Yesterday, President Obama gave a speech to graduating cadets at the US Coast Guard Academy in which he said that climate change "constitutes a serious threat to global security [and] an immediate risk to our national security."

"Even as we meet threats like terrorism," he said, "we cannot, and we must not, ignore a peril that can affect generations."

This is not the first time the federal government has characterized climate change as a security threat. The fact sheet distributed by the administration yesterday quotes reports going back to 2008. And as Philip Bump writes in the Washington Post, the rhetorical effort to link climate to security goes all the way back to Bill Clinton.

Each time it comes up, large swaths of the media treat it as a bold new argument, which is probably a good sign that despite all the efforts, it's still not a natural, instinctive connection for most people. It sounds novel.

Still, Obama's speech was the clearest articulation of the argument to date. So what should we make of the claim? Does climate change threaten our national security? What does that even mean, exactly?

It's complicated. I've been thinking about this on and off for several years now, and I've come to four conclusions. (A top five list would have been more satisfying, but it turns out I only know four things.)

1) Climate change is a threat; whether it's a "national security threat" is a matter of semantics

Brad Plumer wrote yesterday about an IMF report that uses the word "subsidy" in a slightly novel way, to refer to money not charged to fossil fuel companies to pay for the environmental and health damages of their products. Brad asked, Is that a good way to use the word?

And that's the right question. Language evolves; familiar words and concepts expand or take on new meanings. There's no right answer to whether an uncharged tax "really is" a subsidy. It is if we choose to use the word "subsidy" that way. The question is whether viewing the phenomenon through that lens is useful, whether it clarifies or illuminates.

Something similar is going on in calling climate change a national security threat. It's best seen not as a neutral description of the world (there is no such thing) but as a kind of conceptual gambit, an attempt to change the way we think — about climate change and about national security.

Certainly climate change isn't like other things we think of as security threats. It won't invade a neighboring country or plant a bomb on a plane. It has no intentions, no volition, no ill will. It is not even an "it," really, so much as a statistical trend.

Instead, climate change is a "threat multiplier" that can, in the fact sheet's words, "exacerbate existing stressors, contributing to poverty, environmental degradation, and political instability, providing enabling environments for terrorist activity abroad."

Climate change doesn't directly cause, say, armed conflicts. It raises the probability of armed conflicts, by increasing the likelihood of drought, water and food shortages, and forced migrations.

It remains to be seen whether this gambit will be politically effective. Lots of people have high hopes for this messaging — and have for a long while — but whatever its effects inside the military, it hasn't changed the political valence of the climate issue in any discernible way.

Republican reaction to the president's speech amounted to mockery and repeated invocations of ISIS. "Our adversaries are not motivated by the weather; they are emboldened by America's withdrawal from the world," said Rep. Mac Thornberry of Texas. Sigh.

Turns out it's easy to get people riled up about threats with scary faces, but getting them riled up about probabilities is a bit more difficult. Then again, messages change things, if at all, not through cleverness but through repetition, so it remains to be seen whether there's any long-term shift in public opinion on this.

2) The US military will mostly deal with climate change preparation and response (a.k.a. "adaptation")

Obama's primary message was that all branches of the military, including the Coast Guard, need to prepare for a warmer planet and a more volatile geopolitics. Especially in poor and low-lying areas of the world, climate impacts will put new strains on military resources.

The US armed forces will be called upon to join peacemaking and humanitarian missions in the wake of resource-driven conflicts and weather-related disasters. They will need to reinforce or relocate military bases, both domestic and international, that are located near coasts. They will be mobilized to protect Arctic resources newly uncovered by melting ice.

We think of security threats as something we fight, but none of this will look, on the ground, like fighting climate change. It will look like coping with a fractious world, as the US military has done for decades. The climate change threat will mostly be visible as a statistical trend, a rise in the number of disruptions.

3) Prevention (a.k.a. "mitigation") is an ineffective national security tool

The tricky pivot in Obama's speech — and in the climate-as-national-security message generally — is from preparation and response to prevention (or "mitigation," as it's opaquely known).

That climate impacts will generate sociopolitical unrest makes sense; that the military will be called on to respond to unrest makes sense; that it should prepare in advance for that task makes sense. The more difficult case to make is that the military ought to be involved in reducing its own emissions — not just responding to climate impacts, but also helping prevent (some of) them.

This is where Republicans are attacking, where they think the greening of the military appears "ideological" and thus vulnerable. Last year, the GOP House of Representatives stuck an amendment onto the National Defense Authorization Act specifying that no defense funds could be used to implement the recommendations of the IPCC, the US government's National Climate Assessment, or, uh, the UN's Agenda 21. "Why should Congress divert funds from the mission of our military and national security to support a political ideology?" asked Rep. David McKinley (R-WV).

This line of attack is potent for the simple reason that mitigation does not actually address any short-term security threats. It doesn't address any short-term anything. There's a time lag in the atmospheric carbon dioxide cycle; it's about 25-50 years between the time carbon dioxide is emitted and the time it begins affecting global temperatures. Consequently, it takes about 25-50 years between the time CO2 emissions are prevented and the time any temperature rise is prevented.

Mitigation is certainly a smart public policy. It makes sense for the US of today to protect the US of 2065. But to have a substantial effect on temperature, the US will eventually have to be joined by almost all the world's countries, in an effort that will take the rest of the century. Compared to humanity's total carbon budget, the amount of CO2 emitted by the US military is a relative pittance. The military alone cannot have any discernible effect on the rate or severity of global impacts through mitigation.

So is reducing emissions an appropriate use of the military budget? The Navy is looking into running its fleet on biofuels, which are four times more expensive than petroleum fuel. Some US bases are looking into becoming zero-carbon and grid-independent — through on-site power generation, storage, and self-contained microgrids — which will cost considerably more than grid power. In a time of strained budgets, these programs will continue to draw controversy, even if the climate-as-security meme catches on.

4) However, mitigation can be a form of adaptation, because fossil-dependence is a vulnerability

A few years ago, I wrote a piece for Outside magazine about a US Marine Corps program aimed at using renewable energy on the battlefield as a way of reducing the need for fuel resupply convoys, which in Iraq and Afghanistan had become a primary point of vulnerability to attacks and IEDs. Carrying lightweight, roll-up solar panels allowed expeditionary units to go longer without battery resupply. Carrying small, unfolding solar panels (that can be hitched to the back of a truck) allowed patrol bases to run their computers without the loud drone of diesel generators.

The broad point, made to me by several people up and down the military chain of command, is that the military's total reliance on oil represents an enormous vulnerability. In the theater of operations, it makes military units heavy and slow-moving. (In the famous sprint to Baghdad early in the Iraq War, units outran their fuel supply convoys and had to stop and wait.)

It's also a financial burden: Secretary of the Navy Ray Mabus told me that every time the price of oil rises by a dollar, it costs the Navy $31 million in increased operational costs. Similar figures are true for every branch of the military -- and the price of oil is not something the US military can control.

In a similar vein, the push to make military bases independent of the power grid is about reducing their vulnerability to brownouts and blackouts — which, given the thoroughly computerized nature of military operations today (ahem, drones), is more than an inconvenience.

Most people I spoke to in the military, particularly the Marines, weren't all that occupied with the environmental damages of climate change. They weren't interested in social causes or ideology. But they were very interested in reducing the military's use of fossil fuels, which amounts, in this case, to the same thing.

So is climate change a national security threat? Well, a hotter world means more disruptions, which means more burden on the military. And dependence on oil means reduced operational effectiveness and vulnerability to unpredictable price swings. So whether or not the climate-as-security meme takes off, or has any effect on domestic politics, it is already thoroughly ingrained in the military itself, and will only become more salient in coming years, regardless of whether political debate changes. The military, unlike Congress, does not have the luxury of treating reality like a sideshow.