Cities have been spreading out at a remarkable pace, and there is little to suggest that the outward push is slowing. New neighbourhoods, wider roads, bigger car parks and taller skyscrapers have steadily taken the place of parks, woodland, marshes and other open ground.
That shift usually leaves cities with fewer plants, less shade and noticeably hotter streets.
Climate change adds another layer to the picture. Rising temperatures, higher carbon dioxide concentrations, shifting rainfall patterns and a greater risk of drought can all place extra pressure on vegetation.
Yet warmer air and additional carbon dioxide can also stimulate faster growth for many plants, limit water loss and extend the growing season.
Recent research indicates these climate-driven boosts can partly counter the loss of plant cover caused by urban development-provided that construction does not proceed beyond certain thresholds.
Balancing growth and nature
Researchers found that cities converting more than about 5.8 square kilometres (2.25 square miles) of land into roads, buildings and other hard surfaces each year typically saw plant productivity drop.
Where development moved more slowly, the vegetation that remained was often robust enough to take advantage of shifting climate conditions, so overall plant productivity could rise even as some land was built over.
The findings indicate that climate change does not simply erase the harm associated with urban growth.
Instead, whether vegetation can keep pace depends strongly on the speed of development and on local climate conditions.
Looking at thousands of cities
The team set out to go beyond earlier work that concentrated on a single city-or just a small number.
“Previous studies mainly looked at one city or a few cities and often focused on the past,” said Earth system scientist Han Chen of Tianjin University.
“We wanted to study many cities around the world [and] include both past and future changes.”
By combining satellite observations with computer modelling, the researchers estimated plant productivity from 1982 through 2100 across 2,126 cities.
Each city in the analysis contained more than 49 square kilometres (19 square miles) of buildings and paved surfaces, enabling the researchers to assess a large share of the planet’s urbanised areas.
Their simulations tested a range of possible future development pathways, alongside low, medium and high greenhouse gas emissions.
Development outpaces nature
Overall, the cities included in the study are currently expanding far faster than the proposed limit. On average, they add more than 20.7 square kilometres (8 square miles) of developed land each year, and that growth rate is still rising.
Cities in North America averaged roughly 26.6 square kilometres (10.3 square miles) of new development annually, while cities in Asia averaged about 35.8 square kilometres (13.8 square miles).
Out of the 2,126 cities assessed, 1,713 experienced declining plant productivity. Shanghai, Chongqing and New Delhi were highlighted as major hotspots where vegetation fell most sharply.
Other places followed a different trajectory. Giffnock, Scotland, stayed under the development threshold, enabling plant communities to remain steady-or even improve.
Across the 413 cities where plant productivity has increased from 1982 to the present day, climate change was responsible for about 69% of that increase.
A changing tipping point
The researchers estimate that, by the end of this century, the development threshold could rise to around 7.3 square kilometres (2.8 square miles) per year.
Put simply, future climate conditions may let cities convert land a little more quickly before vegetation starts to decline.
However, this does not mean urban plants will have an easier time overall. The study estimates that average annual reductions in city plant productivity from now to 2100 will be about 1.5% to 2% greater than the 1982 through 2024 baseline.
Plants are expected to lose around three grams of carbon per square metre each year, reducing how much carbon is held in vegetation.
“The number looks small, but cities cover very large areas, so the total loss can become large,” Chen said.
“It is like a slow leak: After many years it means less plant growth, less carbon uptake, and weaker cooling from urban green spaces. In practice, this can make cities hotter and less resilient.”
Why city plants matter
Urban vegetation offers far more than visual appeal. Trees cool surfaces by casting shade and releasing moisture into the air. Plants can also capture some air pollutants, store carbon, reduce stormwater run-off, provide habitat for wildlife, and support people’s physical and mental wellbeing.
The study also found that dry cities warrant particular attention, because vegetation in these places reacts more strongly to urban expansion.
Cities such as Phoenix may need to safeguard existing green areas more carefully as growth continues.
According to the researchers, rapidly expanding cities should protect parks, forests and other green spaces even as new building projects proceed.
Cities growing at a slower pace, meanwhile, should focus on keeping vegetation in good condition so plants can take advantage of favourable climate conditions.
More than green spaces
Urban green spaces sustain much more than lawns and trees.
“Flowers support bees and butterflies, trees support birds, and fallen leaves feed soil microbes. These food webs help keep the urban ecosystem alive and stable, not just green-looking,” Chen said.
Maintaining the health of these ecosystems also improves outcomes for nearby residents.
“Green areas outside the city cannot provide the same direct benefits to urban residents,” Chen said. “It is important to have strong vegetation inside the city because this is where people live and feel heat, air pollution, and stress every day.”
The full study was published in the Journal of Geophysical Research.
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