In New York City and across the country, more people die from extreme heat than from all other natural disasters combined. To understand how to keep New Yorkers safe from extreme heat, we studied neighborhood factors that affect temperature, and found that grass, shrubs, and trees can play a major role. Let’s take a look.
At 6.00 PM on June 30, 2018, the air temperature at the LaGuardia Airport Weather Station was 89° Fahrenheit (F). At the same time, the air temperature on a block in East Harlem (below left) was nearly 5° hotter than at the Weather Station, and the temperature on a block in Bed-Stuy (below right) was 3.5° hotter.
Manhattan: 93.9° F
Brooklyn: 92.5° F
This is an example of the Urban Heat Island Effect: when some areas in a city are hotter than others. Temperatures can even vary from block to block.
LaGuardia Airport is an open space without many structures surrounding it. It doesn't trap heat the way other places in the city do, so we often use it as a reference temperature. It is almost always cooler than other parts of NYC, where building boilers and vehicle engines generate heat, manufactured surfaces like roofs and roads absorb heat, and there’s less vegetation to cool things down.
What cools your block? Anyone who has looked for a shady spot under a tree on a hot sunny day knows that trees help to cool off a street.
But by how much? How effective are trees at cooling neighborhoods, and what other characteristics contribute to an area’s air temperature during hot weather?
To find out how trees and other characteristics affect “hyperlocal temperature” - block-by-block differences - we worked with the Department of Parks and Recreation and the Mayor’s Office of Climate and Resiliency to install air temperature monitors around the city. This was part of a comprehensive approach to keep neighborhoods cooler during hot weather in NYC.
We reviewed land-use data and satellite imagery to build a profile of each block where we were monitoring temperature. We found that temperature could vary significantly depending on block characteristics. For example, the block we studied in Brownsville, Brooklyn, had very little vegetation (trees, grass, and shrubs), while the block in Edenwald, in the Bronx, had high grass and tree coverage. You can see the difference in vegetation coverage in these satellite images.
We found that the average temperature at the Edenwald spot was more than 2° cooler than the Brownsville spot.
These are the factors we explored that affect hyperlocal temperature:
What factors matter the most? We built a statistical model that predicts the average summer temperature on a hypothetical city block, based on the neighborhood factors we studied in the hyperlocal temperature study? We modeled the average temperature from 6 to 9PM, when the Urban Heat Island Effect is the greatest (the sun sets, but the city retains heat).
Our model looked at how much each factor affected temperature, and it showed that the most important factor for reducing temperature was green space: trees, grass, and shrubs reduce temperature. Having more surfaces that hold heat increases temperature.
See for yourself - drag the green slider back and forth to see what the temperature would be on a hypothetical block if there were more vegetation or more impervious surfaces like roadways, which absorb sunlight and retain heat.
A block covered in trees, grass, and shrubs might have an average summertime evening temperature of 77°, but one completely covered in impervious surfaces would be 79°. More vegetation means lower temperatures.
Cooling a block by 2° may not sound like much, but small differences in temperature - especially at peak summer temperatures - really matter. For example, the average temperature at Central Park increased 3.4° from 1900 to 2013. Cooling blocks by just a few degrees can help offset the effects of climate change at the local level.
As the climate changes, NYC faces rising average temperatures and heat waves that place New Yorkers at risk. The increase in cooling provided from neighborhoods with trees and other vegetation helps increase equitable resilience between neighborhoods in New York, offsets the effects of the urban heat island, and makes daily life safer and more comfortable on hot days and during extreme heat. The modest cooling provided by increased greenery can also reduce the load of air conditioners and lower energy bills, as air conditioning is a vital public health intervention for reducing heat-related illness and death.