Around four percent of the world`s land surface is defined as urbanised, a figure expected to surge as the planet`s human population rises from seven billion this year to as much as 9.5 billion by mid-century.
But unlike forests, urban areas are absent in most calculations of "sinks" where vegetation soaks up CO2 naturally thanks to photosynthesis.
A new study, though, says the contribution can be significant.
British scientists carried out their survey on the central English city of Leicester, which has a population of around 300,000 living in an area of 73 square kilometre.
They measured the carbon-absorbing capacity of its parks, domestic gardens, abandoned industrial land, golf courses, school playing fields, road verges and river banks.
They found that 231,000 tonnes of carbon were locked up this way, 10 times more than expected. It is roughly equal to the average annual emissions of more than 150,000 saloon, also called sedan, cars.
"Currently, once land in the UK is considered to be urban, its biological carbon density is assumed to be zero," said researcher Zoe Davies of the University of Kent, southeast England.
"Our study illustrates this is not the case and that there is a substantial pool of carbon locked away in the vegetation within a city."
Urban "sinks" are not by themselves a solution to the billions of tonnes of carbon emitted globally but can help mitigate their impact, especially if gardeners grow trees, which absorb far more CO2 than grass and shrubs, she said.
In the case of Leicester, most of the publicly-owned or -managed land in the city comprises lawns. But if just 10 percent of this land were planted with trees, the city`s carbon storage would leap by 12 percent.
"If more trees are planted in urban areas for their carbon storage value, they must be the right kind of tree planted in the right place so that they have a long, productive lifespan, and when trees die they should be replaced," Davies cautioned.
Trees have an additional benefit of lowering the temperature locally, providing shade. In contrast, asphalt roads and buildings store solar radiation during the day, encouraging the creation of "urban heat islands" that can add dangerously to the effect of heatwaves on human health.
The new study appears in the Journal of Applied Ecology, published by the British Ecological Society.