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[edit] Carbon Dioxide emissions

Biofuels and other forms of renewable energy aim to be carbon neutral or even carbon negative. Carbon neutral means that the carbon released during the use of the fuel, e.g. through burning to power transport or generate electricity, is reabsorbed and balanced by the carbon absorbed by new plant growth. These plants are then harvested to make the next batch of fuel. Carbon neutral fuels lead to no net increases in human contributions to atmospheric carbon dioxide levels, reducing the human contributions to global warming. A carbon negative aim is achieved when a portion of the biomass is used for carbon sequestration.^[1]

Calculation of Carbon Intensity of Soy biodiesel grown in the US and burnt in the UK, using UK government calculation ^[2]

Graph of UK figures for the Carbon Intensity of Biodiesels and fossil fuels. This graph assumes that all biodiesels are burnt in their country of origin^[2]

Graph of UK figures for the Carbon Intensity of Bioethanol and fossil fuels. This graph assumes that all bioethanols are burnt in their country of originCite error: Invalid <ref> tag; refs with no content must have a name

The calculation of exactly how much Carbon Dioxide is produced in burning biofuels is a complex and inexact process, and is highly dependant on the method by which the fuel is produced and the assumptions made in the calculation. A calculation should include:

• The cost of growing the feedstock
• The cost of transporting the feedstock to the factory
• The cost of processing the feedstock into biodiesel

Such a calculation may or may not consider the following effects:

• The cost of the change in land use of the area where the fuel feedstock is grown.
• The cost of transportation of the biodiesel from the factory to its point of use
• The efficiency of the biodiesel compared with standard diesel
• The amount of Carbon Dioxide produced at the tail pipe. (Biodiesel can produce 4.7% more)
• The benefits due to the production of useful bi-products, such as cattle feed

The graph on the right shows figures calculated by the UK government for the purposes of the Renewable transport fuel obligation^[1] In practice, biofuels are neither carbon neutral or carbon negative. This is because energy is required to grow crops and process them into fuel. Examples of energy use during the production of biofuels include: fertilizer manufacture, fuel used to power machinery, and fuel used to transport crops and fuels to and from biofuel processing plants. The amount of fuel used during biofuel production has a large impact on the overall greenhouse gas emissions savings achieved by biofuels.

A 2007 study by scientists from Britain, U.S., Germany, Switzerland and including Professor Paul Crutzen, who won a Nobel Prize for his work on ozone, have reported that measurements of emissions from the burning of biofuels derived from rapeseed and corn have been found to produce more greenhouse gas emissions than they save.^[2] The advantages of reduced carbon dioxide emissions are more than offset by increased nitrous oxide emissions. Nitrous oxide is both a potent greenhouse gas and a destroyer of atmospheric ozone.

The claim that biofuels result in emissions savings has also been critiqued on the grounds that it overlooks the 'displacement' effects of large-scale biofuel production, in terms of its direct and indirect role in promoting land use changes and soil carbon losses.^[3]

The carbon emissions (Carbon footprint) produced by biofuels are calculated using a technique called Life Cycle Analysis (LCA). This uses a "cradle to grave" or "well to wheels" approach to calculate the total amount of carbon dioxide and other greenhouse gases emitted during biofuel production, from putting seed in the ground to using the fuel in cars and trucks. Many different LCAs have been done for different biofuels, with widely differing results. The majority of LCA studies show that biofuels provide significant greenhouse gas emissions savings when compared to fossil fuels such as petroleum and diesel.^{[citation needed]} Therefore, using biofuels to replace a proportion of the fossil fuels that are burned for transportation can reduce overall greenhouse gas emissions.

The well-to-wheel analysis for biofuels has shown that first generation biofuels can save up to 60% carbon emission and second generation biofuels can save up to 80% as opposed to using fossil fuels.^[4] However these studies do not take into account emissions from nitrogen fixation, deforestation, land use, or any indirect emissions.

This does assume however that the land used for growing the crops would alternatively be desert or paved area. If the land was previously a (tropical rain-) forest, the carbon absorption of this forest should be deducted from the greenhouse gas savings. This implies that the net effect of burning bio-fuels is an increase in greenhouse gasses. This effect should be incorporated in the LCA, to get a proper overview of the total net effect. Using waste material from plantation forests on previous agricultural land could be carbon positive, due to the carbon stored below ground in the root systems.

A 2008 study conducted by the University of Minnesota ^[5] finds that:

...converting rainforests, peatlands, savannas, or grasslands to produce food-based biofuels in Brazil, Southeast Asia, and the United States creates a ‘biofuel carbon debt’ by releasing 17 to 420 times more CO₂ than the annual greenhouse gas (GHG) reductions these biofuels provide by displacing fossil fuels.

The study not only takes into account removal of the original vegetation (as timber or by burning) but also the biomass present in the soil, for example roots, which is released on continued plowing. Another 2008 study by Princeton University ^[6] reaches similar conclusions and finds that:

...corn-based ethanol, instead of producing a 20% savings, nearly doubles greenhouse emissions over 30 years and increases greenhouse gases for 167 years.