The problem is that for the Green Party to become relevant, you need the Democratic Party to not move to the left to appeal to those voters. You can certainly make the case the Democratic Party is not a left-of-center party today, however, it's still relatively liberal for American politics. It's also nominated two fairly liberal (socially, and a bit economically) candidates as their last two nominees - John Kerry and Barack Obama. If you make the party go further to the right, say, under Clinton and then Gore (who was a moderate up until his loss to Bush, it seems), you might see an opening for the Green Party. But it's unlikely that opening is big enough to make them a serious electoral threat.
Even in 2000, at the height of their popularity in national politics, after eight years of moderate Clinton politics, Ralph Nader still only won 3% of the vote nationally.
The thing is, the examples given, namely an environmental disaster, don't necessarily benefit the Greens - and instead, would just force, potentially, more Americans to solidify around the Democrats, who are more mainstream and more appealing, in this scenario, than the Republicans.
With a two-party system, which this country has pretty much been since the beginning (beyond a few election cycles), you need a total collapse of one party for the rise of another. It's hard to write off either the Republicans or the Democrats because they're too established and too mainstream. The Greens and Libertarians aren't on the whole. It's why there is never a legitimate third party challenger because the views of the left & right are just too stark.
So, maybe the Democrats become so toxic that everyone abandons the party. But how likely is that? If the Democrats could survive Jimmy Carter, and the Republicans could survive Nixon and W. Bush, beyond some unforeseen craziness, the party just isn't going to die off any time soon. In 100 years? Okay - but that makes this future history...
