Carbon Neutral: At a fundamental level, all biofuels are carbon neutral, meaning that they do not add any more CO₂ to the atmosphere. This is so because as all biofuels are ultimately produced by the photosynthetic activity of some plant or tree, the plant had already sequestered (removed) a greater amount of CO₂ from the atmosphere while in its growth phase. Therefore when this fuel is combusted and the CO₂ released back into the atmosphere, no extra CO₂ has been added, making biofuels carbon neutral.

While this argument is correct, the point to be careful about is that a significant amount of CO₂ is released in the atmosphere by the process of growing, harvesting and processing these fuels. Tractors for ploughing, pumps for irrigating, harvesters for harvesting, chemical fertilisers and pesticides produced in large power hungry factories and transported to site, and finally the processing plant that converts the raw bio fuel into usable ethanol of biodiesel itself releases a lot on CO₂ and other GHG’s (green house gases).

When all of the carbon foot print is considered, very few biofuels are carbon neutral, which is a very important sustainability criterion.

Carbon Negative: simply means a process that actually reduces the existing amount of GHG’s in the atmosphere. Any process that actually sequesters (removes) CO₂ from the atmosphere is a carbon negative process.
Controlling GHG’s: So now we see two broad lines of attack against GHG’s – one is to sequester CO₂ from the atmosphere (carbon negative) and the other is to work at reducing the further generation of CO₂ (carbon neutral) and other GHG’s.

Pacala and Socolow in their August 2004 paper in “Science” defined seven broad initiatives that they call “wedges” that work on these two lines of attack. A wedge is defined as a strategy that can grow over the next 50 years from 0 to 50 Giga Ton Carbon reduction potential per annum. That roughly corresponds to $2.5 Trillion per wedge cumulated over the next 50 years, assuming prices of $100 per ton of Carbon (prices touched a high of $110 per ton of Carbon in 2005).

Very clearly, they present only two ways of reducing GHG’s and being carbon negative.

CO₂ Capture and Storage: is a sophisticated way of sequestration in which CO2 is compressed and pumped deep underground for permanent storage in subterranean bead rock. This is still in its early days, and is an expensive solution.

Forests and Soils: Trees live for many decades, sequester CO₂ from the atmosphere and store it as bio mass in their trunk system as well as their root system. In addition, they cause carbon retention in the surrounding soil too, as this figure from The Tree Canada Foundation shows. Even after the death of the tree almost a century later, the carbon in the bio mass stays locked, unless it is burnt or decayed.

Therefore, planting trees is a low cost, effective and quick answer to medium term reduction in GHG’s. Only a very small percentage of the total bio mass of the tree is stored as oil seeds – that is released back into the atmosphere when the bio fuel produced from it is combusted.

EROI: Energy Return on Investment refers to tracking the amount of energy spent in growing and refining a particular bio fuel versus the amount of energy that it finally makes available for use.

For example, for a food crop like corn, a lot of intense agricultural inputs are required – soil preparation, irrigation, fertiliser, pesticide, mechanical harvesting and refining of the corn into ethanol. All this energy totals up to more than the energy that the ethanol actually delivers, making it an negative EROI process and a first generation biofuel.