In the first case, the court ruled 5-4 that carbon dioxide and other greenhouse gases are considered air pollutants under the Clean Air Act and that the Environmental Protection Agency has the authority to regulate the gases.

The Supreme Court´s decision in Massachusetts v. EPA rejects a 2003 EPA decision that it lacked the authority to regulate emissions of carbon dioxide and other greenhouse gases. The EPA made the decision as environmental groups and 12 states were asking the agency to regulate emissions of greenhouse gases from automobiles.

The Supreme Court´s decision on April 2 orders the EPA to make a fresh decision on curbing greenhouse gas emissions from new cars and trucks relying on the best available science.

Justice John Paul Stevens wrote the majority opinion, while Chief Justice John Roberts filed the dissenting opinion.

In the second case decided on April 2, the Supreme Court vacated a lower court´s decision in the Environmental Defense v. Duke Energy case pertaining to new source review.

In overturning the 4th Circuit Court of Appeals decision, the Supreme Court unanimously upheld the EPA´s view that changes in power plants that may contribute to air pollution must be done only with a permit if there is an annual increase in emissions.

The ruling was a victory for environmental groups that have argued that power plants should upgrade their emission control equipment if they undertake modifications that increase annual, not hourly, emissions of nitrogen oxides, sulfur dioxide and other pollutants.

Justice David Souter wrote the opinion for the court.