Energy usage by large, old buildings like the Empire State Building represents a huge obstacle to cities’ dreams of carbon neutrality. New York City’s buildings account for 70% of its carbon emissions, for example, and half of those emissions are produced by the largest 5% of its structures. But retrofitting old buildings to make them more energy efficient represents a formidable challenge, both from an engineering perspective and in terms of convincing owners that doing so is in their financial interest.