The downward revision is attributed to lower-than-expected ad spending in Latin America, North America and Western Europe, but global paid media expenditure, boosted by digital advertising, is expected to climb 5.7% this year.
Digital advertising is continuing to show strong growth, with eMarketer estimating that global digital adspend will increase 18% this year to $170.17bn, accounting for almost 30% of total adspend.
North America, which has the world's largest overall ad market worth $195.26bn, will also have the largest digital ad market in 2015, with advertisers in the region expected to spend $62.07bn on digital formats this year, up 16.8% since 2014.
North America also has the largest mobile internet adspend in the world, which eMarketer values at $31.53bn for 2015, or 43.8% of all global mobile adspend.
Both the US and Canada are expected to record double-digit growth in mobile through to 2019, fuelled by increased spending on mobile video, mobile social networks and mobile search.
"Advertising investments in traditional media like TV, newspapers and magazines have been negatively affected by increased spending in digital formats, such as online editions for magazines and newspapers, and digital video replacing traditional TV," said eMarketer analyst Shelleen Shum.
Despite its adjustment for this year, eMarketer stated that the outlook for global ad spending "through 2019 remains optimistic, with advertisers gaining confidence as stability returns to most major economies".
Worldwide ad spending will reach £719.2bn by the end of 2019, the report predicted, although the pace of growth will vary by region. Western Europe will see the slowest growth while Latin America will see the biggest.
Warc's own International Ad Forecast, based on 12 key markets, expects global adspend to rise 2.8% on a US dollar basis this year, although this too is a downgrade from previous predictions. The 12 markets studied comprise some 75% of all adspend tracked by Warc.
Within the all-media total, online ad expenditure is forecast to grow 15.6%, while TV adspend is expected to fall by 0.9% in 2015. The overall contraction in TV ad revenue is largely accredited to an expected 3.3% decline in the US market, where major sporting and political events are absent this year.
Online ad revenue across North America is forecast to total $61.07bn this year, a 15.5% rise from 2014 but slightly behind eMarketer's expectations. Should it come to pass, online will account for just over a third of all advertising spend in North America by year end.
Data sourced from eMarketer; additional content by Warc staff