What Would a Low-Carbon Military Actually Look Like?

The U.S. military has long been aware of the dangers of climate change: how sea level rise can endanger its ports; how extreme weather can damage its infrastructure; and how negative climate trends can intensify the potential for armed conflict. But understanding the threat that the climate emergency poses and restructuring to reduce its severity are two very different problems.

The fundamental problem is that militaries – especially advanced, expeditionary militaries – rely heavily on carbon-intensive systems, which are not easily converted to carbon-neutral equivalents. The impact is not small: According to Brown University’s Costs of War program, the U.S. Department of Defense is the world’s largest institutional carbon emitter and single-handedly contributes more to global warming than Sweden, Denmark, or Portugal.

Is it possible to have a military force that is both effective and carbon-neutral? To find out, we need to examine why military emissions are so high and whether it is possible to find practical mitigations.

The low-hanging fruit is power for stationary facilities: barracks, airbases, forward operating bases, and headquarters. Changing power generation for stationary facilities from fossil fuels to renewables is not exactly easy, but it is certainly possible with existing technology. There might even be some advantages – a distributed array of solar panels, for example, might be more difficult to disable than a single, centralized generator. This process is already underway.

But power for facilities does not represent the bulk of a military’s carbon footprint; a full two-thirds of the U.S. military’s emissions come from vehicles consuming fossil fuels.

Of these, land vehicles are the easiest to convert. With the transition from fossil fuels to renewables already well underway in the commercial automobile sector, there is a substantial technical knowledge base to build from. That said, a main battle tank is a considerably trickier thing to run on batteries than a Volkswagen Golf. Not only is it far heavier and more complex, but the consequences should it not be able to perform to spec under extreme conditions are much more serious. Electric motors – whether powered by batteries or hydrogen fuel cells – do have advantages over internal combustion, including greater simplicity and reliability along with a favorable power-to-size ratio. But batteries are heavy, slow to charge, and offer limited range; hydrogen fuel cells solve some of those problems but bring much greater complexity and cost.

These are technically challenging problems, but again, not insurmountable. Given the rate of progress of electric vehicles over the last decade it is not unrealistic to expect that the technology could be made fit for defense applications in the years to come. Removable, swappable batteries, for example, might solve the charging-time problem, while continuing improvements in lightweight and more energy-dense materials will doubtless make the technology more competitive in terms of weight.

So far, so good. But the remaining tasks – greening ships, aircraft, and spacecraft – are much more difficult.

Of these, there are at least some possibilities for low-carbon ship propulsion. The obvious one is nuclear propulsion, which powers every American aircraft carrier and the bulk of the world’s most capable submarines. Moreover, during the Cold War, both the United States and the Soviet Union operated nuclear-powered cruisers and destroyers; the U.S. even occasionally deployed entire carrier battle groups powered entirely by atomic energy. But building a nuclear navy is only really a possibility for countries with existing nuclear power infrastructures, and even for them, the costs of safely fueling, operating, and disposing of fleets of nuclear ships is prohibitive.

Other solutions are either marginal or fanciful – a frigate running on biofuels is easy enough to imagine but would merely reduce, not eliminate, emissions; whereas a sail-powered one would probably not qualify as operationally effective.

Aircraft and spacecraft are even more difficult to decarbonize. Nuclear aircraft propulsion is a technical possibility – demonstrated briefly by the U.S. Air Force’s NB-36H in the 1950s – but a practical impossibility, on cost, safety, and size grounds. Electric aircraft are very far from feasibility at scale: Batteries that can power a small quadrotor drone do not translate to a power system that works for a fighter plane or strategic bomber or transport aircraft. Lighter-weight construction materials, more efficient engines, and cleaner-burning fuels can make a difference at the margins, but the fundamental challenge of decarbonizing powered flight is unlikely to be solved in the near term.

Assuming for a moment that a military were to maintain its current capabilities while adopting the emissions-limiting technology likely to be available in the next decade, the upshot is that it would have a diminished but still very significant carbon footprint. If the ambitious targets for emissions limitations are to be met – if we are to avoid the very worst impacts of climate change this century – this is not sufficient. And yet there are very few militaries willing to sacrifice capability for carbon-neutrality.

The remaining option is to seek alternative, less-carbon intensive ways of performing the same missions. This is, to put it mildly, a controversial concept. It is possible that, say, a combination of short-ranged drones, advanced sensors, and distributed precision-guided munitions launchers could accomplish roughly the same mission set as a crewed combat aircraft, but the history of “transformational” high-tech weapons is replete with failures and disappointments, which will make military and political leaders extremely wary. Plus, some missions – strategic airlifts, for example – are very hard to imagine carbon-neutral replacements for, absent a generational technological leap. Finally, none of this is happening in a vacuum. Solutions that leverage greater automation and connectivity will inevitably create new negative externalities around meaningful human control and cyber-vulnerability, just to name a few.

In that way, military carbon reduction efforts are likely to echo humanity’s attempts to contain the climate emergency writ large: late, limited, and both beholden to and weakened by innumerable other factors.