(CNN) -- No subject appears to divide as many people in the climate change arena as biofuels. Their potential to positively impact greenhouse gas emissions is undoubtedly enormous.
A Thai worker fills used vegetable cooking oil in to a container before being processed to be used as biodiesel.
But the pursuit of such non-fossil fuel energy replacements has raised concerns over the impact on the global food supply -- and the environment itself.
When people talk about biofuels they are essentially referring to ethanol or biodiesel, the former favored by the Americas (Brazil and the U.S. produce 90 percent of the world's ethanol between them); the latter preferred by Europe (accounting for 89 percent of global biodiesel production in 2005).
Biofuels can be produced from any number of plant crops; most ethanol in the world today being derived from corn, with sugar cane increasingly gaining favor (the latter providing double the yield per acre of the former).
Biodiesel emanates mainly from vegetable oils or animal fats, and waste cooking oil from China's restaurants has been to thank for supplying China's growing biodiesel industry.
Cellulosic ethanol, also fast becoming the darling of the biofuel movement, is produced by breaking down plant cell walls. And as cellulose is the most common organic compound around, it can be sourced from many more places and has the added bonus of turning things that used to be regarded as waste -- corn stalks, wood chips, grasses -- into incredibly useful sources of energy.
Are biofuels carbon neutral?
Biofuels are often referred to as being carbon neutral -- that is, the carbon dioxide (CO2) that is released through burning them is roughly equal to the amount they sequester when in plant form. In that way, growing biofuel crops helps ease global warming, their proponents argue, as they act as "carbon sinks" reducing CO2 levels.
In that way, relative to petroleum, biofuels could reduce greenhouse gas emissions by "more than 100 percent", World Watch says. Switchgrass and other grasses rate the highest in this regard, with the possibility of reducing emissions by between 70 and 110 percent (relative to petroleum fuels) compared to corn and wheat only offering maximum reductions of 40 percent.
According to the U.S. Departments of Energy and Agriculture, the 1.3 billion tons of "biomass feedstock... potentially available in the U.S. for the production of biofuels" is currently enough to replace around 30 percent of the country's existing gasoline consumption.
Most existing cars can use them immediately -- in small doses anyway (most gasoline-fueled cars can take up to a 10 percent ethanol blend in their fuel without any conversions). They are generally much better for your health when burnt too, reducing tailpipe carbon monoxide emissions by up to 30 percent, and fine particulate matter emissions by 50 percent, according to the Renewable Fuels Association.
Growing pains: Food vs. fuel
Biofuels currently only represent one percent of fuel used in transportation globally, but around one percent of the world's fields -- that's around 12 million hectares -- has already been turned over for their production.
But there are already questions over whether there is enough land available for biofuels to be able to replace even 50 percent of our fossil fuel use. According to The International Institute for Applied Systems Analysis, a maximum of 300 million hectares worldwide could be used for biofuels, but even if the biofuel industry used up 290 million hectares of that, it would only meet one tenth of the projected energy demands for 2030.
Europe only has a limited amount of land to set aside for biofuels, and according to the Soil Association, quoting OECD figures, if the EU gave up 72 per cent of its arable land, it would only only be able to serve 10 percent of its fuel needs.
Biofuels will however offer some economic incentives for developing nations. The concern is whether the pursuit of economic growth through biofuel production in developing countries will spur further deforestation and land conversions.
According to the World Food Programme (WFP) more poor African farmers are opting to sell crops like cassava for use as alternative energy instead of food -- purely for economic reasons. Some say, at least they are decreasing their reliance on increasingly expensive oil at the same time as 25 out of the 47 poorest countries on earth import all of their oil.
However others are warning of a vicious cycle being created by a dangerous food versus fuel competition, which will lead to food shortages, driving up food prices, and encouraging even more farmers to choose to grow fuel over food crops to meet the increasing demand (exacerbated by population growth) and clearing more land in the process.
It was on the subject of land clearing that the biofuel industry was chastised this month, in what could be the biggest blow dealt to biofuels yet.
A report from the Nature Conservancy and the University of Minnesota has raised serious questions over how biofuels are grown. Converting rainforests, peatlands, savannahs or grasslands to grow fuel crops releases CO2, in some cases a staggering 420 times more CO2 than from burning fossil fuel, the report says.
And that, they say, creates a "carbon debt" (with the emissions savings of biofuels fully considered) that could take as long as 840 years to cancel out depending on what land you are converting.
The worst land to convert is tropical peatland rainforest (creating a carbon debt of 840 years) or Amazonian rainforest (320 years) with the lowest carbon debt of 17 years created by converting the wetter woodland-savannahs of Brazil's Cerrado.
Creating new problems?
The methods used to grow fuel crops continue to be held under a magnifying glass; the initial furore over this new form of energy quickly dampened by realizations that in the pursuit of climate change solutions, nothing is ever simple.
Some have even questioned the role fertilizer is playing in climate change, with George Monbiot recently pointing out in The Guardian (based on a recent claim by the Nobel Laureate, Paul Crutzen) that using fertilizer on biofuel crops will be emitting enough nitrous oxide (more than 296 times more powerful heat trapping gas than CO2) to "wipe out all the carbon savings biofuels produce."
Biofuel crops could also put an unbearable strain on the global water supply, Sweden's Stockholm Environment Institute has warned. It says replacing 50 percent of the fossil fuels to meet 2050 transport and electricity demands with biofuels would require up to 12,000 extra cubic kilometers of water a year (the total annual flow down the world's rivers is 14,000 cubic kilometers).
China and India are at particular risk of water scarcity issues, the International Water Management Institute has warned.
The "best case scenario" many believe is making biofuels from native grasses and woody biomass grown on lands unsuitable for crop growing. The U.S. Agricultural Research Service recently found, for example, that growing native switchgrass on the prairies of Nebraska and North and South Dakota resulted in cellulosic ethanol yielding 5.4 times more energy than all the energy that went into producing it.
Native plants are also viewed as "carbon negative," as they are able to store excess C02 in their roots and the surrounding soil. But there are even downsides to using some agricultural and forestry waste products, as not only are they carbon sequesters but they provide nutrients for the soil. Removing them can lead to rapid soil erosion.
What Monbiot recently wrote in The Guardian reflects what many are increasingly coming to believe: "Apart from used chip fat, there is no such thing as a sustainable biofuel."
(Sources: New Scientist; Renewable Fuels Association, Treehugger, National Resources Defense Council; Department of Energy: Biofuel News; The Guardian; World Watch: Reuters ) E-mail to a friend