Mandatory emissions trading is only one way to help countries and companies switch to using less, and cleaner, energy.

ECONOMIST: Buttonwood

The terrain in these neighbourhoods is steep, so escalators operate one way in the morning for commuters and switch direction before lunch.

BBC: Living in: Hong Kong

One obvious way for firms in the power sector to reduce their emissions is to switch the type of energy they use, for example from inefficient coal-fired power stations to efficient gas stations.

BBC: Switching from high to low carbon energy is a challenge