ON A SPRING day in 2021 some visitors to the ECB made an unusual entrance. Just before its governing council was due to meet, two paragliders landed on one of its buildings in Frankfurt, unfurling a banner reading: “Stop funding climate killers!” Another banner urged the bank to “Act on climate now!” Implicit in the stunt, timed to mark the release of a Greenpeace report on the bank’s bond-buying, was praise. Politicians might dither but central banks have the power to get things done.