Farrell & Kammen calculations of Ethanol GHG emissions at high end of Conf. Interval are in error.

[JohnWxy]

Farrell & Kammen meta-analysis, Science (Jan 2006)

The Farrell & Kammen meta-analysis calculated a point estimate for GHG emisssions for the production of ethanol for their "Ethanol Today" scenario. But they also computed a GHG emissions estimate for ethanol at the high end of a 95% confidence interval. Apparently, this was in an effort to show what might be possible (at the extreme high end - 2.5% probability) in terms of GHG emissions for the production of Ethanol. However, in the high GHG estimate, they did not compute the GHG emissions for the products displaced by coproducts of ethanol using the same confidence interval. Computing the GHG emission for the production of ethanol (see spreadsheet at link) they used the same point estimate for the GHG emissions for the products displaced by the ethanol coproducts as ws used in the "Ethanol Today " scenario.

The uncertainty inherent in estimating GHG emissions from cultivating corn is the same as for cultivation of the products replaced by the Ethanol coproducts. (The IPCC guidlelines used by Farrell & Kammen are for any cultivated lands where NItrogen fertilizer is used. The point estimate for the GHG emissions for the coproduct credits (that is the estimate for the GHG emissions of the products replaced by the Ethanol coproducts would take care of the difference in Nitrogen application rates between corn and soy-beans (the products considered to be replaced by the coproducts -i.e. DDGS). The uncertainty inherent in the estimate however, applies to soy-beans just as it does to corn. NOt including the uncertainty surrounding the GHG emisssions estimate for coproducts credits while doing so for the corn is inconsistent and produces erroneous results.

Clearly, you cannot legitimately compute one emissions amount (for the corn cultivated) based on a 95% confidence level while using a point estimate for the GHG emissions of the products replaced by the ethanol coproducts. I have corrected their calculation by including a value for the GHG coproduct credit which reflects the high end of the 95% confidence interval for the products replaced by the ethanol coproducts: GHG emmissions for ethanol.

At the link you will find a spreasheet which compares the Farrell & Kammen calculation (using a piont estimate for the coProduct Credit GHG emissions) to my use of the same confidence interval for GHG emissions for the coproduct credit that was used for corn. NOte that the confidence interval is to allow for the uncertainty of the estimate of GHG emissions from cultivated land (with use of NItrogen fertilizer). This estimate uncertainty applies equally to the products replaced by the Ethanol coproducts. When you consider the possible high end error of your estimate of GHG emissions for corn cultivation, you have to also apply the same level of error (or the same confidence Interval) to the cultivationof the products which are replaced by the ethanol coproducts.

Once you include the GHG emissions estimate for coproduct credits at the high end of the confidence interval (95%)(as was done with the GHG emissions for corn to show the possible (with a 2.5% likelihood) high end emissions figure for ethanol), the total GHG emissions for ethanol production at the high end of the confidence interval computes to 92 grams of CO2 equivalents per MJ. This compares to Farrell & Kammen's incorrect calculation of 121 grams of CO2 equiv per MJ of ethanol produced.

This puts the GHG emissions of ethanol at the high end of a 95% Confidence Interval below the average GHG emissions for gasoline. OF course, lest we forget, the probability of the actual GHG emissions from ethanol production actually reaching 2 Standard Errors of the Estimate above the Mean Estimate (or exceeding it) is 2.5%. This would establish this eventuality as what point spreaders call a VERY long shot. In other words the smart money would be on the 77 grams of CO2 equivalents (the estimate from the "Ethanol Today" scenario) which has a much higher probability of occurring.

FArrell and Kammens model on an Excel spreadsheet can be found at:

http://rael.berkeley.edu/EBAMM/
Click on: "Download the model (version 1.1) (1.0 MB)"

go to the tab: "Ethanol Today" for the basic point estimates.

the tab: "GHG Uncertainty" shows the high estimate of GHG emissins for cultivated acreage. See cell AU17 for the high GHG estimate for ethanol production.