Ad-spending growth is poised for a significant slowdown in 2017, due to economic and political uncertainty, according to the latest forecast from Magna Global, the ad-buying agency owned by Interpublic Group of Cos.

The global ad business will see a slowdown to 3.6% spending growth next year, down from the 5.7% growth clip projected for 2016, when spending is expected to total $493 billion, Magna said. The 2017 forecast is an improvement over a June estimate that 2017 spending would grow 3.1%.

U.S. ad sales will face a slowdown to 1.7% growth in 2017, compared to 6.9% growth in 2016, according to the latest forecast.

Weighing on ad spending are several factors, including fewer advertising opportunities with big events like the election and Olympics, as well as uncertainty around the impact of Donald Trump’s presidential election victory and the U.K.’s decision to exit the European Union.

“If you look at the stock market reaction and also the consumer confidence index after Trump’s election, everything seems to be ok [in the U.S.] so far, but it’s too early to say,” said Vincent Létang, executive vice president for global market intelligence at Magna.

Globally, it’s the “lowest growth rate recorded in 15 years outside of the great recession in 2008-2009,” said Magna in its forecast.

Strong digital ad spending growth, especially in social and search, is one bright spot and explains why Magna upgraded its global forecast from June, Mr. Létang said.

The digital spending was fueled by marketing spending outside of measured media budgets, “and not just taken away from traditional media budgets,” he said.

According to Magna, digital spending is on track to surpass linear TV spending in 2017, thanks largely to a surge in mobile ad-spending. By 2021, mobile advertising will have increased to $215 billion, or 72% of total digital budgets, said Magna.