The Obama administration is backing the effort of major utility companies to dismiss a case seeking to impose curbs on emissions of greenhouse gases. A group of plaintiffs including eight state governments and the City of New York has filed suit seeking to force the Tennessee Valley Authority and five other utilities to reduce their emissions each year for a decade. On Thursday, the Justice Department said the case is unnecessary because the Environmental Protection Agency has sought to limit emissions. But environmentalists criticized the move because the curbs have only been proposed, not enacted.