The Federal Reserve and other global central banks need to consider new stimulus tools to deal with what may be permanently lower interest rates worldwide, a top Fed official said on Saturday. With the so-called natural interest rate in the United States now near zero, and equilibrium rates in other countries around the world also lower than in the past, central banks have less room to stimulate their economies in the face of shocks, San Francisco Fed President John Williams said at a conference at University of California Berkeley’s Clausen Center.