Mobile advertising is continuing to grow, but it's doing so at the cost of desktop ad spending, according to a new study.

While research firm eMarketer predicts U.S. digital ad spending to grow 14% this year and reach $41.9 billion, much of that incremental growth ($7.7 billion) will be driven by spending on mobile, not desktop, ads. In fact, spending on desktop ads is expected to decrease after it hits its peak in 2014.

Desktop ad spend will reach its pinnacle in 2014 at $35.39 billion, after which it will steadily decline to $35.26 billion in 2015, $34.4 billion in 2016 and $32.51 in 2017, eMarketer projects.

"The growth of mobile advertising is slowing spending directed toward desktop ad formats faster than expected, according to new figures from eMarketer, raising the question of whether spending on desktop formats like search and banners will ever grow again," eMarketer said in a statement.

Mobile advertising will continue to be a growth industry, however, going from $4.36 billion in 2012 to $7.65 this year, a 76% increase. Spending on mobile ads will increase through 2017 when marketers are expected to spend $27.98 billion on the category.