The figures are contained in the media agency's latest worldwide media and marketing forecast report, This Year, Next Year, which trimmed an earlier forecast of global ad expenditure growth for 2016 from 4.5% to 4.0%.
This downwards revision was largely due to slowing growth in China and Brazil this year. GroupM lopped off 2.5 percentage points from its previous projection for China, suggesting that a "new normal" was still being established.
But at 6.6% growth for 2016 (7.0% for 2017), China remains one of the world's top growing markets with "ample fundamental support tied to growing urbanisation and consumer confidence".
Brazil's downgrade, from 7% growth to just 1%, was rather more dramatic, amid ongoing political tumult, economic recession and rising unemployment.
With China slowing, the US is now set to become the leading contributor of global ad growth this year; GroupM revised its growth estimate for the year up from 2.7% to 3.1%, driven primarily by a healthier TV marketplace that will grow 3.4% rather than the 2.3% earlier predicted.
All these figures are eclipsed by the growth rates forecast for India: increases of 14 -15% in both the next two years will put the country in line to enter the top ten advertising markets in 2018.
Elsewhere, the report noted that the UK's vote to exit the European Union is not expected to result in a dramatic shift in advertising investment.
Adam Smith, GroupM's Futures Director, expected that over the coming six to twelve months companies would invest less and that, consequently, job creation, wage growth and productivity would be lower than it otherwise might have been.
"This is a difference of degree, not magnitude," said Smith. "Worst-case, we still see that UK advertising growth will reach 4.5% this year, propelled exclusively by the growth of digital."
That is broadly in line with Warc's latest analysis, published yesterday, which downgraded growth from 5.5% in 2016 to 4.2%.
James McDonald, Warc's Senior Data Analyst, told Marketing Week that there was "no advertising recession on the horizon" for the UK, despite the outcome of the referendum.
Data sourced from GroupM; additional content by Warc staff