Let's start here: One may have ethical objections to "conventional," non-free-range chicken production, and nothing in this data can change or even shape that conviction. In fact, I find myself in this camp.
But I wanted to be intellectually honest with myself, too. Was conventional chicken production, which has been relentlessly optimized by farmers, more or less energy intensive than free-range chicken raising?
Free-range chicken production appears to be environmentally destructive. It uses more land, puts out more carbon dioxide, and uses more energy. That's according to British researchers A.G. Williams, E. Audsley, and D.L. Sandars, which was summarized in a 2010 paper in the journal Livestock Science, "Comparing environmental impacts for livestock products: A review of life cycle assessments."
It should be noted that beef production was far more resource-intensive than chicken. So, one answer to taking less of a toll on the land, while still buying free-range chicken would just be to cut back on beef (or meat, in general).
I see the utility of this data not just in narrowly saying one method of production is better than another, but in helping us see the systems operating in our world. They are so complex and so often hidden from us that we can't make these sorts of determinations from intuition.
Some researcher (usually a European) has to go out there and actually do the work and report the findings back. This isn't to say that these numbers are perfect or not subject to assumptions baked into the models, but they are best-efforts at knowing the world as it is.
- Global Warming is a measure of the carbon dioxide or equivalent greenhouse gases created by the chicken production.
- Acidification is a measure of the sulfur dioxide (or equivalent) created by the chicken production. These chemicals cause acid rain.
- Eutrophication measures the amount of phosphorous (or equivalent) pollution created by the chicken production. These chemicals can throw of aquatic ecosystems if they end up in lakes or, say, the Gulf of Mexico.
- Land Use is an easy one. It measures how much land is needed for the production.
- Energy is another easy one. It measures how much energy is needed. We've got the unit in joules here, but it might be easier to think about it in gigajoules. One gigajoule is equivalent to the energy stored in an eight-gallon tank of gasoline.