CNN Center in Atlanta. (Kevin C. Cox/Getty Images)

After leading NBC and its parent company, Jeff Zucker moved to CNN in 2013 with a singular mission: jump-starting the original cable news network’s flagging pulse. “Outside of my family and the Miami Dolphins,” he declared at the time, “there is nothing I am as passionate about as journalism.”

Zucker quickly set off on an ambitious remodeling project. He ordered programs such as a food-and-travelogue program starring the engaging Anthony Bourdain and a crime-investigations series with John (“America’s Most Wanted”) Walsh. In the meantime, he began focusing CNN’s still-considerable news resources on a single topic — missing jetliners, crippled cruise ships — sometimes for days or even weeks on end.

The result? Zucker says his changes are paying off, with prime-time ratings surging 10 percent so far in 2015. “We’re having a fantastic year,” he says with evident enthusiasm.

But it may not be such a fantastic decade, given the forces aligning not just against CNN but all of cable news.

The general drift for cable news’ Big Three — or Big Two and a Half, given MSNBC’s precipitous recent decline — has been downward. Cable audiences peaked during the 2008 election and have been eroding since. The 2012 election was something of a watershed; news audiences typically perk up during election years, but in 2012 they failed to come around.

CNN Worldwide President Jeff Zucker is enthused about the network’s ratings gains after his content-retooling efforts. But viewership is falling off across the cable news industry. (Riccardo S. Savi/Getty Images)

Since 2009, the median daily audience for Fox, CNN and MSNBC has fallen 19 percent, according to the Pew Research Center. The overall decline during prime-time hours for all three has been steeper, 26 percent.

Even Fox News, long the king of cable news, is no longer booming. It lost 2 percent of its overall audience last year, according to Pew, and is off 19 percent from its prime-time peak in 2009.

The increasingly cloudy picture suggests that cable news has seen its best days. The news networks’ audience grew steadily throughout the late 1990s and early 2000s as more households signed up for cable and satellite TV. But that avenue is closed; there are few new homes to wire.

At the same time, fast Internet connections have exploded, shifting the demand for instantaneous news from TV sets to smartphones, tablets and desktop computers. With its lengthy, linear storytelling style, cable news seems poorly adapted for social media, which emphasizes bite-size, shareable stories and clips.

Indeed, cable news’ problem isn’t the news part as much as the cable. While “cord-cutting” — the much-hyped trend of dropping or never getting a pay TV subscription — remains a distant threat, there aren’t many signs that young people, in particular, prefer a TV screen in the living room to the screen in their pocket. The networks have recognized this change for years and have been beefing up their Web operations to position themselves for it.

Nevertheless, the changing media landscape threatens not just Fox, CNN and MSNBC but the entire news-and-information category on cable — from business-news purveyors such as CNBC, Fox Business and Bloomberg TV to tabloid-oriented networks such as HLN to smaller upstarts such as Fusion and Al Jazeera America, suggests Derek Baine, chief analyst at the media-research firm SNL Kagan. Even the Weather Channel is vulnerable. “I can’t remember the last time I turned on the TV to get the weather,” he said.

With few new viewers tuning in, the cable-news market has become a zero-sum game. Audience gains for one channel tend to come at another’s expense. CNN’s mini-surge this year appears to be more a result of MSNBC’s falling ratings than of CNN finding new viewers.

MSNBC’s troubles (its overall audience was down 14 percent overall last year and 8 percent during prime time) have become so apparent that the network was the butt of a stinging joke from President Obama during the White House correspondents’ dinner last month. (“The polar vortex caused so many record lows, they renamed it ‘MSNBC,’ ” the president said.) MSNBC declined to comment for this article.

To fight the head winds, CNN has made the most dramatic transformation of the three leading news networks. It has effectively re-branded itself as a nonfiction channel, Baine said, moving away from its traditional news-and-talk format.

Zucker says the strategy is twofold. First, by creating “appointment” shows starring the likes of Bourdain, Walsh and Morgan Spurlock, CNN hopes to foster the kind of predictable and loyal audiences that flock to series on non-news networks. At the same time, CNN hopes to build viewer interest in an unfolding story with extensive live reporting. Although few people watch this coverage for hours on end, the goal is to entice viewers to sample CNN for a few extra minutes each day — a result that would greatly please advertisers.

“Over the next couple of years, with the strategy we’ve laid out, CNN will look like it looks now,” Zucker said. “We will emphasize the big story and the series and documentaries and films we’ve moved to.”

But in five years, the picture could look very different, said Gabriel Kahn, co-director of the media, economics and entre­pre­neur­ship program at the University of Southern California’s Annenberg School. Three macro-trends — the aging of the TV news audience, changing technologies and news-consumption habits — will imperil cable news just as it has devastated the readership and revenue of printed newspapers, he said.

“The [video news] environment will be radically different” in a few years, Kahn said. “Cable TV, and TV news in general, is so ill-equipped for this. They’re still producing content for a screen in the living room” while viewers are moving to mobile screens. Much like newspapers, the cable networks are vulnerable to digital competitors who aren’t burdened by TV’s high costs or traditional storytelling methods, he said.

In time, he said, the cable news networks will face bankruptcy the same way Ernest Hemingway once described a character’s financial demise: “Gradually and then suddenly.”

At the moment, however, the money is still flowing.

Fox News, which also declined comment for this story, remains one of the most profitable channels of any kind on television. It attracted $2 billion in advertising and license fees from cable operators last year, according to SNL Kagan’s estimates. It earned $1.2 billion — for a staggering 60 percent profit margin.

Even woebegone MSNBC, whose daytime audience has fallen to fewer than 300,000 viewers, did relatively steady business, turning a $206 million profit on $501 million in revenue last year, according to SNL Kagan (the comparable numbers for CNN: earnings of $327 million on $1.13 billion of revenue).

Even so, those numbers mask some bearish trends.

MSNBC’s and CNN’s earnings fell slightly in 2014. And MSNBC’s advertising sales slipped 5 percent. Overall, MSNBC’s revenue has been stagnant for the past three years. Even Fox has begun to weaken; its ad sales grew 2 percent last year.

The cable networks’ financial salvation has been the hundreds of millions of dollars of license fees paid by cable operators each year for the right to carry the networks in their channel lineups. These fees comprised nearly 60 percent of total revenue for the networks.

Licensing fees will provide a cushion for several years more, given that they are guaranteed in contracts that run five to seven years.

But after that? Baine, the financial analyst, said the next round of negotiations between networks and operators could be more difficult. “I bet there’s a bit of regret [among cable operators over their current payments] because of the big declines in the cable-news viewing audience,” he said.

It’s unlikely any of the news networks will disappear anytime soon, but they will have to adapt to survive for long, said Frank Sesno, a professor of media and public affairs at George Washington University.

“People haven’t stopped consuming news and information,” he said, “they’re just consuming it differently. So the issue isn’t whether the product or format goes away, but how it’s adapted to the new environment. It’s media Darwinism. It’s not survival of the fittest, but survival of the most creative.”

Sesno knows what disruption looks like from up close. He was a CNN correspondent and host when CNN had the all-news cable market to itself in the 1980s and early 1990s. He was there when Fox News and MSNBC started in 1996, and when the number of cable channels and Web sites began to explode a few years after that. Later, it was YouTube, ­iPhones and Facebook.

Though no longer employed by CNN, Sesno still occasionally hosts shows there. He sees it preparing for change. He notes that CNN recently expanded its digital political reporting team, hiring more than 40 people to cover the presidential campaign for the Web and TV. “And I bet every single one of them was cheaper to hire than a TV correspondent,” he said.

Zucker, for one, says he’s paying attention: “We recognize that people want to get their news on the Internet. So we’re covering more news on our digital platforms. We don’t feel the need to cover every one of those stories on TV.”