A rather vacant Keith Kloor article concern-trolling over the imminent "extinction" of the environmental movement linked to a much better Wonkblog post. The recent 2014 Pew study showed 32% of Millennials identifying as environmentalists compared to 42% of the prior generation - equating to extinction in the mind of Kloor. The Wonkblog post also linked to a 2011 Pew study, the source for the graph above that Kloor somehow missed. The 2014 poll generally showed Millennials as less likely to accept labels for themselves like political party affiliations.
My rule for trying to clarify an argument is to consider a thought experiment where factual conditions are reversed and try to understand that situation. For a trend like whether generational replacement is endangering enviornmentalism, ask whether the reverse trend would lead to a reverse conclusion. So let's look at the graph above together with change in self identification, and posit instead that Millennials were more likely to call themselves environmentalists but less likely to support environmental policies than older generations. Is that better or worse than what's actually happening?
While fewer Milleninals identify as Democrats, they're definitely siding with more liberal policies on many issues (and on other issues, no real change, so Republicans are at best keeping pace in some areas and losing drastically in others). I think what we may be seeing is a refusal to accept the environmentalist label while accepting the viewpoint. Every year, over one percent of the voting demographic shifts from the 47% supporting renewable energy to 71% supporting renewables. I wouldn't call that bad news for environmental politics.
Where Wonkblog may be right about a real environmental problem is that adopting the environmentalist label helps lead to further involvement and leadership. There's an ironic contrast in environmentalism - non-Hispanic whites as a group aren't more pro-environment than other ethnicities. Non-whites are generally equivalent to or slightly more pro-environment than whites, excepting Hispanics who are significantly more pro-environment than whites.
Despite that demographic fact, the environmental movement is disproportionately white (and older). The other demographic groups are supporting the environmental movement when they could be leading it. Making that happen remains a challenge for environmentalists, regardless of the label people give themselves.
One other hopeful note: accoding to the full 2014 study (p. 45), in 1999 about 39% of Gen Xers labelled themselves as environmentalists and now 42% do, so the trend is positive as people get older and more experienced.