- fresh water. We've already seen tensions rise around the basins of the Nile River in Africa, the Indus River in South Asia, and of course, the Mekong River in Southeast Asia.

"Our future national security strategy is also going to be affected by what's going on in the Arctic. The melting of the polar cap is opening sea lanes that never before existed. There is already potential for a global race to exploit the resources of the region."

Consider that almost every country with land borders shares some international river basins with its neighbors. Historically, this has led to more cooperation than conflict. But if the water starts to disappear -- and climate change is expected to significantly alter both access to and the availability of fresh water -- imagine the tensions that can rise. Pressures and demands will steadily increase, and the future may look very different from the past.

Our future national security strategy is also going to be affected by what's going on in the Arctic. The melting of the polar cap is opening sea lanes that never before existed. There is already potential for a global race to exploit the resources of the region. Everybody knows Russia planted a flag on the seabed of the North Pole. Other countries are up there, too -- China and others, with their ships, mapping out the exploitation of resources, including oil, natural gas, fish. Economic riches tend to attract military interest as nations seek to ensure their own rights are protected. And we know, because we track it, that these countries -- like Russia, China, and others -- are active in the Arctic. And that China is modernizing and expanding its navy.

The impacts of climate change can exacerbate resource competition, threaten livelihoods, and increase the risk of instability and conflict, especially in places already undergoing economic, political, and social stress. Climate change raises all the stakes, and we need to ensure that we are taking the necessary steps to prevent new competition from leading to conflict. And because the world is so extraordinarily interconnected today -- economically, technologically, militarily, in every way imaginable -- instability anywhere can be a threat to stability everywhere. The kind of strife that we’re talking about will not be able to be contained by international borders.

"The impacts of climate change can exacerbate resource competition, threaten livelihoods, and increase the risk of instability and conflict, especially in places already undergoing economic, political, and social stress. Climate change raises all the stakes..."

We have to integrate climate considerations into every aspect of our foreign policy -- from development and humanitarian aid to peacebuilding and diplomacy. But that begins by understanding and accepting the complex links between climate change and national security. It begins by better identifying "red flags" of risk around the world, so our diplomacy and development assistance can better target and enable those nations to become more resilient, more secure, and less likely to fall into a full-fledged war or humanitarian crisis.

But none of it happens without American leadership -- leadership that doesn't just accept the truth, but acts on it. When I was in the Navy, we used to have a saying: "Ship, shipmate, self." Putting the greater good above one's personal interests is second nature to our military men and women.  On climate change -- that is our charge, that is our duty -- for our ship of state, for our shipmates, and for the generations that follow in their footsteps, we have to get this right.