The album, by far and away 2011's top seller worldwide, has taken little more than seven months since release to break the 10 million mark and looks likely to reach 13 million sales by the end of the year.
At a time when even the biggest albums globally struggle to sell more than 5 million copies in a calendar year, the XL album's performance has been remarkable. And the story does not end there with 21 on course to reach 3 million UK sales this week in record-breaking time.
"It's just phenomenal," said her manager Jonathan Dickins. "We knew we had a good record and privately we had some targets, but you can't legislate for this."
"We've taken the whole thing very much one step at a time, but no one could have anticipated this," added XL Recordings managing director Ben Beardsworth, who noted the album was this week enjoying one of its best weeks in a number of territories, including the US where 21's sales were expected to rise week-on-week by around 70% following Adele's MTV VMAs performance of Someone Like You.
The scale and speed of 21's success can be put into context when lined up alongside the biggest global sellers of recent years. Eminem's Interscope album Recovery was recognised by the IFPI as 2010's top worldwide album having sold 5.7 million copies by year's end and Syco artist Susan Boyle's I Dreamed A Dream led 2009's listings after selling around 6.0 million copies (it achieved that despite having only been released in mid-November that year).
Not since Blue Note/EMI's Norah Jones debut Come Away With Me, which was 2003's top global seller, has there been a release to dwarf the kind of numbers 21 has been doing.
Adele's rapid march to 10 million has been achieved because her album became a substantial hit in almost every major music territory.
It has been number one in 19 countries, including Australia, Canada, Germany and the US, where it is the year-to-date's top album by some distance, it sold its three-millionth copy there last month, according to Nielsen SoundScan.
It is also the first album by a British artist to reach this landmark in the States since Boyle's debut two years ago.
Unless something extraordinary happens during the last four months of 2011, 21's success will also mean the top-selling artist album globally will be by a British artist for the fourth time in five years following Amy Winehouse's Back To Black (2007), Coldplay's Viva La Vida Or Death And All His Friends (2008) and Susan Boyle's I Dreamed A Dream (2009).
Meanwhile, in the UK 21's sales reached 2.98 million units last week, according to the Official Charts Company, and are set to hit 3 million by the end of the week.
Sales this week were strengthened by Adele's appearance on Jonathan Ross's new ITV1 chat show where she performed Turning Tables. However, she had to pull out of the first two dates of her UK tour last Sunday and Monday because of illness.
Given the album firmly remains a Top 10 fixture in the UK, US and other key markets, there are likely to be plenty more sales to come before the year is up.
"The joy of an album coming out in January is we've had the whole year so far of great sales and we've still got our first Christmas to come," said Beardsworth.
"I imagine even for a record that has sold at this volume there are going to be some great sales at Christmas."