Not sure what you mean by "decline has been happening since 2012."
If you're saying the economy has been declining since 2012, no, we have not been declining since 2012. The US has been in a continuous economic growth/boom since 2011, around the time when much of the economy mostly recovered from the great recession. Stock market levels around 2011 equalized to the levels of 2007 before the great recession hit. So from 2011 to 2019, we have been in 8+ years of continuous growth.
The Nasdaq index in 2016 was at ~2x the levels before the 2007 recession and ~3.5x the levels of the middle of the recession. In 2019, the index is ~3x what it was before the recession and ~5x what it was during the middle of the recession. Right now, the market is at approx twice the levels it was at the height of the dot com bubble.
Since we've been in a growth period for so long, we're due for a recession within 2 years or so if the business cycle over the last 100 years is any indication.
You didn't read The Economist article in my link? Global trade is the topic and that's the decline since 2012.