Complex sentence: Downsizing imagines what might happen if, as a solution to over-population, Norwegian scientists discover how to shrink humans to tall and propose a 200-year global transition from big to small, but with one catch: the procedure cannot be reversed.
Split into simpler sentences: Downsizing imagines what might happen if, as a solution to over-population, humans could be shrunk to a height of, after Norwegian scientists discover how to do just that. A 200-year global transition from big to small is proposed, but there is one catch: the procedure cannot be reversed.