I was just trying to call it up via browser and got those errors. Otherwise it will be cron'd in the scheduled tasks.
You cannot directly execute cron scripts. Instead, go to the Scheduled Tasks section of the Admin CP and run it from there.
You are getting the error because cron scripts needs to be wrapped with a bunch of vB code before they can be executed (in this case, global.php creates $DB_site, and because the file isn't wrapped by vB, it doesn't have $DB_site defined).