He writes that US economic growth will be less than 1 percent for the next 40 years.  This is in contrast to the above 3 percent growth the economy has experienced for as long as we can remember.
People take Grantham seriously because he predicted bubbles in Japanese stocks in 1989, U.S. stocks in 2000, and most risk assets in 2007.
In his words: 
The U.S. GDP growth rate that we have become accustomed to for over a hundred years – in excess of 3% a year – is not just hiding behind temporary setbacks.  It is gone forever. Yet most business people (and the Fed) assume that economic growth will recover to its old rates.
Going forward, GDP growth (conventionally measured) for the U.S. is likely to be about only 1.4% a year, and adjusted growth about 0.9%.
...
The bottom line for U.S. real growth, according to our forecast, is 0.9% a year through 2030, decreasing to 0.4% from 2030 to 2050 (see table on Page 16). This is all done presuming no unexpected disasters, but also no heroics, just normal “muddling through.
Here are some of the reasons he cites for low future growth: