London—ZenithOptimedia said global ad spending will grow 6.1% this year to $406.5 billion, a slight uptick from the 6.0% growth prediction it made in April. Zenith said it raised its growth forecast based on strong economic progress in the U.S. and Europe. The media agency also said ad spending will grow 5.3% in 2007 and 5.6% in 2008, compared with the 5.0% average growth rate of the past decade. The Internet is growing faster than any other medium, according to ZenithOptimedia. It predicted the Internet will account for 7.0% of worldwide ad spending by 2008, up from its 6.5% forecast made in April as well as an increase from its 6.0% forecast made in December. Internet advertising spending will grow 76% between 2005 and 2008, the agency said.