City parks are beloved for the green spaces they provide. They're a respite from the skyscrapers and concrete, a way to get back in touch with Mother Nature and help to keep the heat island effect in check.

They may also be an unassuming front line in the battle against climate change, according to a study published in Carbon Balance and Management.

Indeed, urban forests in London may be storing as much carbon as tropical rainforests.

Trees of Camden soaking up carbon

Trees absorb carbon dioxide as part of their photosynthesis process, and urban trees are especially good at it, as Mathias Disney, one of the researchers of the study explains in a piece for The Conversation.

"Urban trees are particularly effective at absorbing [carbon dioxide]," he writes, "because they are located so close to sources such as fossil fuel-burning transport and industrial activity."

When trees absorb carbon dioxide and use it to create organic matter that they need to thrive, that matter becomes a large portion of their mass, sometimes making up almost half of it, according to Disney. Determining the mass of a tree is a tricky, however. Normally, the diameter of the trunk or the height of the tree is compared to the mass of similar trees, if not the same species of tree, that has been cut down and weighed in the past. As Disney points out, this method relies on the assumption that trees have a consistent size-to-mass ratio.

"But a fascinating property of trees is how variable they can be, depending on their environment," Disney writes. "So inferring the mass of urban trees from their non-urban counterparts introduces large uncertainties."

Hampstead Heath's trees soak up quite a lot of carbon dioxide. Chris Seddon/Shutterstock

To determine just how good the trees in Camden are at absorbing carbon dioxide, Disney and his fellow researchers at University College London (UCL) set out to measure the trees in the London borough of Camden. To do this, they used publicly available airborne LIDAR scans of trees combined with LIDAR scans conducted on the ground.

LIDAR, which stands for Light Detection and Ranging, sends out pulses of laser light every second and then measures how long it takes the light to return after it bounces off the object. You can get a sense of what the researchers saw t when they conducted these scans by clicking here to see Russell Square as an interactive "point cloud".

Armed with this LIDAR data, the researchers not only estimated the biomass of the 85,000 trees in Camden but also created a formula to predict the differences in size-to-mass ratios between urban and non-urban trees.

What the researchers found was that areas such as Hampstead Heath store up to 178 tonnes of carbon per hectare (t/ha) in comparison to the median value for tropical rainforests of 190 tonnes t/ha. Camden, as Disney points out, is also a particularly carbon-heavy borough.

Growing money

The trees of London are worth a lot. lazyllama/Shutterstock

So there's a lot carbon to soak up, and trees are just the organisms to do it. This makes them decidedly valuable, in more ways than one.

According to a UCL press statement about the study, Treenomics estimates that the environmental value of Greater London's trees is around £133 million a year ($176 million), with their carbon storage capacity worth around £4.8 million a year. Disney writes that while that number is well and good, it's still a little difficult to measure.

"This may equate to less than £20 a year per tree, but the real value may be much higher, given how hard it is to quantify the wider benefits of trees and how long they live. The cost of replacing a large, mature tree is many tens of thousands of pounds, and replacing it with one or more small saplings means you won’t see the equivalent net benefit for many decades after."

Disney and his fellow researchers hope that their study proves the value of trees in urban planning and that it will influence such efforts in the future.

The study was funded by the Natural Environment Research Council, in part through the National Center for Earth Observation (NCEO), a government organization that funds environmental science research.