The figures represent a slight decline from the 4.6% growth predicted in March 2015 — but the market should grow again in 2016 by 4.7%, accounting for an additional $25 billion in spend.

Further, Carat's latest forecasts reconfirm the continued solid growth for digital media, fuelled by the rise of mobile and online video spending trends, which should account for 24.3% of the advertising spend in 2015, and 26.5% in 2016.

For ten of the markets analysed, including the UK, Ireland, Canada and Australia, digital is now the principle media used based on spend, with the US market predicted to join this list in 2018 when the digital advertising spend is forecast to overtake TV advertising by more than $4 billion.

From a regional perspective, Carat confirms ongoing positive momentum in 2015 for most regions, although volatility is expected in some individual markets. Western Europe growth is expected at 2.6%, while growth should come in at 4.2% in North America, 4.1% in Asia Pacific and 12.7% in Latin America.

Western Europe's continued positive growth is driven by solid 2015 figures in the UK and Spain of 6.4% and 6.9% respectively, strong enough to withstand the political turmoil in Greece and its revised advertising growth this year of minus 12%, significantly down from the 8% growth predicted in March 2015.

And, despite a slight decline in growth forecasts due to China's economic downturn, Asia Pacific remains strong in 2015 with an above global spend rate of 4.1%, driven by high-performing India at 11% and growing Australia at 2.4%.

Carat's data also reports an encouraging outlook for 2016, with all regions predicted to increase year-on-year spend next year and Central & Eastern Europe to return to positive growth.

Digital continues to be the only channel warranting double digital growth, predicted at 15.7% in 2015 and 14.3% in 2016.

"Carat's latest advertising spend forecast shows optimism balanced with realism during a year of increased volatility in major markets such as Russia and China," said Jerry Buhlmann, CEO of Dentsu Aegis Network. "Noticeably, the landscape is becoming increasingly complex as previously grouped markets, such as the BRIC economies, are now operating differently and economic situations can quickly change markets at pace."

He added: "Digital media continues to achieve outstanding growth as the effectiveness of this medium and results achieved, especially with millennials, warrants the upsurge in spend levels. As digital rapidly evolves into a more established asset and programmatic and search bring stronger performance and efficiency, we continue to add value to our clients by delivering innovative solutions that are different and better."

The figures dovetail with Ovum's forecast that global ad spend will reach $200 billion in 2020.