The Press release from Universal is wrong on the ATYCLB figure as the album hit the 10 million mark in December of 2001 as reported by Billboard magazine using figures from Universal. By mid 2002 ATYCLB was at 11 million sold worldwide, with 4.1 million from the USA, over 500,000 from Canada, over 4 million from Europe, over 50,000 from New Zealand, over 300,000 from Australia, over 400,000 from Brazil.
Thats over 9.4 million in sales from the USA, Europe, Canada, Brazil, Australia, New Zealand. Rest of the world accounted for the remaining sales.
In addition to this, there was another individual in U2's organization or Universal who talked about the success of the album in the Spring of 2002 siting 4 million in sales in the USA with a total of 11 million worldwide.
The Joshua Tree hit the 14 million mark in December of 1987 as confirmed by multiple media articles from the time. At that time the album had sold 4 million copies in the USA(RIAA confirmed). USA sales would go on to improve to 10 million by 1995 as confirmed by RIAA. This adds 6 million to the 14 million mark, making global sales of the Joshua Tree by 1995 at least 20 million. Add in sales outside the USA after December 1987 and the total sales of Joshua Tree go well passed 20 million by 1995.
Then all sales worldwide since 1995 are added in, and obviously the album is well ahead of 20 million in sales and most likely closer to 30 million.