Boston has been ranked fifth in the country and sixth in North America for environmental sustainability, according to the recently released Economist Intelligence Unit’s Green City Index.

The city of Boston’s sustainable investments in building infrastructure, including the Boston Housing Authority’s $63 million energy performance contract with Ameresco to implement integrated water and energy conservation measures, contributed to the city’s high ranking, Boston Mayor Thomas M. Menino said during the Clinton Global Initiative (CGI) in Chicago Thursday.

Boston is also second only to San Francisco in green buildings and recently adopted the Massachusetts Stretch Energy Code, requiring 20 percent energy improvement across all new commercial and residential buildings above the base energy code, according to a statement.

"This study serves as further proof that we are taking the necessary steps to ensure the green future of our city, not just for this generation, but for the ones to follow," Menino said. "Taking care of the environment is and always will be a key component for Boston’s success. Sustainability means a cleaner and healthier environment, one that will encourage residents to stay and attract businesses to help boost our economy."