Now the 102-story building has a new tale to tell: efficiency. A recent $20-million retrofit — which included everything from cleaning and re-insulating more than 6,000 windows to caulking leaks in the building’s facade — reduced energy use by nearly 40 percent, according to air and energy program lawyer Katherine Kennedy of the Natural Resources Defense Council. And the Empire State Building is just the most visible example of a new trend — retrofitting old buildings to be more energy efficient. For example, billionaire businessman Richard Branson’s Carbon War Room and the Ygrene Energy Fund are leading a consortium that will invest as much as $650 million in similar retrofits in Miami and Sacramento.

Globally, businesses, national and local governments, and property owners have begun to retrofit millions of older buildings in a bid to cut down on energy use and reduce greenhouse gas emissions. These retrofits are the most cost-efficient way to combat climate change and save on rising power bills, according to analysts ranging from the McKinsey Institute to the ‘If it will pay out and save energy, building owners will do it,’ says one expert. International Energy Agency. Melbourne, Australia, for example, plans to reduce energy consumption in 1,200 of its office buildings by the end of the decade. In the U.S., the Environmental Protection Agency is running a “Battle of the Buildings” in which 245 facilities compete to save the most on their utility bills through energy efficiency improvements. In recent years, five major international banks have joined four multinational energy services companies and invested $5 billion in retrofitting old buildings in 16 of the world’s biggest cities, from Mexico City to Mumbai.