Environmental Issues in NYC

New York City is known for its extraordinary population density, making it one of the most energy-efficient cities in the United States. However, the city’s dense urban environment has also led to pollution and heat concentration. Air pollutants, including fine particulate matter (PM2.5) and ozone concentration, lead to serious respiratory and cardiovascular diseases. [3] In New York City, exposure to high concentrations of particulate matter causes more than 3,000 premature deaths and more than 2,000 hospitalizations due to respiratory and cardiovascular causes annually.[4]

A darker color on the map indicates a higher concentration of PM2.5.

Urban heat islands are a phenomenon categorized as heat concentration in urban areas. Similar to air pollutant concentration, heat concentration has major impacts on urban administration and citizens’ quality of life. Heat concentration leads to higher cooling costs, air pollution, and greenhouse gas emissions.[5] These costs create a positive feedback loop that intensifies other ecological issues and worsens the economic status of citizens living in an inferior neighborhood due to financial reasons. Urban heat island is also associated with heat-related illness, which disproportionately impacts outdoor workers and residents without a proper cooling system.

The map on the right shows the surface temperature across NYC, where red color shows signs of heat concentration.

New York City's Urban Forest

Urban greeneries like trees, lawns, and parks, are the most well-known types of green infrastructure. While most literature focuses on the hydrology function of green infrastructure, urban greeneries are also known for their contribution to thermal and air pollution regulation in urban areas.[1] Street trees, for example, lower surface and air temperatures through evapotranspiration during heat waves. They also mitigate air pollution by either absorbing gaseous pollutants or intercepting particular pollutants.
In New York City, the urban forest is constructed and maintained by the New York City Department of Parks & Recreation. The Forestry Division within the Park Department and the Borough Forestry offices together monitor and document the nearly 600,000 street trees in New York City. The information on each individual tree is accessible through an interactive Tree Map.

Instead of mapping out individual trees, the map on the right shows the density of trees at the block group level.

Green View Index (GVI) is another indicator of green amenity coverage. It measures the percentage of eye-level visible greenery at a given point. The GVI used in this visualization is calculated using Google Street View images. By using a pre-trained semantic image segmentation model, we extract the objects classified as “tree” or “grass” and calculate the GVI based on their fraction in the images.

Slide to see how we and the Computer Vision algorithm observe urban greeneries. Green pixels to the right is categorized as "trees" and "Grass".

GVI is commonly thought a more comprehensive and accurate indicator in measuring Green Infrastructure coverage. GVI takes into account three-dimensional green infrastructures like walls covered with vines, lawns, and bushes that are not recorded in the TreeMap.[6] GVI also measures tree characteristics that are omitted from the TreeMap, for example, tree canopy size which is positively correlated with their ecological function.
An example of how we get a panorama at a given location by requesting photoes from 6 angles.

The map to the right visualizes the correlation between tree density and GVI. The parcels’ height represents the tree density, and the color represents the value of GVI. It is noticeable that there is not a definitive positive correlation between the two variables. Notice the value difference in parks, where GVI have higher values compared to tree density attributed to the abundant vegetation.

Effectiveness of Green Infrastructure - surface temperature

GVI is strongly negatively correlated with surface temperature. Higher parcel height symbols a higher GVI, and color symbols the surface temperature.
This finding coincides with our literature reviews about Green Infrastructures’ functions that urban greeneries are effective in lowering surface temperature. Most significantly, parks are the spots with the lowest surface temperature. Block groups adjacent to water bodies and parks also have a lower temperature, except for places with low vegetation coverage like airports and ports.

Midtown Manhattan has a notorious high surface temperature, known as the busiest commercial and transportation hub. The high building density limits the green infrastructure coverage in Midtown Manhattan, except for a few pocket parks.

GVI vs. Air Quality

Interestingly, there is a positive, but not statistically significant relationship between GVI and PM2.5 concentration. This means that higher greenery coverage can be weakly associated with a higher concentration of air pollutants.
Instead of arbitrarily reaching the counterintuitive conclusion that “greeneries lead to air pollution concentration”, one possible explanation for our observation is the policy “tree follows pollution”. While tree planting is a commonly acknowledged and implemented method for fine particulate matter reduction, there is no conclusive result on the efficiency of the tree’s air-cleaning function, especially in large cities with high air pollutant concentrations.[7] In places with higher air pollutant concentrations, trees are more likely to be requested and planted. In fact, the New York City Department of Parks & Creation lists “removing air pollutants that can trigger respiratory illness” as one of the reasons for tree planting.

However, it is true that places with higher PM2.5 concentrations tend to have less greenery coverage. Midtown Manhattan also has an above-average concentration of fine particulate matter.

So is Brooklyn Heights.

In terms of effectiveness, Urban Green Infrastructure, i.e. urban forest in NYC, is effective in lowering the surface temperature but not so effective in mitigating air pollution.

But the unequal distribution of green amenities and undesirable facilities had historically put disproportionate environmental burdens on minority and low-income communities through discriminatory zoning and planning. Until now, some marginalized groups still suffer from historical environmental inequalities due to limited green infrastructure.

Even though the NYC Urban Forest program is a free government service, technology gaps and lack of information can still hinder citizens’ access to such services. The tree service can be requested through online portals, phone calls, and texting, while low-income communities may not have access to these technologies. These vulnerable communities may also fail to realize environmental issues’ chronicle health impact or the existence of the program.

Environmental Inequalities in NYC

Socio-economic indicators, for example Percentage of White Population, Average Household Income, and Mean Household Income, are negatively correlated with surface temperature.

This means that majority-minority and low-income communities are disproportionately affected by urban heat.

The effect of historcial discimitory planning carries over to today. Historically redlined communities are still mostly consisted of minority and lower-income residents today. Redlined communities historically have significantly fewer greeneries due to government disinvestment, which resulted in a higher average temperature on summer days. In addition, low-income communities often lack access to or can not afford air conditioning.[8]

The ecological burden also carried over.
Compared to affluent, white-dominated neighborhoods like the Upper East Side...

... lower-income, minority-dominated neighborhoods like the Bronx, are significantly hotter.

The color represents the surface temperature, and the height of the blockgroup represents the percentage of non-white population.

Does the Urban Forest in NYC mitigates the environmental inequalities...?

Based on our observation, the Urban Forest Program have more to be done to address the unequal distribution of green infrastructure due to the divestment in the past.
Percentage of White Population, Median Household Income, and Mean Household Income are all positively correlated with GVI.

This relationship is visualized in the map to the right, where high block groups (high GVI) are often block groups with blue colors (high median household income).

Even though the Urban Forest Program is though to be a free and accessible program to all New Yorkers, the tree-planting outcome did not effectly address the environmental justice issue.

Conclusion & Discussion

In conclusion,

  1. Urban greeneries are effective in lowering the surface temperature but show no signs of significant effect in lowering air pollutant concentrations.

    2. Urban cool spots are spotted in places with higher greenery coverage, suggesting that tree planting can be an effective method in alleviating the urban heat island effect. Air pollutants, especially particulate matter like PM2.5, can only be temporarily intercepted by trees and are eventually re-suspended back into the atmosphere. To address the air pollution issue, cutting the source of pollution, i.e. fossil fuel consumption and vehicle operation, might be a more efficient approach.

  2. Environmental inequalities - the uneven distribution of pollutants and urban greenery - continue to exist in New York City.

    3. The study observes significant correlations between socioeconomic, greenery, and pollution indicators. Surface temperature and PM2.5 concentration levels are consistently higher in minority and low-income communities, while the coverage of green amenities is consistently lower in these neighborhoods.

    We suggest that the Department of Parks & Recreation actively engage in combating the existing uneven distribution of green amenities by a) planting more trees in historically divested communities, and b) informing the public about environmental inequalities and methods of alleviation.

Unrelated to the conclusions, we also have some interesting observations.

GVI is (mostly) a better representation of greenery coverage in urban areas. GVI is a better predictor of both environmental indicators compared to tree density, but also with its own limitations.
First of all, since GVI is calculated based on Google Street View (GSV) images, it is less time and labor-consuming compared to conducting a tree census. However, this method may not be applicable in other areas with no Streetview service like in China or other developing countries.
In addition, GVI varies through time. Google Street View images are less frequently updated compared to Treemap. The timeliness issue is not significant in this study due to the high GSV availability in New York City, as most of the street views are frequently updated and captured after the year 2020. In places with lower data availability, the only image researchers can get can from 10 years ago, which will not reflect the real greenery coverage. The calculated perceived coverage varies across seasons. In winter seasons, the calculated value is significantly lower than in the summer seasons due to defoliation and snowing. To address this issue, we only used GSV images from the “green months”.
There is also a possibility for the scene parsing algorithm to misclassify objects, and it is impossible to verify the hundreds of thousands of images we have collected.

What to know more? To see a text-based report with full methodology and statistics, click here.

Bibliography

[1]Grabowski, Zbigniew J, Timon McPhearson, A Marissa Matsler, Peter Groffman, and Steward TA Pickett. “What Is Green Infrastructure? A Study of Definitions in US City Planning.” Frontiers in Ecology and the Environment 20, no. 3 (2022): 152–60. https://doi.org/10.1002/fee.2445. [2]Bruton, Candice, and Myron Floyd. “Disparities in Built and Natural Features of Urban Parks: Comparisons by Neighborhood Level Race/Ethnicity and Income.” Journal of Urban Health : Bulletin of the New York Academy of Medicine 91 (July 31, 2014). https://doi.org/10.1007/s11524-014-9893-4.
[3]“Outdoor Air Quality - NYC Health.” Accessed December 9, 2022. https://www.nyc.gov/site/doh/health/health-topics/air-quality-air-pollution-protection.page.
[4] New York City Department of Health and Mental Hygiene, “Air pollution and the health of New Yorkers: The impact of fine particles and ozone”. https://www1.nyc.gov/assets/doh/downloads/pdf/eode/eode-air-quality-impact.pdf
[5] New York City Department of Environmental Protection, “Analyzing the Urban Heat Island Effect”. https://www1.nyc.gov/assets/dep/downloads/pdf/environment/education/10-analyzing-urban-heat-island-effect.pdf
[6]Ki, Donghwan, and Sugie Lee. “Analyzing the Effects of Green View Index of Neighborhood Streets on Walking Time Using Google Street View and Deep Learning.” Landscape and Urban Planning 205 (January 1, 2021): 103920. https://doi.org/10.1016/j.landurbplan.2020.103920. [7]Chen, Lixin, Chenming Liu, Lu Zhang, Rui Zou, and Zhiqiang Zhang. “Variation in Tree Species Ability to Capture and Retain Airborne Fine Particulate Matter (PM2.5).” Scientific Reports 7, no. 1 (June 9, 2017): 3206. https://doi.org/10.1038/s41598-017-03360-1.
[8]Wilson, Bev. “Urban Heat Management and the Legacy of Redlining.” Journal of the American Planning Association 86, no. 4 (October 1, 2020): 443–57. https://doi.org/10.1080/01944363.2020.1759127.