The global forecast had already been reduced from 4.8% in March to 4.3% in June, and the single currency area was again the culprit in the latest quarterly update.
Advertising spend in the Eurozone is now predicted to shrink by 3.1% this year, worse than the 1.1% retraction predicted in June.
The Publicis Groupe media agency tempered its pessimism with reassurance that this would swing back to 0.9% growth in 2013, albeit "assuming the eurozone remains intact".
It also upgraded its forecast for North America's growth in 2012 from 3.6% to 4.2%, noting a greater than expected outlay on political advertising for the presidential election and higher than expected ratings for the Olympics.
However, there was a cut to the forecast for Asia-Pacific, the second biggest region in terms of adspend, from 6.7% to 6.2%.
Commenting on the global forecast, Steve King, global chief executive of ZenithOptimedia, said: "Advertisers are broadly continuing to invest, despite the global economic concerns and issues. However, they are seeking to ensure that any expenditures are delivering strong return on investment.
"The US continues to deliver solid growth. This, combined with the growth in developing markets and in digital media, has helped mitigate the drop in eurozone spending."
Television advertising is expected to become a $200m global market this year, with its share rising from 40.2% last year to its highest ever at 40.4%.
Zenith commented: "The amount of time viewers spend watching television has increased, and even though viewers are presented with a wider choice of channels than ever, the biggest television events are attracting record audiences. The rise of the developing markets has helped lift television’s share as well, since these markets tend to rely heavily on television."
The inexorable rise of the internet will continue to play a key part in global growth, Zenith believes, tipping the medium for 15% annual growth between 2011 and 2014 at the expense of print.
It noted that the prospects for newspaper and magazine publishers are "not quite as bleak" as the headline figures would make them appear because the print figures do not include advertising in websites, tablet editions or mobile apps, which are picked up in the internet category.
Next year, global ad expenditure is predicted to grow 4.6% to $525bn, driven by growth in developing markets.
|Advertising expenditure by region: major media (US$m current prices)|
|Central & Eastern Europe||24,181||26,151||26,631||28,592||31,089|
|Middle East & North Africa||4,881||4,155||4,198||4,313||4,412|
|Rest of World||10,940||11,592||12,321||13,468||14,812|