After several false starts, mobile advertising has now truly taken off and is forecast to account for 37% of all growth in global ad expenditure this year, and 31% in 2014, according to a recent report by ZenithOptimedia.

Mobile advertising (defined in the report as internet ads delivered to smartphones and tablets, whatever their format) is still relatively small—with a total spend of $14.3 billion worldwide this year, or 2.8% of total ad expenditure.

However, it is growing extremely rapidly: 77% growth in 2013, followed by 56% in 2014 and 48% in 2015, ZenithOptimedia forecasts. By 2015, mobile ad spend is expected to total $33.1 billion and to account for 6% of total ad expenditure.

Below, additional key findings from the September 2013 Advertiser Expenditure Forecasts.

Internet Ad Growth Remains Strong

• Desktop Internet advertising continues to grow more rapidly than any of the traditional media. The report forecasts 8% growth in desktop ad spend in 2013 and 2014, and 7% in 2015.
• Total Internet advertising—both desktop and mobile—is expected to account for 20.4% of all ad expenditure in 2013, up from 18.3% in 2012, and it is forecast to rise to 24.6% in 2015.
• Display is the fastest-growing sub-category, with 20% annual growth, thanks partly to the rapid rise of online video and social media advertising, which are growing at 25% and 33% a year, respectively.
• Paid search is forecast to grow at an average rate of 15% a year to 2015, driven by continued innovation from search engines.
• Online classified ad spend has been subdued, since it depends heavily on property and employment markets in the developed world. The report forecasts average annual growth of just 4% in the upcoming years.