(Note: an unfinished version of this post was accidentally posted previously.)

One of the more interesting panels at the 2012 Aspen Environment Forum was a discussion of whether "nature" was even still a viable concept. It's broadly accepted that humankind has had such a profound impact on the Earth that there remains no part of the planet yet untouched, no part of our ecosystems that hasn't been altered by human activity. Even Antarctic lakes kilometers under glacial ice are subject to human fiddling. The question is, is this a permanent change? Can we do anything to reverse the Anthropocene?

The debate got a bit nasty, as these are positions driven as much by emotion as by science. According to one perspective (call it the "Gardeners"), we face a choice between moderating/steering the Anthropocene versus unsuccessfully fighting against it; according to the other perspective (call it the "Re-Wilders"), it's a choice between reversing the Anthropocene versus surrendering to it (and "surrendering" is definitely the concept -- at one point, E. O. Wilson rather pointedly asked another panelist, Emma Marris, where she had planted her white flag). These appear to be hard-to-reconcile positions. It's easy to see the Gardeners' argument that "you can't step in the same stream twice," that reversing the Anthropocene is simply impossible; at the same time, it's difficult to refute the Re-Wilders' assertion that this would essentially abandon the natural world, as it gives an excuse for limiting efforts to push back against the Anthropocene.

This debate is relevant to my interests, of course, and I have sympathies for both points of view. But there's a fascinating element to the situation that hasn't receive enough attention: perhaps counter-intuitively, any effort to "re-wild" parts of the Earth would require significant levels of geoengineering.

Re-wilding wouldn't just mean leaving things alone, not if the goal is to do more than allow a mix of weakened native species and fast-growing invasive species to fight it out. Existing contamination isn't going to go away, at least not in human timescales. Broken ecosystems would eventually renew, but with new (and newly-adapted) species. Unless the Re-Wilder strategy is to wait until the Earth no longer holds any humans (whether due to extinction or exodus), any goal to re-establish a truly natural form of nature requires that we do so actively.

"Actively" means, primarily, figuring out how to restore and protect viable ecosystems, including the elimination of invasive species and the re-introduction of native plants and animals. This is a seriously complex subject, one that we're only beginning to understand. Re-creating a natural ecosystem means more than just shipping in some potted plants and zoo animals; everything from pollenating insects to soil microbes would need to be restored. Moreover, part of the present contamination of natural spaces comes from human-driven temperature increases, so a fundamental debate within the re-wilding movement would inevitably concern the question of solar radiation management geoengineering. If we don't try to return to pre-industrial temperatures, any ecosystem restoration is doomed; conversely, if we do try to shift temperatures back... well, we're just gardening.

And that's the underlying irony of this debate: the only way to truly re-wild the Earth would be to do active management. In fact, one could argue that a re-wilding strategy that just meant leaving parts of the planet alone without any attempt to clean up and restore ecosystems is even more of an example of "planting the white flag" than any active biosphere gardening might be. Saying "it'll get better if you don't touch it" only works if you don't mind waiting a few hundred thousand years (or more); it may feel good as an ideology, but it sucks as planetary stewardship. If we want to return our planet to pre-Anthropocene conditions, we'll have to get our hands dirty.