Global advertising expenditure is expected to grow 4.1% in 2018 and reach a total spend of $578 billion by year's end, according to recent research from Zenith.

The December 2017 forecast was based on Zenith's global advertising data for past years as well as spend projections through 2020.

Zenith forecasts that advertising expenditure will lag global GDP growth over the next few years, increasing around 4% per year through 2020.

Zenith forecasts that total global advertising expenditure will increase by $72 billion by 2020.

The United States is expected to account for 27% of additional ad expenditure, and China is expected to account for 20%.

Mobile advertising is expected to account for 65% of Internet expenditure and 29% of total ad expenditure in 2020—more than all traditional formats, except for television, put together.

Internet ad spend is expected to grow 10% per year between 2017 and 2020. By 2020, Internet advertising is forecast to account for 44% of global ad spend.

Display (which includes online video and social) is expected to be the fastest-growing Internet advertising subcategory through 2020.

About the research: The forecast was based on Zenith's global advertising data for past years as well as spend projections through 2020.