Sharing data between controllers in Ember

I'm new to Ember and I'm pulling my hair out over this.

The app I'm working on has a set of "alerts" that will be dynamically updated. I have the alerts template/controller/module all working fine. The alerts controller has a method for displaying the number of alerts that have not been dismissed. This works fine within the alerts template.

However, when I access that same method from another controller, it always returns 0.

Here's a pastebin with the controllers, templates and model

Thanks for any help!