Climate change is the left wing doomsday theory of choice, a little smarter than most because it tends to make more nebulous predictions rather than setting a specific date. Ultimatly it all comes down to embracing their specific socio-political philsophy and belief structures which get into goverment, heavy regulation (to protect the enviroment), and ultimatly into anti-capitalist socialism as the solution. You see predictions based on vague time tables, and if nothing happens by a certain point they can always claim that it's because of the efforts of the followers who are involved, but they still need to keep going and force their agenda... etc...
Guys like the Unabomber represent the extreme fringe of this school of thought.
That said, like most things of this sort there is a grain of truth in it. The bottom line is that there are simply too many people out there for the planet to sustain at a reasonable, first world standard of living. We're already having problems with deforestation, strip mining, and other problems trying to meet the demands of the people we have. With third world countries like China (which represents roughly one third of the human race) demanding higher standards of living and more resources it puts an increaing strain with what's out there and also increasingly diverts resources that would otherwise have been going to the Western World which have traditionally been holding the highest standards of living. It all comes down to a "we want to live as well as possible" (everyone does), and "if it's between yours and ours, we pick ours" which has been fueling a lot of the global chaos recently as well. People point fingers at the US and Europe for the global financial problems, but due to our own morality and peace at any price attitudes we tend to overlook the "robber economies" that basically lend money they steal from us, back to us, in order to maintain the peace (which is a whole discussion in of itself).
The point of the rambling is that the basic trends do exist, but rather than embracing real, unpleasant, solutions which involve wars and unheard of numbers of people dying followed by far more strict population controls, this basic philsophy argues that instead people should be willing to give up their lifestyles, technology, etc... and use less resources. The basic idea being that if we sacrifice all of these comforts and live at a very basic level, everyone could in theory live at that level without any need for wars, goverment could then be used to maintain that status quo. It does more or less lead into an anti-capitalist/highly socialist point of view, in many cases in the US harkening back to Jacksonian philsophy, and the view of people properly existing as a loose confederation of peasant yeoman farmers who work for substinance with little in the way of luxuries or comforts.
To be honest I find it kind of ironic that a lot of the philsophies similar to the one mentioned above (there are a lot of them, all vageuly similar, that go to the same basic place) largely tend to thrive in social media, yet as a sticking point try and act like capitalism and the same infrastructure that spawns them has lead to increasingly little value being assigned to humanity by other humans, socially or otherwise. To be blunt people are probably a lot more interconnected than ever before, however that same level of connection has lead to a lot of those problems since it presents an enviroment where everyone's differances can come out to the forefront, where they otherwise might not be as easily noticed. That, and we begin to see the dark side of tolerance, when you argue that everyone and everything should be accepted the lowest human denominator sinks incredibly low, and there is little in the way of social force making people try and improve, there is no real "darwinism" (all jokes about awards aside) within society anymore, no matter to what depth you might sink society is expected to adapt to you. This creates increasing numbers of divides and what amounts to elitism on the part of those who can claim even average amounts of intelligence. Something like Youtube or Facebook kind of illustrates exactly how far humanity without standards continues to sink.... not to mention again, simple overpopulation. When humans are literally everywhere and we're all that heavily connected there is little value to contact, and the simple laws of supply and demand make it so that competition is going to become increasingly fierce.
At any rate, all this rambling aside, the bottom line is I tend to be an extremist of sorts myself. I pretty much think killing off 90% of the population would be a good thing, it's one of the reasons why I think little of suggestng wars, atrocities, mass murder, etc... when conducted for the right reasons. Getting rid of people can be in of itself a worthy objective from my point of view. Sometime walk into a crowded store, mall, etc... or on packed streets full of traffic or whatever and imagine if 9 out of 10 of those people were gone. There would still be a lot of people around, but a lot more space, resources would be stretched a lot less, and everyone would have more room to live. Less competition and more space would also make people more social. Keep things at roughly that level until we manage to develop the technology to colonize other planets and spread out our population and I think people would generally be a lot happier, and the benefits over say a thousand years of that would be incredible. Of course a lot of people don't really want to look at things that rationally, especially seeing as there is always the question of cost.
Right now the bottom line is that few people are going to agree with me, or the extreme other view (like in that article), which is why the current degenerative cycle continues, and things get worse and worse. Nobody wants to live in a neo-Jacosonian "back to nature" socialist Utopia, they like the creature comforts they have now (like this Internet thing), at the same time while a lot of people might like the idea of a world with less people, killing off 90% of the population is something most people aren't willing to embrace simply because they don't want to be the ones to die, or to lose their loved ones (many of which would die). That said, things are going to get worse before they get better, and to be honest I'd expect enviromental scare tactics to be used by the whole "regression" movement with increased vigor as time goes on, not that I expect it to go anywhere, after all the bottom line is the enviroment WILL get worse as long as the population stays where it is now, or continues to grow, especially if people want to live with anything approximating the standard American lifestyle (which is pretty reasonable overall).