ZenithOptimedia’s optimistic predictions for online is holding steady. The Publicis Groupe media buyer’s latest forecast expects global internet ad spend to grow 26.7 percent and break through the 10 percent share barrier this year—a year earlier than Zenith predicted just three months ago. By 2010 Zenith predicts online will attract 13.6 percent of all advertising, well ahead of the company’s previous prediction of 12.3 percent. As for actual dollar amounts, Zenith sees worldwide online ad spend this year of $52.2 billion, $64 billion in 2009 and $78.1 billion in 2010.

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The sunny views for online advertising are included in a report that expects darkening clouds for growth in North America and Europe: Zenith has downgraded its forecast for the former to 3.5 from 3.7 percent, while growth for the latter is expected to end up 3.7 percent higher instead the earlier prediction of 3.9 percent. But thanks to developing world’s growing ad spend, Zenith has nevertheless upgraded its overall spending forecast slightly, expecting a 6.6 boost in 2008, up slightly from the 6.5 percent growth predicted in its March forecast.

The increasing economic uncertainty that Zenith notes is swirling in Europe and the North America will accelerate the shift to online, Zenith said. Aside from that, the report cited improved online video and targeting abilities as further reasons for marketers to shift more of their budgets to the web. Still, Zenith’s positive predictions for online ad spend comes after several other, less sanguine industry reports. Just over a month ago,  Lehman Brothers analyst Doug Anmuth said online ad spending in the U.S. will be up 23 percent, down from his previous call for 24 percent growth. Meanwhile, display growth has been trending lower, according to TNS’ look at Q1, when display gained only 8.5 percent compared to the previous year’s doubly-digit quarterly growth rates.