AT&T has started pushing out a software update for the iPhone 5 and the iPhone 4S in the US, which adds support for Wireless Emergency Alerts.
Wireless Emergency Alerts are part of the FCC’s CMAS program and are mandated by law. You may turn off alerts (except for Presidential alerts) if you choose. Go to Settings >Notifications>Turn On/Off.
Once you install this update, your phone will be able to receive these alerts, which are issued by the government. You will receive notifications for things such as AMBER (America’s Missing: Broadcasting Emergency Response) alerts, emergency alerts for man-made or natural disasters or Presidential alerts.
While the software update is mandatory, you are not charged for the data to deliver it and Wireless Emergency Alerts will never count against your messaging plan. The update will be sent to users with iOS 6.1 or higher.