• by Wayne Friedman , September 2, 2009

After three years as anchor of ABC's "World News," Charles Gibson announced he will step down at the end of the year.

Sawyer will move from "Good Morning America," where she has been since January 1999, initially co-anchoring the show with Gibson. She will become the second woman to solo anchor a network evening newscast. Katie Couric became anchor of the "CBS Evening News" in April 2006.

ABC's "World News" pulled in the most advertising dollars of any evening newscast last year: $157.4 million, according to TNS Media Intelligence. "CBS Evening News" was second at $156.4 million, and "NBC Nightly News" came in third at $145.9 million.

Over the last three years, "CBS Evening News" grabbed $479.2 million in advertising revenue; World News" took in $459.5 million; and "NBC Nightly News" nabbed $449.5 million, according to TNS.

During Gibson's tenure, "World News" gained the top spot among viewers. For a four-week period starting in early 2007, it averaged 9.69 million viewers a night to NBC's 9.65 million. "CBS Evening News" trailed at 7.6 million. The last time "World News" went to No. 1 was ten years earlier.

Soon after ABC's spike, "NBC Nightly News with Brian Williams" reclaimed the top position, which it has held ever since. For a recent week -- August 24, 2009 -- NBC's "NBC Nightly News" pulled in 7.86 million viewers; ABC's "World News" averaged 7.08 million viewers; and CBS' "CBS Evening News" took in 5.39 million.

Overall, early-evening advertising spending on the three early-evening newscasts has been trending down over the years -- from $486.4 million in 2004 to $459.6 million last year, according to TNS. Media executives estimate that 30-second unit costs for early evening news are around $40,000 to $50,000 a commercial.

Gibson took over as interim anchor of World News after the illness and death in August 2005 of veteran anchor Peter Jennings. Then ABC News president David Westin named the co-anchor team of Bob Woodruff and Elizabeth Vargas to replace Jennings.

But less than a month later -- after Woodruff was severely injured in Iraq in January 2006 -- Gibson stepped into the anchor chair alongside Vargas. Later, Vargas became pregnant and announced she would step down. Gibson has been the sole anchor since February 2007.

"It has not been an easy decision to make," Gibson wrote in an e-mail to ABC staffers. "This has been my professional home for almost 35 years. And I love this news department, and all who work in it, to the depths of my soul."

Gibson said he had intended to step down as anchor of "Good Morning America" in 2007 before Westin enlisted him to anchor the nightly newscast.

ABC hasn't decided as yet who will replace Sawyer at the morning show.