Large ships anchored in Japan's Tokyo Bay are being encouraged to turn off their engines to reduce greenhouse gas and airborne pollutants, reports said.

The Tokyo metropolitan government is to build a power transmitter at Hinode Pier in Minato Ward, and will start supplying power to moored ships, for a fee, by early 2011.

Moored ships commonly keep their engines running to generate power for onboard operations such as lighting and air-conditioning.

Anchored ships plugging on to shore-side electricity for power, or cold ironing, can help reduce emissions of carbon dioxide (CO2), sulphur dioxide (SO2) and nitrogen oxide (NOx).

The land power supply will initially be limited to domestic vessels such as cargo ships, large ferries and sightseeing boats that consume large amounts of bunker fuel. About 4,000 vessels fall under this category annually.

About 25,000 domestic vessels and 6,500 foreign-registered vessels call at Tokyo Bay each year, according to the Tokyo government's Ports and Harbors Bureau.

Calculations from the Construction and Transport Ministry suggest that if domestic vessels shut down their engines in Japanese ports, CO2 emissions could be cut by 1 million tonnes a year – about half the current levels of emissions, even after taking into consideration the land-based power generated for moored ships.

''[In this way], we might be able to reduce airborne pollutants by 90% or more,'' a ministry official was quoted saying.

The concentrations of NOx and other airborne pollutants along the edge of Tokyo Bay is reportedly 20-30% higher than the average across the capital's 23 districts.