For comparison, the great recession resulted in a 3.4% drop in GDP. Now, a 9% drop in work will result in less than a 9% drop in GDP (wealth is increasingly produced by machines, not people, resulting in a lower labor share of income), but even so, 9% is big.
By the way, as I always ask when this topic comes up, can someone link to actual studies? I.e., hard data rather than an innumerate reporter breathlessly advocating a proposal because it sounds cool?
Incidentally, a back of the envelope calculation (http://www.chrisstucchio.com/blog/2013/basic_income_vs_basic...) suggests that it would be vastly cheaper to adopt FDR's employer of last resort policy. If we are going to adopt a radical overhaul, lets consider the full search space.
[edit: some sources about the third world are cited on the right, not the bottom, and I missed them on first reading. Thanks davidx.]