Nature scarcity in cities has been observed for centuries. Only recently have we detailed the acute symptoms of separation anxiety felt by urban society. Urban greening creates projects that simultaneously restore our health and invite plants and animals to reclaim their place among us.
The first step to reconnecting to the natural world may be rethinking the appropriate daily dose. The Biophilic Cities Project, led by Tim Beatley at University of Virginia, proposes everyone would benefit from consuming his or her “minimum daily requirement of nature,” similar to caloric intake requirements. The reconstructed nature pyramid dishes out portions from a bite-sized serving of walking to work under a street forest canopy, to annual banquets of vacations in awe-inspiring wilderness areas.
Some leading cities are making a balanced nature diet part of their long-term planning by engaging residents in multi-use projects. The spread of innovative projects is matched by a growing body of research. Here are some of the highlights.
1. Air filtration: nitrogen dioxide and other airborne pollutants etch away at human health and the resulting changes in nitrogen availability may also give an advantage to some invasive plant species. A modeled test of deposition rates in street canyons found green walls and roofs could reduce NO2 levels by up to 40 percent over conventional bare brick and concrete buildings.
2. Natural born coolers: heat waves and urban heat island warming can spike summer energy consumption for air-conditioning. Trees and other vegetation cool the air around them through shading and evapotranspiration while feeding and sheltering wildlife. The cumulative effect of park cooling islands across an entire east coast city was estimated to lower maximum daily temperatures by 4.1 degrees on the Kelvin scale.
3. Co-use corridors: greenbelts and bluebelts allow wildlife to disperse between larger habitat patches and also provide more enjoyable and friendly walkways and bikeways for people. An analysis of 54 greenspaces in a single Canadian city found a need for 325 corridors to connect just half of the nodes. Weaving such a expansive network would have to include backyards, planted medians, and other fine-scale patches provided by homeowners in addition to public openspaces.
4. Community cohesion: civic pride may facilitate neighborhood greening, but sometimes people need a helping thumb to get started. The Pennsylvania Horticulture Society started planting trees and grass in vacant lots around Philadelphia in 1999. Over a ten-year period, gun assaults and vandalism around the planted lots decreased and residents claimed to exercise more. One step up from repairing broken windows.
5. Carbon storage: among the other benefits of urban trees is their capacity to store and sequester carbon. Total annual carbon sequestration in U.S. communities was estimated at 25.6 million metric tons, worth $2 billion in savings. Although only 3.2 percent of U.S. tree-bound carbon is in cities, programs like Million Trees NYC will increase urban carbon storage and sequestration from plantings.
6. Food for all: urban farming takes advantage of underutilized spaces on the ground and overhead to bring nutrition to people and pollinators in the food desert. Scaling up U.S. production to cover 12 percent of a city area and provide 100,000 jobs, like urban agriculture does in Havana, faces some challenges, mainly a lack of knowledge. Adapting the agriculture practices of rural areas to the built environment and new technologies are now bringing Americans up to speed.
7. Biodiversity in mind: densely populated urban centers are infamous for dominance by a few, often overabundant species such as starlings and house sparrows. High species richness may be as important as total abundance for putting our minds at ease. Conceptually, the resiliency and vigor of a diverse ecosystem will inspire the same for human mental health. More research on that link may change how we see, and how often we see, nature at home and farther afield.