At one point the US economy was almost half of the world’s economy. Current trends suggest that US economy is loosing ground to other emerging economies.
China for instance has seen an exponential increase in its share of the global productivity where its share of the global GDP rose from around 2% in 1990s to almost 9% by 2008 (see graph below).
As for the BRIC economies, they continue to grab an increasing share of the global GDP as well, which is reflected by the steep increase in their share of global productivity. BRIC economies are account for 16% of the global output (see graph below).