Extending its gains for the fourth straight session, the Sensex at the Bombay Stock Exchange climbed 133 points, or 0.7 percent, to a fresh 31-month closing high of 18,800, amid firm global markets, today. The Nifty at the National Stock Exchange rose 32 points, or 0.6 percent to 5,640.Stock markets in Japan, Hong Kong, South Korea and Singapore rose between 0.3 percent and 0.8 percent.