We first need to nuke the CO₂ from orbit to kickstart a global warming and get some atmosphere going. It's the only way to be sure.
Planting trees is insanely interesting though. Building a habitat can be done, and the main issues will be financial – and I would wager the shipping will cost more than the habitat itself. And that exercise may not be popular or successful without a clear idea of what to do next.
In contrast, attempting to plant trees is an endeavour of significant importance. Even only as an experiment to test the compatibility of plant life to extraterrestrial soils, it is something that needs to be done because of its pure scientific value. It will be the coolest and most interesting thing done in space research since ages. There could be ways to do it with an unmanned probe, and thus much more cheaply than attempting to build a colony – a self-inflatable green house that can collect gases from frozen water and CO₂ ices might do it. You can stack it with seeds so you can bruteforce which plants work. Or perhaps not, but it's still easier than getting us there.
And if it works – because it may also fail – it will be a major step in the path towards self-sustainability. Self-sustainability, of course, is a pie in the sky – you can't do that even on Earth. But getting the plants growing will be as significant as you can have it now in that direction. A bigger, but more difficult step would be bootstrapping an industry...
Now, that's the thing. Advances in 3D printing, robotics, mixed with the ability to mine materials in situ on Mars might double as a way to build a habitat before the humans get there, and a way bootstrap the future industry, while also making it cheap enough to make future self-sustainability sound less insane than it presently does. (Though it's not like they can solve the problem, it's just that I can't imagine a solution to the problem without them. I am not implying that you can just throw some robots at it, and it will happen.)
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The truth is, a planetary or deep space colony is something that has neither been attempted before, nor seems reasonably doable at this point. That also means that we can't predict how or when it will happen, and that we don't know exactly how the means that we are lacking look like. Any impressions that we have of what it might take might be way off, whether for optimistic or pessimistic prospects. Even the article's crazy notion that we may accomplish it in a couple of decades isn't impossible, because we have shitload of new, still evolving technologies that haven't been applied on this problem yet.
The two big things I can foresee some potential progress on would be:
- Reducing launch/trip costs, as well as getting rocket fuel on Mars. SpaceX are making some effort there.
- Figuring out new ways to build stuff, on Mars, with Martian resources. That would involve any clever ideas on how to tackle this logistic nightmare, whether they use high-tech solutions (3D printers) or low-tech solutions (digging caves).
