Some critics of the Environmental Protection Agency’s new requirements for power plants argue that forcing emissions reduction will curtail economic growth. But the recent experience of states that already cap carbon emissions reveals that emissions and economic growth are no longer tightly tied together.

One of the ways that states will be able to meet the new E.P.A. standards is by joining a Northeastern cap-and-trade program known as the Regional Greenhouse Gas Initiative, which first put in a carbon cap in 2009. In a cap-and-trade system, the government places a ceiling on total carbon emissions and issues permits for those emissions, which companies can buy and sell from one another.

The nine states already in the program — Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New York, Rhode Island and Vermont — have substantially reduced their carbon emissions in recent years. At the same time, those states have had stronger economic growth than the rest of the country.