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As the climate warms, it’s not just scientists and academics taking notice. New York City is one of many cities developing plans to protect residents from the effects of climate change.

More than 8 million people live in New York City, but they’re not all at equal risk of heat stress. Some areas are hotter than others—urban heat hotspots within an overall urban heat island. These hot spots, coated with impervious manmade materials, soak up and hold more solar radiation than the trees and grass in cooler areas. Another inequality, according to the health department: Most of NYC’s heat-related deaths occur because of time spent indoors, not outdoors—inside homes with no air conditioning.

On average, extreme heat kills more people in the U.S. than any other weather hazard. The Centers for Disease Control and Prevention (CDC) considers all heat-related deaths and illnesses preventable but estimates that heat kills about 700 people each year.

A New Strategy, a New Tool for NYC Council

The NYC Council wants to save more lives. In March 2020, Council Speaker Corey Johnson issued an ambitious list of recommendations in a report called “Securing Our Future: Strategies for New York City in the Fight Against Climate Change.”

The resiliency section included efforts to combat heat-related deaths. This gave the NYC Council Data Team—which analyzes data on key Council issues and creates ways to display it—a reason to try a tool new to them: USGS Landsat satellite data. The team ultimately delivered an interactive heat map, sharing the same hope of more experienced Landsat users—the hope of helping people cope with a changing world.

Before the recent development of a Landsat dataset for land surface temperature, USGS remote sensing researcher Dr. George Xian used weather station records for temperature data, but coverage was limited. Landsat’s coverage of every point on earth offers a large-scale look at temperatures, and hot spots, within cities. Xian works at USGS Earth Resources Observation and Science (EROS) Center, which has archived nearly 50 years of Landsat data.

“As time goes by, we’re going to see higher and higher temperatures in the urban area,” Xian said. “… As more and more people are moving to the city areas, this becomes a huge impact to human health. This also can impact landscape and ecosystem health issues.”

Useful or a Dead End?

Soon after he started working as resiliency planner for the NYC Council in 2019, James Cottone began searching for a way to display how heat risk can differ by neighborhood.

Cottone first realized Landsat data could help when he came across a temperature map in a study on mitigating New York City’s urban heat island effect. Someone suggested he ask the data science team to help with the project. “We have a data science team?” he responded—and reached out.

The nine people on the NYC Council Data Team tackle plenty of projects, from addressing homelessness to reducing buildings’ greenhouse gas emissions.

But Landsat, data scientist Ben Witte said, “for many of us, it was a very new phenomenon, and it did require a little bit of trial and error.”

The team first thought they would need a lot of temperature data from multiple years, so they spent time getting familiar with the USGS user interface EarthExplorer and learning about available datasets and the bulk download option. After downloading hundreds of scenes from the freely available Landsat Analysis Ready Data’s Provisional Surface Temperature product, Witte created a map showing surface-temperature ranges across the city. But Cottone and the team weren’t sure it showed what they wanted.

“There were all these factors, like not having a great deal of experience with this kind of remote sensing data, which really made us nervous that maybe this was going to be a dead end,” Cottone said.

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Key Approach: Relative vs. Actual Temperatures

Connecting with a couple of people associated with Columbia University helped them form a new strategy for using Landsat data. One was Dr. Christian Braneon, a remote sensing specialist serving on the New York City Panel on Climate Change.

Braneon also works at NASA’s Goddard Institute for Space Studies (GISS) housed at Columbia. In addition to collaborating with colleagues, he engages with people outside of academics, such as city leaders. He explains how satellite data can help their city plan for climate change.

Braneon urged the data team to consider mapping relative temperatures rather than actual temperatures. He explained that actual land surface temperature may vary considerably from day to day because of factors such as clouds or recent rains. But the hottest areas of a city on one summer day tend to consistently remain the hottest areas on any other summer day, regardless of the weather conditions or actual surface temperatures.

“Run with the relative land surface temperature. Don’t get fixated on whether the land surface temperature was 110 degrees or 112 degrees Fahrenheit,” Braneon recalled telling the team, mindful that some people try to draw meaning from high values alone.

“That really simplified our approach into a much more manageable way,” Witte said.

“We were struck by how little data he actually thought was necessary in order to do this,” Witte added. Instead of hundreds of scenes, they blended data from only three days in the summers of 2018 and 2019.

Revealing the Hot Spots

From that data, they created a heat map showing temperature deviation from the mean, with a range of blues showing cooler areas and red showing warmer areas. One additional layer shows the percentage of residents with the comorbidities of hypertension or COPD; another shows the percentage of households with air conditioning.

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The heat map revealed interesting results.

“It’s amazing how the parks are consistently always cooler,” said senior data scientist Rose Martinez.

Coastal areas are cooler, too, Cottone said, with wetlands generally being the coolest. Lower and Midtown Manhattan turned out cooler than he expected, possibly because of building shadows or the surrounding water.

Then there were the hotter areas—in Queens, Brooklyn and the Bronx.

“The temperature map correlates somewhat … with race and income, where warmer places are often more Black and Latino and lower income than some of the cooler neighborhoods,” Cottone said. Higher comorbidities and lower access to air conditioning followed the same correlation.

Cottone’s observations line up with what the CDC has found in other cities, along with a racial/ethnic association with heat-related deaths.

Planning for a Warmer Future

Council Speaker Corey Johnson’s report contains recommendations to achieve cooling equity by developing a comprehensive cooling plan, targeting cooling center locations in hotter neighborhoods, providing assistance for cooling homes and better measuring heat-related deaths.

Public measures to cool down an urban heat island often include adding vegetation such as trees and grassy areas, and even “green” roofs.

It’s important for cities to plan for a warmer future, Braneon said. He works with GISS’ Urban Climate Change Research Network, which has helped cities from Paris, France, to Durban, South Africa, understand how to use global climate models and remote sensing datasets in resiliency planning.

In addition, Xian has been working on a pilot study of temperature trends and urban heat island intensities in three US cities—Atlanta, Minneapolis, and Sioux Falls, South Dakota—using extensive Landsat data from 1985 to 2018. He hopes to extend the pilot study area to 50 major U.S. cities and eventually the entire country to give cities a tool for planning.

The Council Data Team has a role in helping New York plan for the future, and they’ll keep Landsat in mind. “It is an incredible resource that has a lot to offer, and I think there’s definitely an intention to revisit it,” Witte said.