New York City mayor Michael Bloomberg slammed national governments worldwide yesterday for their lack of action on climate change, saying their empty words on global warming had left a 'vacuum of leadership' which cities must fill. 'We've seen how national governments have struggled, both at home and on the international stage, to take climate-change action,' he told delegates at a four-day conference in Hong Kong yesterday. 'Cities must learn from that experience. We must be bolder, we must be more collaborative and we must be more determined. Together, we must fill the vacuum of leadership ourselves.' Bloomberg made the comments during his first address as chairman of the global climate-change network C40, taking over from outgoing chairman Toronto mayor David Miller. The network, which formed in 2005, has 40 member cities including Hong Kong, Beijing and London that share information about their climate-change initiatives. Bloomberg borrowed Miller's mantra 'nations talk, cities act' as he outlined how the group would tackle climate-change issues while national governments lagged behind. 'Over the past five years, our cities have, in short, demonstrated that we are prepared to boldly confront climate change. As mayors, we know that we don't have the luxury of simply talking about change without delivering it.' He said the group represented one-fifth of global gross domestic product and about 12 per cent of the world's carbon emissions, so the group's actions were significant. Bloomberg said he was heartened by the willingness of mainland mayors to talk about the environment. 'The interesting thing in this trip is to see mainland Chinese mayors, all of a sudden, worried about the environment. Even in China, they are, all of a sudden, talking about fewer coal-fired plants.' Bloomberg listed four key strategies for C40 cities over the next two years, such as a broader agenda for sustainable living, compiling comparative data and more co-operation and collaboration. Plans to retrofit buildings for energy efficiency and switching to electric-powered vehicles were important priorities, he said, but other areas such as water conservation and waste recycling needed attention, too. He said climate-change concerns differed from city to city, so the C40 network would develop solutions 'as well-tailored to those realities as a Hong Kong suit'. After the conference, the billionaire mayor took the subway from Wan Chai to Central before transferring to the airport line to Tsing Yi. 'I take the subway in New York everyday,' Bloomberg said. 'I think they're both great systems. Somebody said to me 'do you like the subway?' and I said if you like the subway, you should see a psychiatrist.'