As sales of CDs plunged over the last decade, the music industry clung to one comfort: downloads continued to sell briskly as people filled their computers and iPods with songs by the billions.

Now even that certainty seems to have disappeared, as downloads head toward their first yearly decline.

So far this year, 1.01 billion track downloads have been sold in the United States, down 4 percent from the same time last year, according to the tracking service Nielsen SoundScan. Album downloads are up 2 percent, to 91.9 million; combining these results using the industry’s standard yardstick of 10 tracks to an album, total digital sales are down almost 1 percent.

After enjoying double-digit growth in the years after Apple opened its iTunes store in 2003, song downloads began to cool several years ago. But the rate of decline this year — weekly sales began to lag in February, and the drop has accelerated rapidly in recent months — has caught the business by surprise.