Imagine cities as living, breathing organisms. They inhale and exhale through plants – exchanging oxygen and carbon dioxide (CO2), energy and life. Understanding the delicate balance between photosynthesis and respiration in urban environments isn’t just science – it’s one of the keys to unlocking sustainable, profitable, and greener cities.
Photosynthesis: the city’s green engine
Photosynthesis is nature’s way of turning sunlight into life. During the day, urban plants absorb CO2 from the air, using sunlight to convert it into oxygen and energy. This process is like a city’s solar power plant, quietly working to clean the air and cool the environment.
In urban areas, photosynthesis is a game-changer. Trees, bushes, and hundreds of thousands of small gardens– 3.8 million garden plots in London – act as natural air filters. For example, a single mature tree can absorb up to 22 kg of CO2 annually. Multiply that by thousands of trees and you’ve got a powerful, self-sustaining system that reduces emissions and combats climate change.
In a city such as London, where there are 8.4 million trees, it’s even more pronounced. London’s trees absorb around 168,000 tons of CO2 every year during the day – around 1% of the entire city’s CO2 emissions – and that’s just one type of vegetation.
But photosynthesis isn’t just about cleaning the air. It also mitigates the urban heat island effect. By releasing water vapor through their leaves, plants cool their surroundings. It’s like a natural air conditioning system that saves energy and money.
Respiration: the city’s nighttime breath
While photosynthesis only takes place during the daylight hours, respiration happens 24/7. At night, when the sun sets, plants fully switch roles. They now absorb oxygen and release CO2 much like humans do. This process is essential for their survival, but in densely populated urban areas, it can tip the balance.
Think of respiration as the city’s nighttime breath. It’s subtle, but it matters. In areas with limited greenery, the CO2 released at night can accumulate, especially when combined with emissions from traffic and industry. This creates a double-edged sword: plants help during the day, but at night, they can add to the problem.
The balancing act
The interplay between photosynthesis and respiration is a delicate dance. In well-planned urban environments, the benefits of photosynthesis far outweigh the effects of respiration: a lot more CO2 is absorbed through photosynthesis than is emitted through respiration. But in concrete jungles with sparse greenery, the scales tip the wrong way.
Concrete and other impervious surfaces influence how much heat is absorbed during the day and emitted at night. Concrete and asphalt absorb a lot more heat and it’s not always good for natural plant growth. That is because it heat-stresses plants beyond their normal growing conditions.
Impervious surfaces also prevent water from reaching root systems, and this reduces the amount of evaporation from the plants above. Hard materials also limit the transfer of carbon and nutrients to the soil and roots. In some cases, hard surfaces reduce the amount of oxygen that can reach biomass under the soil, and this can prevent adequate respiration.
Local spikes in CO2 concentration can impact urban heat effects in non-negligible ways (e.g. CO2 959 ppm in Patna India, January 2025, more than twice the global average of ~426 ppm). These concentration spikes can be compounded by plant respiration at night; the cumulative effect can have a significant impact on cold-blooded herbivores such as moths for example. Impacts on lower order animals can impact the entire ecosystem.
The bottom line is that urban plant health is often measurably impacted, and that leads to less CO2 absorption and water transpiration.
This is where data-driven solutions come in. By mapping CO2 fluxes – the rate of movement of carbon between sources and sinks – we can identify areas where photosynthesis dominates and where respiration has a more pronounced effect – during the day and night This knowledge allows us to identify constraints posed by urban spaces and optimize them for photosynthetic activity – ensuring they are enabled to do as much as possible to absorb CO2 and reduce heat.
The opportunity
Urban green spaces aren’t just nice to have; they’re a necessity. Cities like New York have already proven this. In 2022, researchers from Columbia University found that urban plants eliminated 40% of the city’s CO2 emissions during summer months. This included all traffic-related emissions.
The UK has a similar opportunity. By leveraging high-resolution data and advanced modeling, we can pinpoint which urban areas are already net-zero and which need improvement. This isn’t just about saving the planet – it’s about saving money as well. Councils, businesses, and homeowners can make low-cost adjustments to their green spaces, potentially saving millions of pounds in unnecessary spending.
Be part of the cycle!
From carbon to oxygen: be part of your city’s natural cycle. Plant, protect, and contribute to changing the urban air and ecosystem with every seed you sow.
