Recently PepsiCo, which owns the Tropicana brand, calculated that the equivalent of 3.75 pounds of carbon dioxide is emitted into the atmosphere for each half-gallon carton of orange juice it produces. As they tracked the carbon footprint to the consumers breakfast table it revealed that the biggest single source of greenhouse gas emissions turned out to be the act of growing oranges, not transportation or production.

Orange groves use nitrogen fertilizer, which requires natural gas to make. It can turn into a potent greenhouse gas when it is spread on fields. PepsiCo undertook this step as they are among one of the first companies to provide consumers with an absolute number for a products carbon footprint. The result of this study has revealed to PepsiCo the effort they will need to find ways to grow oranges using less carbon.

PepsiCo’s experience is a harbinger of the complexities other companies may face as they come under pressure to calculate their emission of carbon dioxide, a number known as a carbon footprint, and eventually to lower it.

“The main thing is helping us figure out where the carbon is in the chain,” said Neil Campbell, president of Tropicana North America, a division of PepsiCo. While acknowledging that protocols for measuring greenhouse emissions are far from perfect, Mr. Campbell said, “you can end up doing nothing if you let that stop you.”

PepsiCo, a manufacturer of soda, salty snacks and cereal based in Purchase, N.Y., is among a growing number of companies that hope to get ahead of potential government mandates and curb their energy use as prices and long-term supply grow less certain.

They also want to promote supposedly low-carbon products to consumers anxious about rising global temperatures; such labeling has already appeared in Europe.