Viewership for the opening ceremony in London in 2012 was down from Beijing in 2008, which drew more than 1 billion viewers.
Viewers' habits change during every edition of the Games, with more searching for Olympics news online or through social media, fragmenting the traditional broadcast audiences that advertisers still pay a premium for.
U.S. ratings for the opening ceremony were down significantly compared to London.
Broadcasters boosted their Olympics coverage 25 percent to 125,000 hours from 100,000 hours of output in London, Lumme said. They also increased digital hours 2.5 times to 81,500 hours available, compared to London.
The IOC estimate that half the world will watch some part of the Games is in line with the last Summer Games in 2012, when the IOC found that 3.6 billion people around the world watched at least one minute. The world's population is roughly 7 billion.
Lumme said ratings are strong in the Americas, thanks to a favorable time difference, as well as in Brazil. Globo TV, the main Brazilian rights holder, had its highest ratings since the World Cup in 2014.
When asked about ratings being down in the United States compared to London on NBC's main broadcast channel, he said that ratings are on par with London if all of the platforms viewers are watching are combined, such as cable and online.
"People are just consuming the Games differently now," Lumme said.
He noted that NBC viewers had already watched 2 billion streaming minutes, more than the streaming activity of the five previous Olympics combined.
Lumme negotiates the IOC's long-term deals with broadcasters globally, which is the greatest source of revenue for the Games.
The IOC has said that Olympic broadcast revenues from 2013 to 2016 will be $4.1 billion, a 7.1 percent increase from the previous four years.
(Reporting by Liana B. Baker in Rio De Janeiro; Editing by Meredith Mazzilli)
