In 1989, many nations signed a treaty aimed at halting the expansion of a massive hole in Earth’s ozone layer. Nearly thirty years later, the Montreal Protocol has done just that. But, according to a new study, it has also done something its architects never intended. It has become one of America’s most effective tools in the fight against climate change. Between 2008 and 2014, the Montreal Protocol led to greenhouse gas reductions with roughly half the benefit of all other U.S. climate regulations enacted.