Climate Change on Your Block

By Karuna Meda | Illustrations by Ollie Hirst

In Philadelphia, summer is nearly 22 degrees warmer in some neighborhoods than others — researchers and city agencies are teaming up to address this environmental disparity.

Wisps of pastel pink and orange tinge a dark blue sky as dusk settles on a summer’s eve in the city of Philadelphia. The soothing hues belie the intense heat that researcher Radika Bhaskar, PhD, can feel radiating off the pavement. She kneels to place a temperature sensor in the ground, moving cautiously on the hot surface. Across the street, her colleague Megan Heckert, PhD, does the same. “It’s so much more bearable here under the trees,” she calls out. Dr. Bhaskar nods — with a research journey traversing both ecology and environmental engineering, she knows all too well the cooling properties of plants. A beep on the sensor app catches her attention and she’s taken aback by the reading — a blistering 95°F, even at sun down. She stands and surveys the surrounding houses in concern, thinking about their inhabitants who will have to sleep through this uncomfortable heat. It’s a troubling snapshot of the planet’s warming in our own backyard.

In just the last thirty years, Philadelphia’s average summer temperatures have increased by 3°F, making it almost as hot as Atlanta, Georgia. Average annual rainfall has also increased. In 2021, Hurricane Ida was a terrifying example of the havoc wreaked by severe storms, with record flooding along the Schuylkill River that displaced hundreds and killed five.

These local patterns are reflected globally. In fact, some of the most devastating effects of climate change have happened in the past year alone — floods in Pakistan that submerged a third of the country; record-breaking drought in China that dried up dozens of rivers and reservoirs; massive wildfires in Europe that destroyed more than a million acres of land. The latest report from the Intergovernmental Panel on Climate Change, the world’s leading body on climate science, indicates we are now on track to surpass acceptable limits of warming as early as 2037. We have reached a ‘code red’ and extreme weather events and their cascading effects will likely happen more frequently, and severely.

The rate of warming that has brought us to this precipice has indisputably been driven by humans burning fossil fuels, which emit heat-trapping gases. Yet, while all of humanity’s fingerprints are present, the impact will be felt unevenly. According to the Environmental Protection Agency, Black and Latino communities in the U.S. are 40–50% more likely to live and/or work in areas with the highest projected increases in temperature and flooding compared to other demographic groups. These populations also experience higher incidences of conditions like hypertension and asthma, symptoms of which are worsened by rising temperatures.

Understanding this environmental injustice, at least in cities, requires focusing on the urban environment. For the past four years, Dr. Bhaskar, an engineering professor at Thomas Jefferson University, has embarked on an ambitious partnership with researchers in geospatial mapping and industrial design, and Philadelphia’s Water Department and Office of Sustainability, to study how climate change impacts the city’s hardest hit neighborhoods. The team is combating the local trends of a warmer, wetter planet by combining human engineering with tools from Mother Nature herself.

Using Green Infrastructure to Combat Increased Stormwater

The approach of integrating natural systems and engineered systems has been a driving force behind Dr. Bhaskar’s research, from studying how to use plants to pull pollutants from the air and soil, to measuring fluid dynamics in trees that live in drought conditions. “I’m always thinking about how we can bring ecology into our urban environments,” she explains. “How do we then measure the different functions of these nature-based solutions?”

These were similar questions that Philadelphia’s Water Department sought to answer in their "Green City, Clean Waters" initiative. Started in 2011, the project aims to combat stormwater overflow. Two-thirds of Philadelphia is served by an older water-drainage system called the combined sewer system, meaning a single pipe collects both household sewage and stormwater. During increasingly wetter seasons due to climate change, this system often overflows and billions of gallons of stormwater and diluted sewage pollute local waterways. This harms aquatic life and hinders recreational activities like swimming.

“The urbanization of our environment has resulted in more impervious surfaces,” explains Matthew Fritch, an environmental engineer in the Water Department. “So when it rains heavily, stormwater has nowhere to go.”

“Green City, Clean Waters” applies green tools or infrastructure that 1) use the natural properties of plants to both absorb rain and to physically intercept it before it hits the ground and 2) also contain the stormwater in an underground catchment area made of rocks and absorbent material, to ensure it has time to soak into the soil and release more slowly into the sewage pipes. The city has already installed nearly 3,000 structures across the city which have kept almost three billion gallons of sewage overflow out of Philadelphia's waterways over the past 10 years.

“Philadelphia is a national, if not international, leader of green stormwater infrastructure,” says Fritch. “But it is challenging — we are used to dealing with pipes, not plants. It’s a different world.”

A world, however, very familiar to Dr. Bhaskar, whose knowledge of engineering and plants’ adaptive mechanisms made her an ideal collaborator. In 2018, Fritch teamed up with her to test different materials and plants for a green roof, one of the many types of “Green City, Clean Waters” installations. They wanted to evaluate how well the roof was absorbing water, using sensors placed in the soil. They also wanted to measure a process called evapotranspiration, whereby water taken up by the plants’ roots is released back into the atmosphere. This moisture-laden vapor cools the surrounding air.

“So plants not only act like sponges, soaking up the excess rainwater, but also like air conditioners,” explains Dr. Bhaskar. It’s not the only way plants cool — they cast shade, and reflect sun rays off their leaves, preventing paved surfaces like asphalt from heating up as much. This combined cooling means that urban green spaces can reduce surface temperatures by 1–4°F during the day.

As they collected the measurements from the roof, it occurred to Dr. Bhaskar, could these green tools be used to combat another major effect of climate change — rising temperatures?

A view from North Philadelphia

The Urban Heat Island Effect & Environmental Injustice

While this question percolated in her mind, Dr. Bhaskar serendipitously met Saleem Chapman at a panel on climate change in early 2019. At the time he was the chief resilience officer in the Office of Sustainability and recently became its director. The conversation quickly turned to warming in Philadelphia.

Local climate projections predict that by 2030, the number of days with temperatures reaching 95°F or higher is expected to double in Philadelphia, from 21 to 42, which would be nearly half of the summer season. But warming is not distributed evenly across the city — neighborhoods that have more buildings, pavement and black rooftops are warmer than those with more trees and parks. Because of deep-seated inequities and discriminatory practices like red-lining, tree cover or canopy is not equitably distributed in Philadelphia, with 40% in some areas and 3% in others. The latter experience an intense “urban heat island” effect, and on a peak summer’s day, they can be nearly 22°F warmer than the coolest neighborhood. It’s like living in two different climates.

These heat islands face other stressors. In 2018, Chapman’s office and the Department of Public Health created a Heat Vulnerability Index that combines data on daytime temperature, availability of resources like pools and cooling centers and socioeconomic factors like income, age and incidence of disease. It’s a tool that several cities across the world have used to pinpoint hotspots to keep residents safe during the summer.

An area is more vulnerable if it is both very hot and the people who live there are more sensitive to the effects of high heat (see map below) — for instance, older citizens, people with pre-existing medical conditions and those who don’t have access to or cannot afford air conditioning. The index indicates that regions in North and West Philadelphia are the hottest, and that Black and Latino communities and people experiencing poverty are disproportionately vulnerable to that increased heat.

As Dr. Bhaskar and Chapman discussed the injustices of these urban hotspots and the sensors on the green roof, they arrived at the same questions — could they use similar sensors to measure the temperature at green infrastructure sites and identify a possible cooling effect? And if so, how could this encourage the placement of future green infrastructure in areas that are vulnerable to both increased heat and stormwater?

“Heat mitigation was not a primary consideration of the Water Department’s green stormwater infrastructure program,” says Chapman “But it is a potential co-benefit that could promote collaboration between city agencies, exactly the kind of approach we need to confront multiple environmental stressors.”

They started a research partnership to explore tackling two climate change birds with one green stone. After more discussions and successfully acquiring funding from the William Penn Foundation, they began assembling a team.

Visualizing Heat Across Philadelphia

The researchers created a map of temperature variation across Philadelphia using data provided by the Water Department. Each shape represents a census block group and those in orange and red are the hottest neighborhoods of the city. The researchers chose study sites within these areas (in green) to measure the cooling effect of green infrastructure. Many of these blocks overlap with the areas identified as most vulnerable to heat by the city’s Heat Vulnerability Index, due to the prevalence of conditions exacerbated by heat like diabetes and hypertension, and lack of cooling resources. Many of the darkest blue or coolest areas correspond to the presence of big parks and/or water bodies. The cooling effect of these spaces spread to adjacent areas, in lighter shades of blue.

Zooming in on Heat, at Night

Dr. Bhaskar brought in the expertise of Dr. Megan Heckert, an urban geography researcher at West Chester University who uses geographic information systems and spatial analysis to explore issues like sustainability and tree equity. They shared the goal of collecting information on small scales, more relevant to the experience of urban warming.

“Much of the research on urban heat relies on satellite data, which while informative, is not at the resolution we need to understand temperature differences across short distances,” explains Dr. Heckert. “What is it like to live on a shady street or city block, compared to one with fewer trees?”

Many green stormwater infrastructure installations are considerably smaller than city parks, the types of urban green spaces typically studied for mitigating urban heat. So their potential cooling effect might be limited, both in magnitude and geographic reach. This also motivated the researchers to take a finer scale approach to understand what level of cooling can meaningfully impact the on-the-ground experience of heat.

A significant aspect of that experience is the effect on health. During the day, extreme heat can lead to dehydration, cardiovascular stress and increased risk of heatstroke. But the danger lasts into the night. Surfaces like concrete and asphalt can bake up to 140°F degrees on a hot day and radiate that stored heat back into the air at night. It forces residents to incur higher energy costs by running air conditioning or fans through the night, which is prohibitive for some. Without the ability to cool down sufficiently, higher nighttime temperatures disrupt sleep, thereby increasing susceptibility to disease and worsening existing conditions like hypertension. In fact, one analysis showed that elevated nighttime temperatures were a major factor in heat-related mortalities in Philadelphia from 1983–2008.

“Residents living in these urban hotspots can maybe escape the heat of the day by being in an air-conditioned place of work or cooling center,” says Dr. Bhaskar. “But at night, they’re literally trapped.”

Average summer-night temperatures in Philadelphia have increased by nearly 4°F since 1970. But unlike daytime temperatures, less is known about how this increase is distributed across the city, let alone a city block. The researchers therefore decided to gather their temperature measurements after sun down, hypothesizing that the effects of shading and evaporative cooling provided by the green infrastructure persist into the night. The next step was to determine how and where.

Camouflaging Sensors on a City Block

During the summer of 2020, Dr. Bhaskar’s team and Philadelphia Water Department engineer, Matthew Fritch, rigorously tested low-cost sensors against high-end ones used by Philadelphia’s Air Management Services Laboratory to compare accuracy and repeatability of measurements. Once a reliable sensor was identified, the next challenge was to camouflage it into the urban environment. The sensors needed to be placed where people are actively living, working, playing, etc. in order to capture the on-the-ground experience of heat. This proved to be challenging — some sensors were stolen, others succumbed to the elements. Dr. Bhaskar turned to Eric Schneider, an industrial design professor at Jefferson, and his students to help create a casing that would easily disguise the sensor in areas of high foot-traffic, but not hinder its accuracy.

“The students had to design for a hybrid environment, incorporating features that could blend in with concrete as well as soil and trees,” says Schneider. Ultimately, two designs were chosen — one that looked like a rusty, utility box and another that could double as a plant label or mounting stake. Both held up well in the field – the sensors logged data accurately and remained undisturbed.

Heat sensor installed on a street sign

Measuring Climate Change on a City Block

With reliable sensors ready to be deployed (see above), Dr. Heckert mapped out potential locations, prioritizing green infrastructure sites in neighborhoods with high heat vulnerability, low surrounding tree cover and high energy costs. The team decided on two sites with green infrastructure built by the Water Department in Upper North Philadelphia, one of the hottest parts of the city: the Panati Playground, which has a rain garden with trees, and the Cecil B. Moore Recreation Center, which has tree trenches incorporated into the sidewalk.

In the summer of 2021, Dr. Bhaskar and researchers in her lab, including rising senior Cianna Quintana, installed the sensors in and around the study sites. Quintana, who grew up and still lives fifteen minutes away from the study sites, was shocked at the stark temperature differences from block to block. “We have lots of trees where I live, so even on a really hot day, the shade provides relief,” she says. “But as I commuted to the study sites, I would see fewer trees, and not even a strip of grass in some places. There’s no escape from the heat.” The experience has inspired her to develop a pavement material that will release less heat at night for her senior team project, and seek a career path in climate resilience.

One of the first things they noticed was a difference in nighttime temperatures based on geography: At night, the temperatures of their study sites in Upper North Philadelphia were nearly 10°F warmer than Chestnut Hill, one of the coolest neighborhoods in the city. While this is less than the daytime disparity (22°F), it provides evidence that nighttime heat should be factored into existing health disparities. It could also account for the six-fold increase in heat-related mortality expected to hit Philadelphia by the end of the century.

They then compared surface temperatures of the green infrastructure sites to their surroundings. On average, the temperatures within both sites were approximately 4°F cooler compared to the adjacent sidewalk at night. The Panati Playground rain garden was more than 10°F cooler than the sidewalk across the street. In fact, the spatial analysis showed that this cooling effect reached the paved surfaces at the corner of the playground, which were also cooler than the sidewalk across the street. “It suggests that a green stormwater infrastructure’s cooling could have a reach at the level of a city block, but more needs to be done to determine the precise geographic reach,” says Dr. Heckert.

“This magnitude of cooling was quite surprising, especially considering how small these green spaces are and how hot the study locations are,” says Dr. Bhaskar. “It shows us that, at least on a short time scale, these green tools have a measurable cooling effect at night.”

How Trees Can Manage Rain and Influence Heat

Trees have multiple benefits: Water taken up by the tree roots is released as moisture-rich vapors by the leaves, cooling the surroundings like an air conditioner. Trees also act like a sponge, with their canopy absorbing rain and thereby reducing the amount of water entering storm grates. They also intercept the sun’s powerful rays and provide shade.
A sidewalk with fewer trees gets uninterrupted heat from the sun and is much hotter than a shady sidewalk. The heat absorbed by the sidewalk during the day is then radiated back into the air at night, dangerously increasing nighttime temperatures.
Green stormwater infrastructure combines plants’ natural properties with man-made engineering. In tree trench structures, water coming from a storm grate infiltrates an underground storage area or reservoir made of rocks and other material. This stored water soaks into the soil where it is taken up by the roots of the tree, ensuring that stormwater is released more slowly into the sewage pipes and prevents overflow.
Trees have multiple benefits: Water taken up by the tree roots is released as moisture-rich vapors by the leaves, cooling the surroundings like an air conditioner. Trees also act like a sponge, with their canopy absorbing rain and thereby reducing the amount of water entering storm grates. They also intercept the sun’s powerful rays and provide shade.
A sidewalk with fewer trees gets uninterrupted heat from the sun and is much hotter than a shady sidewalk. The heat absorbed by the sidewalk during the day is then radiated back into the air at night, dangerously increasing nighttime temperatures.
Green stormwater infrastructure combines plants’ natural properties with man-made engineering. In tree trench structures, water coming from a storm grate infiltrates an underground storage area or reservoir made of rocks and other material. This stored water soaks into the soil where it is taken up by the roots of the tree, ensuring that stormwater is released more slowly into the sewage pipes and prevents overflow.
Trees have multiple benefits: Water taken up by the tree roots is released as moisture-rich vapors by the leaves, cooling the surroundings like an air conditioner. Trees also act like a sponge, with their canopy absorbing rain and thereby reducing the amount of water entering storm grates. They also intercept the sun’s powerful rays and provide shade.

Engaging City, Community and Health Stakeholders

Much more needs to be done to build on this work — the team hopes to replicate their findings at other green infrastructure sites in the city, and determine their geographic reach of cooling. But it was an important proof-of-concept for how to approach climate change. “We’re dealing with moving targets, so we have to be nimble and action-focused,” says Elaine Montes, who recently joined as a program manager of infrastructure resilience in Philadelphia’s Office of Sustainability. “This means maximizing our resources, and getting buy-in from the agencies that manage those resources.”

The cost of building green stormwater infrastructure can run into the six-figure range, then requires continuous maintenance by grounds crews. It also relies on coordination with the departments of Parks and Recreation and Streets. Because of silos among government agencies, communication is key. Montes has been working with Dr. Bhaskar on developing messaging around their pilot study for key city stakeholders.

For Dr. Bhaskar, it has underscored the complexities and roadblocks of government work around climate. But she has appreciated the urgency with which the city has approached this project. “It’s made me realize how vital it is to have civic collaborators for research like this,” she says. “I’m excited to co-design more studies with those who shape climate policy, in the hopes that it translates into action.”

An equally important priority is starting dialogues with the public. “When we were out in the field, nearby residents would sometimes stop and ask what we were doing,” recalls Quintana. Some people were curious, others were mistrustful and worried the green infrastructure would pose a nuisance. But many didn’t even know the city had installed these tools. “Without engaging community members, it’s hard to convince them that a tree is going to have a positive impact. Especially when they are facing so many other stressors.”

Community outreach is one of the main goals of the city’s first Environmental Justice Advisory Committee, announced in February 2022. Dr. Bhaskar is one of 17 members who will make recommendations to the mayor’s office and be ambassadors for climate equity. Through the committee, Dr. Bhaskar met Mariel Featherstone, DPM, a student in the master of public health program at the University of Pennsylvania. There she’s done work with Dr. Eugenia South, showing that urban green spaces significantly improve mental health. She and Dr. Bhaskar aim to have conversations out in the community to see if green infrastructure has influenced well-being. “There is a direct link between a person’s surrounding and their mood,” says Dr. Featherstone. Violent crime increases on hotter days, and people who face climaterelated natural disasters frequently struggle with mental health problems. “Climate change is no longer just an environmental issue, it’s a public health issue.”

Learning From Ecological Resilience

Dr. Bhaskar hopes involving other health experts, students and community members will bring varied perspectives to a multilayered challenge. Her approach is inspired by an ecological concept she studied in her postdoctoral work called response diversity.

The theory describes how diversity can confer resilience: an ecosystem is made up of various species with different properties — some that tolerate drought, some that withstand high temperatures, and others that handle extremely wet conditions. Any one of them might struggle in one of these extreme conditions, but as long as the others thrive, the ecosystem as a whole can still function. “We can learn from how living systems with diverse responses can withstand a changing climate and apply it to engineered systems like green infrastructure.”

But Dr. Bhaskar fears we are dangerously close to a tipping point, a permanent shift that even the most resilient ecosystem will not be able to prevent. “It is urgent that we take action now, or the most vulnerable will continue to pay the price,” she says. “Drawing on different areas of expertise and experiences, we can strive to build equitable climate resilience.”

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