It is very appropriate that a conference on sustainable cities should devote a whole day to energy. Because if energy systems are not sustainable, then in the end nothing can be.
Energy sustainability is no longer about running out of fuels.
That was the argument of the seventies. The limit now is the capacity of the atmosphere to absorb the wastes from the use of fossil fuels. There is already enough fossil fuel, mainly coal, discovered and accessible, to raise the carbon dioxide content of the atmosphere ten fold. And yet the work on climate change is worrying about the severe impacts of merely doubling it. And unfortunately more fossil reserves are being discovered all the time.