Unlike all other developed countries, save Australia, the United States, the world's top greenhouse gas emitter, has not agreed to regulate emissions of heat-trapping gases.

But even without regulations, some US companies, organizations and individuals fearing future rules, or wishing to reduce their impact on the environment, have begun to offset their carbon emissions. They do so by investing in projects like burning powerful greenhouse gas methane, at old coal mines, or preserving forests, which absorb carbon dioxide.

The power plant agreement was the "latest sign that smart energy solutions like wind power and energy efficiency are gathering steam," Carl Pope, the Sierra Club's director, said in a statement.

KCP&L will add 400 MW of wind power, 300 MW of energy efficiency, and take other actions to cut emissions, which could include shutting other units. The company will also take steps to reduce overall carbon emissions by 20 percent by 2020 and cut other pollutants, such as nitrogen oxide.

The Sierra Club will drop appeals that sought to stop the construction of the unit. Coal emits more carbon dioxide, the main gas scientists link to global warming, than any other fossil fuel.

The deal follows one Sierra struck last year with the municipal utility of Springfield, Illinois. That utility agreed to retire one of its coal plants, purchase 120 MW of wind, invest US$4 million in energy efficiency, and cut soot, smog and mercury emissions.