The assessment by UN-Habitat said that the world's cities were responsible for about 70% of emissions, yet only occupied 2% of the planet's land cover.
While cities were energy intensive, the study also said that effective urban planning could deliver huge savings.
The authors warned of a "deadly collision between climate change and urbanisation" if no action was taken.
The Global Report on Human Settlements 2011, Cities and Climate Change: Policy Directions, said its goal was to improve knowledge of how cities contribute to climate change, and what adaptation measures are available.
Joan Clos, executive director of UN-Habitat, said the global urbanisation trend was worrying as far as looking to curb emissions were concerned.
"We are seeing how urbanisation is growing - we have passed the threshold of 50% (of the world's population living in urban areas)," he told BBC News.
"There are no signs that we are going to diminish this path of growth, and we know that with urbanisation, energy consumption is higher. (read more)