Air Carrier Captain reported a 'cargo door open' EICAS illuminated during initial climb. After troubleshooting and conferring with Dispatch the flight executed a safe air return where Maintenance put the cargo door indicator on MEL and released the aircraft.

Narrative

On our scheduled flight from ZZZ to ZZZ1 we had an air return due to a cargo door 'open' indication. We ran our checklists and had no door open indications prior to pushback. The ramp gave their speech before brake release confirming doors and latches were closed and secure. We then had a normal and uneventful departure. At approximately 8000 feet in the climb we received a 'cargo door open' caution EICAS (engine indicating and crew alerting system). Leveling off at 14;000 feet the FO (first officer) continued to fly the aircraft while I ran the ECL (emergency checklist). We both decided that the pressurization of the aircraft seemed to be operating normally but we could hear a rhythmic noise at the approximate location of the cargo door. I confirmed with Inflight that they were not moving around the galley carts which might have caused that noise. After conferring with dispatch I opted to do an air return back to ZZZ. The landing and taxi to the gate was normal. After arriving at the gate the ramp supervisor confirmed that the forward cargo door handle was ajar. Maintenance put the cargo door indicator on MEL and after refueling we made our way to ZZZ1 with no subsequent indications.