welcome to hardtalk. i'm stephen sackur. when it comes to the global response to the climate change challenge, it pays to differentiate between words and deeds. at last month's cop26 summit, the chorus of concern from world leaders was deafening. but the actions? well, tough decisions on deeper emissions cuts to stave off catastrophic warming were put off until next year. my guest is the us special envoy for climate, former secretary of state john kerry. his mission remains — to restore american leadership on the biggest existential challenge facing our planet. is that mission impossible?