Biofuels are carbon neutral. They require land, but a single engineered biofuel crop could grow on land that other crops couldn't. It's a promising use for the Canadian tundra that's melting.

Sails are very promising - a cargo ship would need enough fuel to maneuver in port and to generate electricity en route, but for most of the trip it could rely on (computer-controlled) sails.

Neither of these options are mature today, but they're a lot closer than nuclear-powered cargo ships.