greener

Reader's Digest 10月号の記事 '12 Tips for More Sustainable Living' からの引用です。
Don't crush your cans.
First off, when you're on the go, drinks in cans are the way to go. Recycling aluminum is much more efficient than recycling plastic (about 85 percent of which ends up in landfills anyway), using less than 5 percent of the energy it takes to manufacture the ofiginal product (versus 67 percent for plastic). Glass bottles are also a far greener choice than plastic, since, like aluminum, glass can always be melted down and made into something new.

"greener" は環境に優しいの意味での "green" の比較級ですね。　辞書にこの意味の説明があるのか確認します。

・Oxford English Dictionary: connected with the protection of the environment; supporting the protection of the environment as a political principle: Try to adopt a greener lifestyle.
・Collins Dictionary: If you say that someone or something is green, you mean they harm the environment as little as possible.: ...trying to persuade governments to adopt greener policies.
・Cambridge English Dictionary: relating to the protection of the environment: a green campaigner/activist