Dean, a dairy farmer in Modesto, California, is a rarity. Unlike most milk producers, including his father, he can see himself replacing his entire herd of cows with the supposed villain of the dairy sector: almonds.

Such a move would seem to be in keeping with current consumer trends. More than one-third of American consumers are looking to incorporate more plant-based foods and beverages into their diets. And milk has been one of the products on the front line of this change.

While sales of cows milk have fallen by one-third in the U.S. since the 1970s, sales of plant-based alternatives have grown 6 percent since 2012. Worth an estimated $2 billion last year, they now account for around 10 percent of the total milk market. Almond milk is the most popular nondairy alternative, accounting for 64 percent of that market.

Those motivated by environmental reasons to cut out cows milk argue they want to reduce the planetary footprint of what they consume. Cows are a major user of land and resources, and are a key source of methane emissions, a potent greenhouse gas.

Yet, when it comes to choosing between cows and almond milk, the decision is complicated.

The climate footprint of dairy can vary hugely depending on technology, location and farm management. Some dairy farmers in California are trying hard to improve their sustainability credentials. Take Albert Straus, owner of the Straus Dairy brand , who has installed a methane digester and is generating energy from cow manure.

Im trying to create a model that is viable financially, sustainable to the land and humane to animals, says Straus.