The more challenging outlook for 2015 reflects downgrades in half of the 12 major markets covered in the report, including the US (-3.0pp) and China (-1.5pp), the two largest economies in the world. The 12 markets studied comprise some 75% of all advertising expenditure tracked by Warc.
Russia (-15.1pp), Canada (-2.9pp), Italy (-0.7pp) and the UK (-0.3pp) make up the remaining four major markets where adspend growth has been downgraded.
However, despite its modest downgrade, the UK is still expected to post 6.6% growth at current prices this year, making it the third fastest-growing major market behind China (9.0%) and India (16.1%).
There is a bleaker outlook for Russia, which is suffering from low oil prices and decreasing consumer spend. Adspend in Russia is expected to decline 13.1% in 2015, making it the only market other than France (-0.2%) where adspend is projected to fall.
The US, the world's largest ad market, is expected to record adspend growth of 1.4% in 2015 despite its downgrade of 3.0pp since Warc's previous forecast.
US adspend growth is being dragged down by the performance of its TV sector, which experienced a weak 'upfronts' period earlier this year and, unlike last year, has not benefitted from the stimulus of major sporting and political events. US TV adspend is expected to decline 3.3% this year.
More positively, all markets apart from Russia are expected to post adspend growth in 2016 with India rising at the fastest rate (+12.4%) followed by Brazil (+7.7%), which is likely to benefit from the Rio de Janeiro Summer Olympics.
Taken together, adspend across all 12 major markets is expected to total $413.7bn at current prices by the end of the forecast period in 2016.
In real terms, after accounting for inflation, the total is expected to be $325.3bn, or $8.5bn below the peak that preceded the global economic downturn in 2009.
Commenting on the report, Warc data analyst James McDonald said: "After a strong rise in global advertising spend last year, the outlook for 2015 is more sobering.
"With three of the world's largest economies now in recession, and slowdowns seen in the US and China, we expect a degree of caution when it comes to committing ad budgets this year.
"That said, advertisers have increased spend every year since the 2009 crash, with the influx of money for internet ads underlining the growing range of options for marketers."
A free sample of Warc's International Ad Forecast can be downloaded here.
Data sourced from Warc