Can America lead an effective global response to the climate change emergency? At last month’s COP26 summit in Glasgow the chorus of concern from world leaders was deafening, but the really tough decisions on deeper emissions cuts to reduce global warming were put off until next year. Stephen Sackur speaks to the US Special Envoy for Climate John Kerry. His mission is to restore American leadership on the biggest existential challenge facing our planet. But is that mission impossible?(Photo: John Kerry in the Hardtalk studio)

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