Associated Press

The federal government’s position on ethanol fuel is that it must contribute less to global warming than gasoline does, or why bother promoting it. Yet by some calculations, ethanol is worse because it encourages the destruction of forests to make way for new farmland, many assert. Burning trees releases carbon dioxide into the atmosphere; what is more, the trees are no longer there to absorb carbon dioxide.

Now, two professors at the University of Nebraska counter that gasoline is an even bigger source of heat-trapping gases than previously believed. While most attention focuses on the obvious sources of gasoline-related emissions — drilling wells, transporting oil, refining it into gasoline and finally burning it in a car engine — they argue that the military activity that goes into protecting and acquiring oil imports from the Middle East takes an emissions toll that doesn’t get factored into comparisons of gasoline and ethanol.

In a paper to be published on Tuesday in Environment Magazine, Adam J. Liska, whose specialty is industrial ecology, and Richard K. Perrin, who focuses on agricultural economics, write that fuel burned in warplanes and ships — and the carbon dioxide released in manufacturing those planes and ships — should be counted in gasoline’s total.

Figuring out which military emissions should be credited to gasoline’s account is difficult, they acknowledge.

Entering the war in Iraq, for example, was not ostensibly about protecting oil supplies, but about blocking Saddam Hussein from developing nuclear weapons. Yet there “is now growing consensus among economic, foreign policy and military analysts that oil played a large part in the United States-led invasion of Iraq,” they add.

Leaving aside debates about lives lost and billions of dollars spent, the war in Iraq alone is producing 43.3 million metric tons of carbon dioxide per year, they estimate. Of Americans’ daily consumption of about 19.4 million barrels in 2008, the year on which the study focused, about 2.4 million barrels of oil was imported from the Middle East.

One potential weakness of their argument, the authors acknowledge, is that even if the United States halved its imports from the Middle East to, say, 1.2 million barrels a day, it would take just as many ships, planes and soldiers to defend the remainder, so the carbon dioxide emissions related to military activity would not change.

And if the United States stopped importing oil from the Middle East, some other consuming nation would probably start burning jet fuel to defend access, they write.

Figuring out how to calculate military emissions “is not without its conceptual and assessment difficulties,’’ the authors say. Deciding what proportion of the military emissions are side effects of importing oil “can only be estimated,’’ they write, and there are “considerable judgment and substantial uncertainties.’’

But between military acquisitions and spending on direct military activity, about 289,000 tons of carbon dioxide is released per billion dollars spent, the researchers estimate.

They conclude that in weighing the relative benefits of using biofuels rather than oil to rein in greenhouse gas emissions, policymakers must gain an understanding of military emissions related to oil. Indirect changes in emissions that will ensue from any change in course “must surely be included in rational policymaking,’’ they write.