Is there such as thing as a custom multi-session global variable?

I have a WP page of “Upcoming Events” that the website ADMIN uses to post schedules and schedule changes. If a change is very critical I would like a way to display an “alert” icon on the homepage (that would alert users that they should read the “Upcoming Events” page.

In order to make this process as easy as possible on the website ADMIN is there a way to have a “custom global variable” that, for example, I could create a simple SHORTCODE that the ADMIN could include in the text on the “Upcoming Events” page when he/she is posting a critical event.

All that this shortcode would do would be to set a “custom global variable” to a value of “alert”. Meanwhile, the homepage would always look at this same “custom global variable” and if it finds that its value is “alert” then it would display a message such as “Scheduling Alert: check your “Upcoming Events” page for more info” (normally this message would be not visible on the homepage).

Is this doable or, if not, is there some other way that I can do this without forcing the ADMIN to manually update both the homepage and the Schedule page?