Do you think we will ever find life on extraterrestrial planets in our life time. That would be an amazing discovery to live to see.
If so then thickening the atmosphere would suffice for terraforming, since Low Earth Orbit is below the effect of the protective magnetic field, and the only difference at that point is the atmosphere. We would have to ensure it has an ozone layer, though.
But I believe there is a caveat here; how bad does it get when a CME hits Mars? Has Curiousity experienced one of those?
A CME was my first thought as well. I think a few inches of water provide adequate protection, and if Mars has water we can use, the explorers can live in a terrarium inside an aquarium, but talk about cabin fever. It'd be almost like living in Antactic winter year round, pretty much trapped indoors except for quick scurries outside in heavy and restrictive clothing. No suit provides much protection from radiation.
Anyway, if by "terraforming" is meant giving it enough of an atmosphere to sustain life, it won't be any Earth life, except perhaps from some very hardy single-celled critters. Certainly no grass or trees. And then there's the matter of the lack of a magnetosphere, meaning that the atmosphere would be blown off into space by the solar wind.
Face it, Mars will always be a kind of desert. I'm not sure it's worth the considerable effort of attempting to terraform it.
A very strong CME could devastate the surface of earth as well by simply overwhelming the magnetosphere. It's happened in the past. One in 1859 produced auroras even at the equator and would have had devastating effects to any electrical grids exposed to it if it happened today. The really bad news is that CME's 10-20 times greater than even that one are possible. (source)
All true. But with no magnetosphere at all any CME would likely be devastating to anyone not in shelter, right? Perhaps even with a thick post terraforming atmosphere. That's the theory at least, but I'd like to know what actually turns out to be the case. We've already heard surprising stuff from Ray R about what Curiosity is picking up without a CME (basically the difference between Mars and Earth could be explained solely by Mars' thin atmosphere).