According to the study, corn ethanol's carbon intensity (CI) had decreased by 23 percent from 58 to 45g CO2e/MJ over the same period.

Among the reasons for this, the study said, are improved corn yields while decreasing intensities in chemical and energy usage in corn farming, increasing ethanol yields and lower energy usage as well as ethanol co-products such as distillers grains, corn oil and CO2. 

In particular, it said ethanol plants have reduced production emissions by 30 percent over the 15-year period by reducing the energy inputs for each gallon of ethanol produced.

As for corn farming, reduced intensities of chemical and energy inputs have resulted in a 17 percent reduction in farming-related emissions. 

Since distillers grains displaces conventional animal feed and corn oil replaces soybean oil to produce biodiesel and renewal diesel, the study said both co-products receive emissions credits under the GREET model, thus reducing the emissions burden of ethanol production. 

It said additional measures exist to reduce corn ethanol's GHG emissions further such as sustainable farming practices and the use of renewable energy at ethanol plants. 

Read the full study here.