Bernstein Research expects 2008 online advertising revenue to grow 20% in the United States and 22% overseas. Even if the macro-environment worsens significantly in the second half of this year, the analyst firm expects at least 19% revenue growth in the U.S. and 21% overseas.

Investors are most concerned about specific sectors cited in the recent earnings calls, where several companies referred to weaknesses in advertising, according to the report. Google, for example, cited weakness in auto and home financing, real estate and travel, describing the three sectors as economically sensitive.

Despite evidence of a downturn in automotive advertising online, the opposite is true of financial services. Outside of mortgages, the percentage of online advertising for financial services has increased significantly since the beginning of 2008.

While Bernstein Research has found that auto advertising has been "cut drastically," its 8% share of online ad revenue still is dwarfed by the 20% share of financial services, which continue to do well despite the mortgage crisis.

Finance, Insurance and Real Estate ranks as the largest sector, accounting for approximately 29.6% of online advertising overall. Media and Entertainment follows with 25.2%. Retail comes in at No. 3, with 13.8%. Other ranks No. 4 at 12.9%, and automobiles ranks No. 5, accounting for 8.6% of total online advertising. The research shows a slowdown in growth rate from the 27.6% for automotive advertising during the last four quarters to 3.8% in the first quarter of 2008.

During the first part of 2008, automakers reduced advertising on TV by about 7.4%; print by 12.8%; and radio and outdoor, by 9.1%. Compared with overall cutbacks, "online advertising held up relatively well," notes Jeffrey Lindsay, senior analyst at Sanford C. Bernstein.

The research firm also examined three economic scenarios to forecast total and online advertising growth for each that Lindsay dubs "current," "pessimistic," and "positive."

• The current economic scenario assumes a mild economic recession in the United States and a significant economic slowdown overseas. It provides a 2008 projection of 20% growth in the U.S. and 22% overseas, up 21% globally overall from 2007.

• The pessimistic scenario sees a drop in real U.S. GDP of between 1.5% and 1.9% for three to four quarters caused by a declining housing/mortgage crisis, combined with resurgence in oil prices through the end of 2008. While it trims worldwide online advertising growth to 20%, it reduces the 2009 growth forecast to 17%.

• The positive scenario assumes strong business performance through the end of 2008 and generally lower energy prices. In this case, the 2008 growth projection for global online advertising increases to 23% and the 2009 projection rises to 20%.

Although Lindsay notes a downturn in certain advertising categories referenced by Internet companies such as Google and Yahoo on second-quarter earnings calls, the effects have been offset by persistent growth in online ads, which gives a more positive outlook on the overall ad market.