A silent eco-revolution is happening! Cities are capturing heat emitted by computer servers and using it to warm everything from government buildings to college dorms. What was once waste heat is now seen as a renewable energy source, and it is no surprise that such systems are becoming more popular by the day. “You’re basically reusing something that’s currently being dumped, so there’s no additional climate impact to it,” says John O’Shea, who leads the heat and electricity division at Dublin’s energy agency Codema. His team estimate that their project is saving 1,400 tons of CO2 emissions every year, which equals a reduction of about 60 percent compared to the previous system of individual on-site boilers. And the trend seems to be building momentum, as this is just one of a growing number of district heating systems using waste heat from data centers. Many more are developed all over the world, with the most ambitious project yet aiming to deliver heat to 100,000 Helsinki residents, saving 400,000 tons of CO2 every year.