A very interesting piece! I've frequently noted the intense, seemingly religious fervour that often accompanies discussion of environmental issues. Inquiring minds will ask: but what is a "religion"? Good question! I cover this in my Intro to Philosophy of Religion course. The two most helpful attempts at definition I know of are as follows. Both can't get rid of certain borderline cases, but they're helpful nonetheless.
First, the authors of this excellent book, say
“a tentative, working definition... [a] religion is constituted by a set of beliefs, actions, and experiences, both personal and corporate, organized around a concept of an Ultimate Reality.” (p.7)
By that definition, many forms of environmentalism will count, as they revolve around an idea of ultimate reality as either (1) the natural world (as science reveals it), or (2) the natural world understood as including the Earth as a living thing - "Gaia", or Mother, etc., other living things being our siblings, as it were.
Philosopher Keith Yandell gives what I think is more helpful, functional definition of what a religion is, in this book.
A religion proposes a diagnosis (an account of what it takes the basic problem facing human beings to be) and a cure (a way of permanently and desirably solving that problem): one basic problem shared by every human person and one fundamental solution that, however adapted to different cultures and cases, is essentially the same across the board. Religions differ insofar as their diagnoses and cures differ.”(17)
Environmentalism, then, will be the religion defined by something like the following diagnosis and cure. Diagnosis: as things stand now, our Mother Earth is dying, and thus our race is doomed. Cure: we can save her, and thus our race, by recycling, by voluntary lifestyle changes, and through increased governmental control of earth-harming industries, traditions, and practices. That does seem to me like the central narrative of the universe defining the lives of many current-day people.
"Religions" are complex things. We philosophers tend to focus on the belief element in religion, but late philosopher of religion Ninian Smart (yes, smarty pants, he was very smart) pointed out that religions have six dimensions: doctrinal, mythic, ethical, ritual, experiential, and social. Various streams in the Environmental Movement clearly have all six dimensions. Thus, they aren't simply like religions, or quasi-religions - they are religions - they compete with others in the religious area. This can be obscured by the fact that people syncretize environmentalist religion with other kinds.