A tree in Highgate Cemetery, as seen using LiDAR. The cemetery is rich with old trees, and LiDAR has revealed just how much carbon they are storing. University College London.

City-dwellers usually think of their impact on the global climate in terms of energy consumed or products bought. They may, however, be underestimating the importance of something right outside their doors. The parklands of London store nearly as much carbon as tropical rainforests, making urban forests a much-overlooked frontline in the fight against global warming.

Street trees and parks remove an astonishing amount of pollutants from the atmosphere, improving the health of the urban residents. They also cool the local environment, causing people to go easier on their air conditioners, and make an important contribution to city-dwellers' psychological health.

Still, the comparison with mighty old-growth forests comes as a surprise. A typical tropical rainforest holds 190 tonnes of carbon per hectare (85 tons per acre). As these are trashed to make way for cattle ranches or palm plantations, most of this carbon is released into the atmosphere, amounting to an estimated 15 percent of human-induced global warming.

Hampstead Heath, a large park in London, stores 178 t/ha. The finding is particularly remarkable because so much of the heath is made up of open grassland rather than forest. However, it is consistent with previous research showing temperate forests are several times more carbon-dense, although much less biologically rich, than tropical rainforests, in part because fallen wood takes much longer to rot.

The conclusion is drawn from a study published in Carbon Balance and Management of all 85,000 trees in Camden, the London Borough that includes Hampstead.

“We were able to map the size and shape of every tree in Camden, from forests in large parks to individual trees in back gardens," said first author Dr Phil Wilkes of University College London in a statement. "This not only allows us to measure how much carbon is stored in these trees but also assess other important services they provide such as habitat for birds and insects."

The mapping was done using light detection and ranging (LiDAR), where laser pulses are fired at an object to produce a detailed three-dimensional structure. The team have previously used LiDAR – combined with measurements of the amount of carbon held by tree trunks, roots, and branches – to estimate rainforests' carbon density.

on top of RIEGL from Phil Wilkes on Vimeo.

Based on these measurements, the environmental economics social enterprise Treeconomics estimates that the carbon storage of each tree in greater London provides a value of £17.80 per year. This is, however, small in comparison with the total benefits a typical urban tree provides, which are estimated at £493 per year.

The study goes far deeper than the averages, however, revealing the differences in carbon storage that the trees display depending not just on species and size but location. The work may give pause to councils that sacrifice large numbers of trees for redevelopment.