Cities are one of our greatest inventions, they bring us together and drive wealth - but as more of us flock to live and work in them, we need to find ways to make them greener, more sustainable and efficient places to live. Physicist Dr Niraj Lal explores some of the engineering solutions driving change. To see how we’ll keep our growing population moving he visits the construction site of the largest public transport system ever undertaken nationally and discovers how engineers are finding ways to replace concrete - one of the biggest CO2 emitters in the world – with greener alternatives.