How to preserve global variables in javascript when using Closure compiler with advanced optimization?

I have my own Javascript library, which I want to minify by using Google closure compiler with Advanced optimization. By looking at the docs I see how to declare functions which are used outside of the library.

However I couldn't find a way how to preserve global variables declared in my library. Closure compiler just removes them, because it thinks they are never used. Anybody can help ?

Edit: example code:

var variable_1 = true;

This is defined globally right at the beginning of my library, but it's never used in the library itself. It is used outside the library when it is included in some page. But that Closure compiler doesn't know and thats the reason it removes these declarations.