It's interesting to see how environmental literature has changed from decade to decade. It began with the romantics: folks like Wordsworth, Rousseau, Thoreau, Gilbert White, Muir. These guys -- often mixing poetry, philosophy, and naturalist observation -- more or less championed the goodness of nature or wilderness.
In the early 20th Century, we see folks like Muir and Roosevelt fighting for specific things, whether it be the preservation of an animal or a landscape in the form of a national park. By the middle part of the 20th Century, we got books like Rachel Carson's Silent Spring and Aldo Leopold's A Sand County Almanac, which depict a world that is an interconnected ecological network, in which an insecticide like DDT could have a massive impact on the whole ecosystem.
Nowadays, in the 21st Century, we're seeing books about food and food production in, say, Michael Pollan's The Omnivore's Dilemma and Jonathan Safron Foer's Eating Animals. There aren't as many premier environmental books calling for more wild spaces (with the exception of E.O. Wilson's recent Half Earth). Rather, we see books like Elizabeth Kolbert's The Sixth Extinction and Jared Diamonds Collapse, which are grim reality checks about our destructiveness as a species and our increasingly fragile planet, now challenged by overpopulation, thinning resources, and an unstable climate.
The earlier books, surprisingly, are just as readable and relevant today, as their call to embrace nature is, I think, a necessary step our society must take to surmount our much scarier environmental problems today. For a wonderful survey of all environmental writing, from its birth to its much more diverse form today, I strongly recommend American Earth, a collection of writings edited by Bill McKibben.
So what exactly is a 21st Century environmentalist? It may be a person who combines all the environmentalisms of yesteryear: someone who has the aesthetic nature-loving sensibilities of the naturalist-romantics, the desire to preserve wild places and animals in perpetuity, the understanding of a vast and complex ecological system that needs careful management, and the concern for animal welfare and the fate of our planet.