Usually, it is assumed that via impact or some other mechanism enough of an atmosphere could be released on Mars to shield against radiation....

No one then works through the effects of volatile releases on Mars, which have occured naturally in the past due to impact.

What we can see from the current "sandstorms" starting at Hellas is that only a limited increase in Mars atmospheric density is enough to produce an extremely unstable environment - one dominated by sandstorms.

The assumption always adopted by never explicitly stated by terra-formers is that once an atmosphere is released on Mars, the resulting environment would be climatically stable, and amenable to man's presence - in other words that the "weather" would stablize into some non-threatening "enviroment"-

But there is no reason why the resulting "weather" would not be dominated by storms of incredible force... and as can be seen with the current storms, very good reasons to suspect that exacdtly this would be the case...

ep