What is the real temperature to caramelize sugar?
a lot of recipes caramelize sugar in a simmering water.


Calling the solution "simmering water" isn't a good characterization.   The boiling point of pure water is 100C.  But the boiling point rises as the concentration of sugar in the solution increases.  Once you're above 75% or so, the boiling point increases significantly.  For 90% sugar (still 10% water remaining), it's up to around 120C.  

As the water evaporates, the sugar concentration, the boiling point, and therefore the temperature all increase.  When you reach 160C, there's probably less than 1% water and the decomposition of the sugar (carmelization) rate starts to increase significantly.

There are some charts/tables for different concentrations here, but they only goes up to 90%