To recap the decade that was, Billboard is looking at one major theme from each year and explaining how it dominated that 12-month period. Below, we continue with 2011, a year in which a singular star emerged from a lagging music business and nearly turned the whole thing around single-handedly.

An NPR headline from January 2011 summarized the state of the music industry the previous year rather bluntly: "2010 Was A Very Bad Year For Trying To Sell Music." The findings of Nielsen's year-end report, as summarized, were indeed bleak. The total of 326.2 million albums sold that year was the lowest number since SoundScan first compiled those numbers 20 years earlier, and 13% lower than the year before. Just as discouragingly, digital sales had plateaued, rising just a single percent. Where once there was hope that the volume and accessibility of iTunes' virtual library could help make up for the closure of physical record stores across the country, now it was clear that the sales chasm was too wide for it to bridge.

THE 2010S WERE THE DECADE THAT...

2010: Turbo-Pop Ruled the Radio | 2012: EDM Infiltrated Everything | 2013: Streaming Became Unignorable 2014: Cultural Appropriation Dominated the Pop Discussion | 2015: 2015 Was the Year That... Canadians Ran Pop Music | 2016: Every Major Album Release Was an Event | 2017: Latin Pop Took Over the U.S. | 2018: Hip-Hop Took Its Victory Lap 2019: Lil Nas X's 'Old Town Road' Put a Bow on the Decade

Katy Perry's "California Gurls" was listed as the best-selling song of 2010, but the digital success of "Gurls" and Perry's other myriad Teenage Dream smashes raised the question of if it was coming at the expense of LP sales. While Dream notched a staggering five No. 1 hits, by the end of 2011, the set had only been certified double-Platinum by the RIAA -- an impressive number for a regular pop album to be sure, but not a particularly resounding one for the album that made Hot 100 history with its radio dominance.

Conversely, the top album of the year on Billboard's 2010 year-end chart was from an artist with essentially zero radio presence: I Dreamed a Dream, by golden-throated 49-year-old Susan Boyle, who parlayed a viral appearance on U.K. reality show Britain's Got Talent into a surprisingly successful recording career. The implication of the numbers was clear -- teen pop fans raised on file-sharing and digital singles no longer had much interest in paying for full albums, but their parents still might.

In 2011, an artist emerged who reached maximum appeal with both groups. English singer-songwriter and BRIT School grad Adele had achieved some stateside success the previous decade with her 2008 debut album 19, named after her age upon its release. She had arrived as part of a wave of retro-minded, adult-friendly U.K. singers who found their way to American Top 40 stations and VH1 video playlists in the mid-to-late '00s -- led by Amy Winehouse, whose singular voice and force of personality made her a legitimate pop icon on both sides of the Atlantic. Adele's star presence was less immediately intoxicating than Winehouse's volatile charisma, but her pipes were second to no one's, and 19 single "Chasing Pavements" -- a bleary-eyed post-breakup torch song -- became a modest crossover hit, reaching No. 21 on the Hot 100 and winning Adele a Grammy for best female pop vocal performance.

As popular as 19 was, nothing could have prepared Adele or anyone else for the leap she would take with her sophomore set, 2011's 21. Recorded largely in the aftermath of a devastating breakup, 21 doubled down on the heartache of "Chasing Pavements" with a set of despairing ballads and overpowering breakup anthems, with only occasional mid-tempo relief. Few would have tabbed it off the bat as the stuff of blockbuster dreams, but the album was a runaway success almost immediately: In February, the set debuted at No. 1 on the Billboard 200 albums chart with sales of over 350,000, a first-week number usually reserved for veteran hitmakers and established stars, not young throwback imports with one major hit to their credit.

However, by that point, Adele was well on her way to the second major crossover hit of her career, with 21 lead single "Rolling in the Deep." A tensely chugging pop-soul kiss-off with one of the decade's great chorus releases, "Deep" proved a slow-burning Top 40 juggernaut. The song hit No. 1 on the Hot 100 in May in its 18th week on the chart, staying there for seven weeks, and ultimately topping Billboard's year-end Hot 100 for 2011. The chart performance of "Deep" only gets at a fraction of its cultural ubiquity that year: Instantly becoming a pop standard, the song was covered and reinvented over the course of the year by artists ranging from Patti Smith to Lil Wayne to the Glee Cast to Linkin Park. And as the single's profile continued to grow, so did 21's, as the album logged ten weeks at No. 1 over the year's first half, never leaving the Billboard 200's top three in its first 24 weeks of release.

"Rolling in the Deep" was an unexpected chartbuster, but it quickly turned out to be far from a fluke. "Someone Like You," a tearjerking power ballad performed with only piano for accompaniment, profiled even less like a radio hit than "Deep," but was pulled as the set's second U.S. single in August. After a captivating performance of the song at that September's MTV Video Music Awards, it jumped leaped from No. 19 to No. 1 on the chart, and by November, the impact of "Someone" was sizable enough to inspire a Saturday Night Live skit featuring multiple cast members listening to it and bawling their eyes out. ("Do you do it too?" Nasim Pedrad's character asks Kristen Wiig's at one point. "Everyone with a heart and an iTunes account does," the latter responds.) The sweeping "Set Fire to the Rain," released as single three in later that month, would follow "Deep" and "Someone" to the Hot 100's apex the next February.

By that point, 21's run was beginning to verge on the historic. The album spent 13 weeks total atop the Billboard 200 in 2011, and then returned to the top spot for 10 of the first 11 weeks of 2012 -- boosted by a Grammys victory lap in which Adele took home six combined trophies, including song and record of the year for "Deep" and album of the year for 21 -- to become easily the longest-reigning No. 1 album of the 21st century. That February, Adele mania was so strong that even debut album 19 returned to a new peak on the chart, reaching No. 4 after getting no higher than No. 10 during its original run. And by the end of 2012, 21 accomplished something many industry prognosticators thought impossible for a new album in the 2010s: It was certified Diamond by the RIAA for shipments of over 10 million copies, the first album released since Usher's Confessions in 2004 to notch such an achievement.

How did 21 do it? Well, if you listened to the traditionalists at the time, she did it by bringing "real music" back -- the kind of live instrument-based, feeling-first, soulful pop music that listeners weren't getting from a radio market increasingly saturated with Max Martin and Dr. Luke's brand of heavily compressed, synth-soaked, aggressively catchy turbo-pop. But while Adele's music did have a feeling of timelessness -- mostly meaning it just didn't sound unmistakably like the specific moment in time it came from -- it wasn't from a different mainstream galaxy, either. In its own way, "Rolling in the Deep" sounded as punchy on the radio in 2011 as "Firework" or "We R Who We R," while "Someone Like You" was of a piece with the piano balladry of Bruno Mars or OneRepublic (whose Ryan Tedder even produced and co-wrote fourth 21 single, "Rumour Has It"). The set snowballed commercially because it could speak to everyone in the family minivan -- the kids listening to Top 40, the parents reared on Motown and classic rock, maybe even the grandparents longing for the days of crooners on the radio.

But mostly, Adele worked because of Adele. She was a unique presence not only in 2011, but in all of 21st century pop: a preternaturally gifted singer and songwriter with a leave-it-all-on-the-floor approach to recording and performing -- and also an earthy, relatable, and strangely unassuming personality both on and off the stage. Even as Adele rose to a strata of sales that not even Lady Gaga or Beyoncé could claim, she never seemed like she was becoming part of the pop machine: She felt fresh, sincere, unworked. Due to the combination of her peerless instrument and her unquestioned personal and artistic integrity, Adele's heartbreak anthems connected on a wavelength entirely of their own, as if no one had ever quite meant it like her before. It was an unrepeatable formula, because the mere act of trying to repeat the formula would muck up the essential charm of the whole thing.

Thanks in large part to the assistance of Adele and the 5.8 million copies moved by 21, album sales in the music industry actually improved 1% in 2011 -- the first year where the numbers went up since 2004. Digital song sales saw even more robust improvement, raising 8.5% to a then-record 1.27 billion downloads. While most execs approached the evidence of music industry growth with caution --“It’s encouraging... but we’d be silly to jump up and down,” said Rob Stringer, then-chairman of Columbia, Adele's American distribution label, in a New York Times year-end recap -- others wondered if maybe there was finally a light at the end of the tunnel for the long-suffering music biz. “I think it’s the sign that the music industry is finally starting to come to figure out the digital present and future, at least when it comes to download sales," Gartner media analyst Michael McGuire said in the same Times article. "Perhaps we’ve seen the bottom.”

Such optimism would prove largely unfounded over the next few years. Album sales dropped 4% in 2012, a period in which 21 was again the year's best performer with 4.41 million copies sold -- the first time in the SoundScan era that an album had been the overall best-seller two years in a row. Album sales took an 8% tumble in 2013, and perhaps more alarmingly, digital song sales also dipped 6%, their first overall decrease over a full year since SoundScan began tracking them in 2003. The slippage got steeper in 2014, when album sales and digital song sales fell 11% and 14%, respectively. While blockbuster albums were released by major stars over this period -- Taylor Swift's crossover-completing 1989, Justin Timberlake's long-awaited comeback LP The 20/20 Experience, Beyoncé's game-changing, surprise-dropped self-titled set -- none of them even approached 21's stratospheric sales figures, leaving industry experts to wonder if any artist would be able to match that album's anomalous performance again.

In 2015, they got their answer, and once again it was Adele. Returning from a multi-year absence following the extended 21 run (and vocal hemorrhaging that required her to cancel the final dates on her ensuing world tour), the singer-songwriter released her follow-up 25 that November. The immediate returns were beyond even what most optimistic predictions would've expected: a jaw-dropping 3.38 million copies sold in its first week, zooming past the previous record of 2.42 million, set by *NSYNC's No Strings Attached during the CD-era boom times of 2000.

The set would move 7.44 million copies by the end of 2015, according to Nielsen Music, nearly quadrupling the year's second-place finisher. But this time, Adele alone wasn't enough to offset the turning tides: Overall album sales still dropped 6% from 2014, with digital song sales also sliding 12%. If the question was whether or not Adele could save the music business -- as many publications wondered at the time -- the answer was, despite her best efforts, a resounding no.

Although Adele wasn't able to single-handedly turn the industry around, she still taught it an important lesson about the unpredictable nature of pop stardom, and how some recipes for it are only makeable once. The hunt for the next Adele would be conducted by record execs and anxious fans for the entirety of the decade to come, but while some artists in her general mold found success, none could replicate the singular momentum that 21 did when it first swept audiences around the world away in 2011.

As the 2010s come to a close and the list of artists who can reliably sell even one million copies of an album in a calendar year continues to dwindle, Adele continues to lurk in the background, still in a commercial class by herself, still untainted by the great machine of pop music. And when she returns next, you can bet she'll once again give the industry a taste of the glory days of big sales -- though it'll ultimately prove a fleeting one.

Next, in 2012: A genre's full mainstream emergence changes the sound and direction of pop music, and gives the decade its early identity.