The environmentally friendly New York mayor, Michael Bloomberg, made his largest ever gift to the green cause on Thursday, giving $50m(£31m) to a campaign to shut down America's coal-burning power plants.

The sheer scale of the gift, to the Sierra Club, promises to transform the organisation's Beyond Coal campaign.

But it still falls short of the sums the Koch family has dispensed to defend oil and coal interests. According to Greenpeace, the billionaire oil brothers have donated $55m to the climate change scepticism cause.

America gets nearly half of its electricity from coal. The campaign aims to cut production by 30% by 2020, through shutting down the oldest and dirtiest plants and stopping the highly destructive process of mountain-top mining.

The Sierra Club claims to have already stopped the construction of more than 150 new plants.

With Bloomberg on side, it can put more coal plants in its sights.

The donation will account for one-third of the campaign's $150m budget over the next four years.

The Sierra Club called it a game changer, enabling the organisation to double its staff to 200, and to expand the anti-coal campaign from 15 to 46 states.

New campaign posters, with pictures of children as "filters" for coal pollution, went up throughout Washington Metro stations this week.

Since the collapse of efforts to get a climate change law through Congress, a number of US environmental organisations, such as Sierra Club and Greenpeace, have devoted more resources to the fight against coal.

The Sierra Club realised it needed more resources, however. Given Bloomberg's track record as New York mayor, where he has pushed to green the city's taxi fleet, the organisation decided to approach his charitable foundation for support.

For Bloomberg, the anti-coal campaign is a chance to step in where Congress has failed to act.

He said in a statement: "If we are going to get serious about reducing our carbon footprint in the United States, we have to get serious about coal. Ending coal power production is the right thing to do, because while it may seem to be an inexpensive energy source the impact on our environment and the impact on public health is significant."