By Neta C. Crawford

Scientists and security analysts have warned for more than a decade that global warming is a potential national security concern.

They project that the consequences of global warming — rising seas, powerful storms, famine and diminished access to fresh water — may make regions of the world politically unstable and prompt mass migration and refugee crises.

Some worry that wars may follow.

Yet with few exceptions, the U.S. military's significant contribution to climate change has received little attention. Although the Defense Department has significantly reduced its fossil fuel consumption since the early 2000s, it remains the world's single largest consumer of oil — and as a result, one of the world's top greenhouse gas emitters.

CC BY-ND The largest sources of military greenhouse gas emissions are buildings and fuel. The Defense Department maintains over 560,000 buildings at approximately 500 domestic and overseas military installations, which account for about 40 percent of its greenhouse gas emissions. The rest comes from operations. In fiscal year 2016, for instance, the Defense Department consumed about 86 million barrels of fuel for operational purposes.

Why Do the Armed Forces Use So Much Fuel? Military weapons and equipment use so much fuel that the relevant measure for defense planners is frequently gallons per mile. Aircraft are particularly thirsty. For example, the B-2 stealth bomber, which holds more than 25,600 gallons of jet fuel, burns 4.28 gallons per mile and emits more than 250 metric tons of greenhouse gas over a 6,000 nautical mile range. The KC-135R aerial refueling tanker consumes about 4.9 gallons per mile. A single mission consumes enormous quantities of fuel. In January 2017, two B-2B bombers and 15 aerial refueling tankers traveled more than 12,000 miles from Whiteman Air Force Base to bomb ISIS targets in Libya, killing about 80 suspected ISIS militants. Not counting the tankers' emissions, the B-2s emitted about 1,000 metric tons of greenhouse gases.

Quantifying Military Emissions Calculating the Defense Department's greenhouse gas emissions isn't easy. The Defense Logistics Agency tracks fuel purchases, but the Pentagon does not consistently report DOD fossil fuel consumption to Congress in its annual budget requests. The Department of Energy publishes data on DOD energy production and fuel consumption, including for vehicles and equipment. Using fuel consumption data, I estimate that from 2001 through 2017, the DOD, including all service branches, emitted 1.2 billion metric tons of greenhouse gases. That is the rough equivalent of driving of 255 million passenger vehicles over a year. Of that total, I estimated that war-related emissions between 2001 and 2017, including "overseas contingency operations" in Afghanistan, Pakistan, Iraq and Syria, generated over 400 million metric tons of CO2 equivalent — roughly equivalent to the greenhouse emissions of almost 85 million cars in one year.

Real and Present Dangers? The Pentagon's core mission is to prepare for potential attacks by human adversaries. Analysts argue about the likelihood of war and the level of military preparation necessary to prevent it, but in my view, none of the U.S.' adversaries — Russia, Iran, China and North Korea — are certain to attack the U.S. Nor is a large standing military the only way to reduce the threats these adversaries pose. Arms control and diplomacy can often de-escalate tensions and reduce threats. Economic sanctions can diminish the capacity of states and nonstate actors to threaten the security interests of the U.S. and its allies. In contrast, climate change is not a potential risk. It has begun, with real consequences to the U.S. Failing to reduce greenhouse gas emissions will make the nightmare scenarios strategists warn against — perhaps even "climate wars" — more likely.

Reposted with permission from our media associate The Conversation.