How can I detect significant changes in my data?
Looking for "outliers" will not give you the appropriate answer because you have a trend. 
Moreover the trend is not linear, so the answer will depend on the trend model.

What you should do is :

(1) fit the trend. I choose a logistic function with a MAD but an exponential with LSE growth would have lead the same results

(2) compute the residuals $e(t)$ as the observed data minus the model.

(3) search for the outliers on the residuals. I tool Tuckey's fence with $|e(t)|$ greater than 1.5 times the interquartile.

The attached graph shows that obviously you should choose an exponential growth with a multiplicative error, thus minimizing the MAPE and computing the errors in percentage.