Researchers led by Timothy Searchinger at Princeton University said their study showed greenhouse-gas emissions will rise with ethanol demand. U.S. farmers will use more land for fuel, forcing poorer countries to cut down rainforests and use other undeveloped land for farms, the study said.
Searchinger's team determined that corn-based ethanol almost doubles greenhouse-gas output over 30 years when considering land-use changes. Bob Dinneen, president of the Renewable Fuels Association in Washington, said the study used a flawed model and overestimated how much land will be needed.
The intuitive case for ethanol is based on the fact that the carbon you release in burning it was trapped in the corn just a few months ago.
Let's back up. The oxygen combustion of carbon fuels, no matter how efficient (ie, no matter how little particulate pollution is produced) produces carbon dioxide. Yearly averages of carbon dioxide levels are rising because we burn lots of fossil fuels -- essentially, highly compressed prehistoric swamps. This is carbon that was in the atmosphere millions of years ago, but has been out of the atmosphere since the beginning of human civilization. Hence we say that fossil fuels result in a net emission of carbon dioxide.
The intuitive case for ethanol is based on the fact that, rather releasing carbon that was trapped by plants while our shrewlike ancestors were trying to steal eggs from dinosaur nests, burning ethanol releases carbon that was trapped by corn a few months ago. So, over the scale of a couple years, ethanol results in zero net emissions of carbon dioxide.
However, you can't just magically find some unused tract of land and grow lots and lots of corn for use in producing ethanol. At least, the land that's not currently being used is in environments that are too hostile for corn to grow -- the middle of the Sahara, the tundra, etc. You have to take land that's currently in use -- namely, for growing corn or other crops for food -- and change its use -- to growing corn for ethanol.
This results in an increase in food prices -- nothing's changing the demand for food (if anything, it grows as the global population increases), and the supply's just dropped a few percentage points. And, because of globalisation, an increase in food prices in the US increases food prices in places like rural Brazil.
The people of rural Brazil are, generally speaking, already quite poor. So if food prices go up, there's now a (larger) gap between how much food they can afford to buy (it's not like they've gotten any less poor) and how much food they need to survive. It's a lot easier for them to clear-cut another X number of acres of rainforest and grow enough food to fill that gap than it is for them to scrounge together enough additional money to maintain their level of food consumption. But the X number of acres of rainforest would have absorbed a certain amount of carbon from the atmosphere. This counted against the net emission of carbon from burning all the fossil fuels (and, I might add, raising all the methane-producing cattle we do). Effectively, a mandate to switch a certain amount of US land from food production to ethanol production does actually increase net emissions of carbon.
The question is how big of an increase, and how that compares to the decrease in net emissions from burning ethanol instead of petroleum. And the answer, it appears, is enough to double the emission rate in 30 years.
Part II, on externalities, when I feel like writing it. Link from Paul Krugman.