Global property services company Knight Frank is out with its 2011 Wealth Report, and although it’s beautifully designed, it comes bearing bad news for the D.C.-as-global-city crowd. Washington D.C. is ranked number 12 in the Global Cities Index, the same as last year. But based on an “attitudes survey,” analysts expect D.C. to slip to 15th place by 2020, as cities like Sao Paolo, Mumbai, and Shanghai rise in the ranks.

If you’re an American declinist type, most of that slippage will come in the “political power” category, where D.C. still ranks number one. If you’re an optimist, it might not be so bad, if D.C. manages to get its economic act together and raise its ranking in that category from 31st place to something more respectable.

One might argue that Washington’s popularity with the wealthy shouldn’t matter as much as livability and affordability for all classes, but I’d counter that a city with excellent quality of life, global political reach, world-class educational institutions, and economic girth offers more opportunities for everybody.

Anyhow, the whole thing is worth a read.