With the leaders of some of the world's most polluting countries - the US, China and India - set to skip it, is the COP climate conference irrelevant? The event in Brazil next week marks three decades of target-setting and financial pledges - yet temperatures are still rising and so are emissions. Were all those promises a load of hot air? Have dreams of net zero gone up in smoke? Niall is joined by our science and technology editor Tom Clarke who is at the summit. Producer: Araminta Parker Editor: Mike Bovill