Auckland mayor Len Brown insists the city must reduce its carbon emissions or risk getting left behind.

Brown has returned from UN climate talks in Paris, where countries agreed to limit global warming to two degrees, and he says Auckland needs to reduce its emissions in line with the agreement.

The mayor is also facing criticism over his trip to the climate talks, which he attended as part of the New Zealand delegation, and at the invitation of UN special envoy Michael Bloomberg.

Mayors of most major cities attended the Paris talks to discuss the role local councils can play.

Four Auckland Council delegates also attended the talks.

Brown said reducing emissions is good for the environment and thinks a low carbon footprint will be attractive to visitors, investment, and foreign companies.

He praised Auckland's transition to electric trains, reducing the city's emissions.