I use green to mean stuff that is less bad – and I chose my words carefully – for the planet and the flora and fauna that live on it.
Sustainable, on the other hand...means able-to-be-sustained.
It means, for all intents and purposes, that whatever you are talking about can keep on doing what it is doing, and can do so essentially forever.Now, these two terms are obviously related, and pretty much anything that can be defined as sustainable in Anderson's terminology would also qualify as green. Yet some activities that clearly are green—even extremely green—just as clearly are not sustainable. The example that came to my mind was freeganism. Freegans, as you may or may not know, are people who try to live without buying anything at all. They Dumpster-dive for everything from furniture to food. They won't drive or use mass transit, but will engage in hitchhiking and train-hopping since these activities don't add to the overall number of cars, buses, or trains on the road. They "squat" in vacant buildings and forage for food in state parks.
Now, no one could deny that the way these people live is green. But it's not sustainable. The reason it's not sustainable is that freegans are dependent on the waste of others. An entire society of freegans wouldn't be able to function, because there would be nothing in the Dumpsters to dive for. There would be no drivers to hitchhike with and no trains to hop, and the state parks would quickly be picked clean of all their edible plants. Freeganism is only possible in a wasteful society; it can't be the foundation of a sustainable society.
Less extreme forms of reuse, likewise, are green but not sustainable in the long term. Or rather, to clarify, it obviously is a sustainable practice to keep individual products in use so long as they are still usable—either by repurposing them or by passing them on to others. But it's not sustainable for all the products we use to be reused, because individual products will eventually wear out. Sooner or later, some new goods will have to be produced.
Or take the recurring environmental issue of population growth. Steady growth in the global population is, obviously, not sustainable, because we've only got one planet, and it has a limited amount of space. But steady decline in the global population isn't sustainable either, because eventually, the population will dwindle down to nothing. A decline in global population would be a good thing in the short term, because it would mean less competition for limited resources; in short, it would be green. But it wouldn't be sustainable, because sooner or later the population would have to stabilize. (It could, of course, stabilize at zero, but that wouldn't exactly be a desirable outcome for us human fauna, no matter what a relief it might be for some other species.)
Of course, maybe these aren't really so much examples of unsustainable activities as examples of a problem with this definition of sustainability: what you can call sustainable changes depending on the size of the system you're looking at. Freeganism and reuse might be considered sustainable if we look at them both as just parts of a larger system, one that involves manufacturing as well. In the same way, our backyard compost bin is not sustainable when viewed as a system unto itself: the amount of yard and garden waste that we put into it just isn't enough to produce the volume of compost we need for all the plants we grow. Yet our little compost bin could be a component within a much larger self-sustaining system, one in which all the vegetable waste in (say) the entire country gets processed into compost and used to grow new plants. (On the other hand, this system might still be unsustainable if it's impossible to grow all the plants we grow now, including food crops, with nothing but compost to fertilize them. Which means that a sustainable agriculture system might, as this article suggests, actually require the use of synthetic fertilizers—something we don't usually think of as green.)
Another problem with Anderson's definition of "sustainable" is that when you say you can go on doing something "essentially forever," you are assuming that everything else about your system remains unchanged. This is why Anderson cites "substituting renewables for coal-fired power" as an example of a non-sustainable practice: "Just because coal is not sustainable does not mean windmills are sustainable at the scale needed to replace coal." But in making this claim, he also makes the assumptions that (1) the sources of renewable energy we have now are the only sources we will ever have, (2) the sources of renewable energy we have now will always remain as resource-intensive to produce as they currently are, and (3) improved efficiency will not allow us to reduce our energy needs to the point where they can be met by renewable power. Yet at least two of these assumptions are clearly not supported by the available data. The cost of solar power, for instance, has dropped dramatically since 1980 and remains on a downward trend, while improvements in efficiency—based solely on the technology we have right now—could cut our nation's power use by over 25 percent. Put these two facts together, and it's quite easy to envision a future in which we can meet all our energy needs sustainably without having to scale back our usage to merely "keep[ing] a few lights on, maybe some critical refrigeration" as Anderson suggests in the comments. (I did try to make these same arguments in a comment of my own, but for some reason, Anderson's site has repeatedly refused to publish it.)
So while Anderson's definitions of "green" and "sustainable" sound reasonable, I'm not sure if they're ultimately all that useful. His idea of sustainability is one that only works if nothing ever changes, and the one thing you can pretty much be sure of in life is that things will change. But perhaps more disturbing is that, in defining "green" as merely "less bad" rather than actually good, he seems to dismiss it as having no real validity or use. I would argue, on the contrary, that "green" is actually a useful and necessary step on the road to "sustainable." The only way to get to the long term, after all, is through the short term.