The State We're In

Mar 21, 2024

By Jay Watson, Co-Executive Director

Ever heard of a “sponge city”? Sponge cities are urban areas with a superior ability to absorb rainfall and prevent flooding, thanks to abundant natural features like trees, lakes, and parks – and good infrastructure design.

As climate change brings warmer temperatures, and stronger and more frequent storms, “sponginess” is becoming an increasingly desirable city attribute.

A recent study analyzed the sponginess of 10 major world cities: New York, Montreal, Toronto, London, Singapore, Mumbai, Auckland, Sydney, Shanghai, and Nairobi. The international design firm Arup evaluated cities on their amount of green and blue space, soil types, and water runoff potential. Each was given a sponge rating of 1 to 100 percent, according to the amount of water it can absorb.

Auckland came out top with a 35 percent rating, due to its storm water systems and plentiful parks, golf courses and gardens. New York City was in the middle of the pack at 30 percent, tied with Singapore, Mumbai and Toronto. London came in second to last at 22 percent and Sydney was ranked the least spongy at 18 percent. London and Sydney both have low percentages of green and blue space, and moderately high runoff.

Arup’s analysis was not a competition, but was intended to get urban leaders to think of nature as an asset: valuable infrastructure that should be protected and enhanced.

Fortunately, almost any city’s sponginess can be improved through natural solutions like adding trees, parks, meadows and greenery. Boosting a city’s absorbency not only makes it more flood-resistant, it also makes it more resilient to droughts because the ground holds water longer.

Adding trees is also critical to reducing the urban “heat island effect” during the hot days of summer. Temperatures in city settings can be up to 7 degrees hotter than those in nearby suburbs because of a lack of trees and greenery, and an excess of heat-absorbing asphalt and concrete.

Shade cast by a full tree canopy reduces surface temperatures, while evapotranspiration from tree leaves cools the surrounding air. Trees also filter particle pollution from the air, muffle city noise, and sequester carbon from the atmosphere.

Here in New Jersey, many cities are striving to become greener, cooler and spongier. Thanks to new sources of funding, urban greening is experiencing a renaissance.

In 2023, the New Jersey Department of Environmental Protection awarded $24 million in “Natural Climate Solution Grants” to plant thousands of trees throughout the state – especially in cities – and boost the carbon-capturing abilities of salt marshes and wetlands. The funding came through the Regional Greenhouse Gas Initiative (RGGI) program. The federal Inflation Reduction Act (IRA) of 2022 provided $1.5 billion in funding for urban and community forestry projects across the United States, including 10 in New Jersey.

Recently, New Jersey Conservation Foundation organized an “Urban Forest Partners” meeting between the national nonprofit American Forests and over a dozen groups working in four New Jersey cities – Trenton, Newark, Camden and Elizabeth. The goal was to share information and strategies to bring more trees to the Garden State’s cities, making them healthier places to live, work and play. American Forests has been chosen to distribute $50 million of the IRA funding.

An important topic discussed by the Urban Forest Partners group was “tree equity” – the idea that trees are essential to public health and well-being, but not always distributed equitably among populations.

American Forests has created an online Tree Equity Score calculator to help reduce environmental inequities by identifying the places most in need of more trees. Users can click on any location in the nation, down to the census block level, to find out which areas have sufficient tree cover and which don’t.

“While tree shortages can be found in cities all over the country and the world, in the U.S. they are found primarily in already disadvantaged communities due to discriminatory zoning laws, racial segregation and ongoing wealth disparities,” says Benita Hussain, Chief Program Officer, Tree Equity, at American Forests. “The data shows us that lower income communities are often more vulnerable to extreme heat and climate impacts than higher income areas, creating environmental and health inequity between neighborhoods. Tree planting is nature’s most effective solution for reversing these trends while improving a city’s sponginess.”

Areas with abundant trees are shown in green on the Tree Equity Score map, and places lacking trees are shown in orange. Not surprisingly, most New Jersey cities display as orange. For example, downtown Trenton – a neighborhood with many buildings and parking lots – is shown to have a tree canopy cover of only 22 percent. This contrasts sharply with many surrounding suburbs that have tree canopies covering 50-60 percent of their land area.

The Tree Equity Score calculator also provides demographic information about each census block, including percentages of people of color, people living in poverty, children and elderly, and people experiencing linguistic isolation because English is not their first language. It also includes information on each area’s heat burden.

For areas most in need of action to increase tree cover, many potential solutions exist. Street trees can be planted in neighborhoods, new parks can be created, and vegetated islands can be added to parking lots. Post-Covid, when fewer people work out of an office full-time, it may also be possible to reduce the size of many parking lots, replacing asphalt with trees.

For the health of all New Jersey residents, let’s keep planting trees along city streets, in urban parks, and anyplace else we can! We also must thoughtfully plan for the long-term stewardship of newly planted trees and our existing, maturing canopy. Urban trees must be cared for – watered, pruned and loved – so we can continue to enjoy their benefits for decades or centuries to come.

To find out how your city or town ranks on the Tree Equity Score calculator, go to www.treeequityscore.org/. To learn more about urban and community forestry grants, go to www.fs.usda.gov/managing-land/urban-forests/ucf/2023-grant-funding. To learn more about sponge cities, go to www.arup.com/perspectives/publications/research/section/global-sponge-cities-snapshot#tab1-5.

To learn more about preserving New Jersey’s land and natural resources, visit the New Jersey Conservation Foundation website at www.njconservation.org or contact me at info@njconservation.org.