Wondering what a fuel’s score really means? The carbon intensity of fuels is the life-cycle greenhouse gas emissions per unit of energy delivered, so a lower number means a cleaner fuel.
| Item | Value |
|---|---|
| California ethanol carbon intensity, Q1 2016 | 72.9 gCO2e/MJ |
| California ethanol carbon intensity, Q4 2022 | 59.2 gCO2e/MJ |
| Decline in that ethanol score, 2016 to 2022 | 19% |
| Corn ethanol land-use change factor from 2016 | 19.8 gCO2e/MJ (30 gCO2e/MJ through 2015) |
| Carbon capture and storage effect on ethanol | About 31 gCO2e/MJ lower |
| US advanced biofuel minimum life-cycle reduction | At least 50% versus its fossil counterpart |
Figures come from Choices Magazine’s analysis of California ethanol and a 2025 Congressional Research Service report. They can change, so check the sources before you rely on them.
What a carbon intensity score measures
A carbon intensity (CI) score adds up greenhouse gas emissions across a fuel’s whole chain. That covers growing or extracting the feedstock, processing, transport and burning. The total is divided by the energy in the fuel, usually in grams of CO2 equivalent per megajoule.
Dividing by energy matters. It lets you compare a litre of ethanol, a kilogram of hydrogen and a kilowatt-hour of electricity on the same scale, even though they are sold in different units.
Policy designers use the score in two ways:
- As a pass or fail threshold, where a fuel must beat a fossil baseline by a set percentage.
- As a ranking for credits, where cleaner fuels earn credits and dirtier ones create deficits.
The California program works the second way. According to Transport Policy’s summary of the California Low Carbon Fuel Standard, it “relies on life cycle analysis to estimate the carbon intensity of transportation fuels.” Its credit system lets providers generate tradable credits through low carbon fuels and imposes deficits for higher-carbon fuels.
Why the same fuel can have different scores
A score is not a fixed property of a fuel such as ethanol. It belongs to a specific production pathway, meaning one feedstock, one plant and one energy supply. Two ethanol plants can legitimately report different numbers.
Research on California corn ethanol shows how large the swings can be. In that analysis, the average score fell 19% between Q1 2016 (72.9 gCO2e/MJ) and Q4 2022 (59.2 gCO2e/MJ). Plant-level choices drove much of the spread:
- Drying distiller grains instead of leaving them wet raises ethanol’s score by about 8-10 gCO2e/MJ.
- Combined heat and power using biomass cuts it by 3-4 gCO2e/MJ, or 1-2 gCO2e/MJ if it runs on natural gas.
- Replacing half the natural gas with renewable natural gas from livestock manure lowers it by about 10 gCO2e/MJ.
- Carbon capture and storage can lower it by about 31 gCO2e/MJ.
So when you see a single score for a fuel type, ask which pathway it describes and whether it is an average or a plant-specific value.
Land use change: the contested part of the score
Direct emissions are fairly easy to measure. The harder piece is land-use change, the idea that growing more fuel crops can push farming onto new land and release stored carbon elsewhere. That effect cannot be observed at a single plant, so it is modelled.
The choice of model changes the answer. The same Choices Magazine article notes that California assigned corn ethanol pathways 30 gCO2e/MJ for land-use change through 2015. From 2016 it reduced that factor to 19.8 gCO2e/MJ after updated modelling.
The US Congressional Research Service reports that the EPA itself has acknowledged its “previous [lifecycle analysis] framework is comparatively old.” Its model comparison exercise also found that “estimates of land use change (LUC) vary significantly among the models used in this study,” as described in the CRS overview of the Renewable Fuel Standard.
For a closer look at how these boundaries are drawn, see this guide to the life cycle assessment of biofuels.
Thresholds versus scores in the US Renewable Fuel Standard
Not every program publishes a gram-per-megajoule number. The US Renewable Fuel Standard instead assigns each fuel category a percentage reduction against a gasoline or diesel baseline. Per the CRS report, “Advanced biofuel is renewable fuel, other than corn starch ethanol, with lifecycle greenhouse gas emissions at least 50% lower than lifecycle greenhouse gas emissions from its gasoline or diesel counterpart.”
That is a pass or fail test. Meeting the emissions threshold alone is not sufficient: corn starch ethanol is excluded from the advanced biofuel category.
Why the baseline matters as much as the score
A score only becomes meaningful next to a reference. A fuel that emits less than its fossil counterpart is an improvement, and the size of that gap is what regulators reward. Change the reference fuel and the same biofuel can look better or worse.
That is why percentage reductions and absolute scores can tell different stories. A percentage depends on the baseline chosen, while an absolute value stays comparable across programs only if the boundaries match. When you read a headline claim, look for both.
Three practical consequences follow:
- A fuel can pass a threshold in one program and miss it in another because the baselines differ.
- A falling score over time may reflect better plants, updated models, or both.
- A very low score is not automatically a very large real-world benefit if the volumes are small.
How to read a score responsibly
A low number is a useful signal, but it is only as good as the method behind it. Before you compare two fuels, run through this checklist:
- Check the system boundary: does it include land-use change and transport?
- Check the baseline: is the comparison to gasoline, diesel or a different reference?
- Check whether the value is a default, an average or a verified plant-specific figure.
- Check the date, since scores are updated as models and plants change.
Scores also say little about scale, cost or other environmental effects such as water or biodiversity. Comparing very different energy carriers needs extra care, which is the theme of green hydrogen vs biofuels. For the wider context of low-carbon energy, read what is renewable energy.
Frequently asked questions
What is a good carbon intensity score?
Show answer
Lower is better, and the useful comparison is against the fossil fuel the product replaces. In the California example above, ethanol averaged 59.2 gCO2e/MJ in Q4 2022, down from 72.9 in Q1 2016, according to Choices Magazine.
Is carbon intensity the same as a carbon footprint?
Show answer
Not quite. Carbon intensity is emissions per unit of energy from one fuel, while a footprint is the total emissions of a person, product or organisation.
Why do scores for the same fuel differ between programs?
Show answer
Programs choose different baselines, system boundaries and land-use models. The CRS report says land-use change estimates vary significantly among models.
Can a producer lower its score?
Show answer
Yes, through process changes. Research summarised by Choices Magazine finds carbon capture and storage can lower ethanol’s score by about 31 gCO2e/MJ.
