The reason you are wrong is because you are not taking the length of time involved in these temperature changes. The Ice shelf, in a "normal" era of warming, would break apart over thousands of years. This breakage is happening in 10s of years. This is called "orders of magnitude" of difference. Orders of magnitudes are referenced whenever the range over which you are comparing two variables is so large, the only way to figure them efficiently is to divide every number by a common denominator and then figure those products. Its like the earthquake scale: Richter. a 1.0 earthquake has 1/10th the power of a 2.0 Earthquake and 1/1,000,000 the 7.0 earthquake. 

The basic explanation is, when something like this happens within our lifetime, and we can all sit around staring at it happen like a bunch of dumb clucks with our thumbs in our mouths, it ain't normal.