Negative feedback: reversing carbon emissions

It is increasingly clear that putting less carbon dioxide in the atmosphere will not be enough to combat climate change; we take a look at the effort to actively remove the stuff from the air. Our correspondent takes a ride on Chicago’s Red Line, whose length represents a shocking level of inequality. And why a push to go organic in Turkey isn’t everyone’s cup of tea. For full access to print, digital and audio editions of The Economist, subscribe here www.economist.com/radiooffer