WASHINGTON — Worsened drought stress, changing rainfall patterns, flowers and pollinators thrown out of sync: these only scratch the surface of the ways climate change challenges plant life. But warmer air and higher carbon dioxide levels can also fuel faster plant growth, limit plants' water loss, and extend growing seasons — enough so, in some cases, to offset the paving-over of green spaces in cities.
From 1982 to now, a new study finds, cities converting more than about 5.83 square kilometers (2.25 square miles) of land to impervious surfaces within their boundaries each year tended to see the total productivity of their plant communities decline. But in slower-developing cities, the ecosystem retained its ability to recover from land conversion, with climate conditions invigorating the remaining greenery enough for overall plant productivity to rise.
The offsetting effect doesn't apply equally in every climate. But knowing when and where it kicks in could help planners adjust the pace of development to conserve city greenery in the context of climate change, the researchers hope. Urban plants keep city air clean and cool, support city ecosystems, capture carbon dioxide and generally make cities more pleasant to live in.
"The speed of city growth matters. If cities expand too fast, they lose green land faster than plants can benefit from warming and higher carbon dioxide," said Han Chen, an Earth system scientist at Tianjin University and lead author of the study. "Slower and smarter growth can help cities keep stronger vegetation and better climate benefits."
The study will appear Thursday, July 2nd in Journal of Geophysical Research: Biogeosciences, AGU's journal for research on the biogeosciences of the past, present, and future Earth.
Cities, don't grow up too fast
Overall plant productivity in most cities drops as humans replace parks, lawns, farmland, wetland and patches of urban forest and grassland within city limits with buildings and pavement, canceling out any benefits the plant community as a whole might derive from warmer, more carbon-rich air due to climate change. Chen's team wanted to know whether a city's development rate affects how easily that canceling-out occurs.
"Previous studies mainly looked at one city or a few cities and often focused on the past," Chen added. "We wanted to study many cities around the world [and] include both past and future changes."
Accounting for the elevated carbon dioxide levels found in most cities, the researchers used a vegetation model and satellite observations to simulate plant productivity from 1982 to 2100 in 2,126 cities around the world, each with more than 50 square kilometers (19 square miles) of pavement and buildings to ensure they were covering the bulk of the global urban environment. The simulations considered various future scenarios of urban development and low, medium, and high greenhouse gas emissions.
Collectively, the cities in the study overshoot the team's "speed limit," developing an average of over 21 square kilometers (8 square miles) within their boundaries each year and climbing. North American and Asian cities build especially fast, averaging 26.6 and 35.8 square kilometers per year, respectively. Of the cities studied, 1,713 showed declining plant productivity, with Shanghai, Chongqing, and New Delhi emerging as notable hotspots.
But some cities fall below the threshold, mostly in Europe: In Giffnock, Scotland, a moderate urban development rate has helped keep the plant community stable or rising and able to take advantage of any potential climate boosts. Across the 413 cities where plant productivity rose from 1982 to the present, climate change accounted for approximately 69% of that trend.
The researchers project the "tipping point" may rise to 7.18 square kilometers (2.8 square miles) per year as climate change progresses through this century, allowing cities to convert land slightly faster before incurring vegetation declines. But those declines may be steeper, with the gap between greening and de-greening cities widening as urban development accumulates, too. Represented in terms of the carbon plants lock into their tissues as they grow, the team estimates, average annual city plant declines from now through 2100 will rise roughly 1.5% to 2% from the 1982-2024 baseline, with plants losing around three grams of carbon per square meter per year.
"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."
Urbanites need to touch grass, too
The researchers hope their findings will help inform urban development strategies. To maximize plant productivity, they write, fast-growing cities should make sure to preserve green space as they grow, while slower-growing cities should focus on managing existing plant life to take full advantage of climate conditions. Arid cities like Phoenix may want to take extra care since, as the team's analysis found, dry climates make plant communities disproportionately sensitive to urban expansion.
Efforts like these can help maintain plants as food and habitat for city ecosystems, Chen said. "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."
That stability allows plants to keep the city air clean and cool, store carbon, and make outdoor spaces more comfortable over the long term. "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."
