Too soon, we move out of pleasant spring weather and into another hot American summer, which odds are will be hotter than the historical normal. Last summer, there was a massive heatwave across much of the west and the southwest of the United States, stretching across 12 states.
Climate change means heatwaves like this are becoming more frequent, with the average number of dangerously hot days likely to triple by 2050. Heatwaves already kill as many as 1,300 people in a typical American summer.
Trees can be one solution to reduce air temperatures. Trees’ canopy cools the air by shading surfaces like concrete and asphalt, preventing them from heating up in the sun, and by transpiring water as they grow, cooling the air just as the evaporation of sweat cools you off on a hot day.
But we know from case studies in particular cities or small sets of cities that tree canopy is very unequally distributed in America, with low-income or people of color (POC) neighborhoods generally having less tree cover.
We decided to map tree inequality on a national scale, to answer the simple question: How widespread is this trend of tree inequality?
A Problem Everywhere
For this study, we wanted to measure the extent of tree cover inequality and its effect on temperatures for a large sample of thousands of communities throughout the United States.
Even a few years ago, this kind of remote sensing and image processing would have been a daunting task, but now we were able to use open-source cloud computing on the Google Earth Engine Platform to map tree cover and summer surface temperatures at a 2m resolution for almost 6,000 communities across the United States. We then overlayed maps of forest cover and summer surface temperatures with US Census data on income, race, and ethnicity.
We found that tree inequality is essentially everywhere.
In 92 percent of US communities, low-income neighborhoods have less tree cover than high-income neighborhoods. The rich have, on average, about 15 percent more tree cover and live in neighborhoods that are around 3 degrees F than the poor. This trend extends to race as well: In 67 percent of US communities, POC neighborhoods have less tree cover than white neighborhoods, even after accounting for trends in income.
Why is There Tree Inequality?
Why American communities have such widespread tree inequality is a complex and multifaceted story that varies somewhat from community to community.
Important elements of the story are the historical patterns of de jure segregation, followed by redlining for decades and the (ongoing) segregation of cities by income. Our research shows that one key pattern associated with tree inequality is the gradient from urban to suburban. In most American cities, richer, predominately white households have fled to the suburbs, where there are larger lots with high tree cover. This has left households in city centers predominately low-income and POC, living on smaller lots that have more pavement and fewer trees.
Regardless of the historical mechanisms leading to tree inequality, it is clear from our data that tree inequality leads to an inequality in temperatures near people’s homes.
In a world of altered climate, where heat waves will get more frequent and more intense, tree inequality becomes an inequality in climate risk.
Another research study of mine has estimated that trees in American cities save around 1,200 lives a year, a number likely to grow as climate change worsens. However, our new national survey of tree inequality suggests that most of these lives saved are in high-income, predominately non-Hispanic white neighborhoods. Trees are already protecting households from climate change, but they are primarily protecting the rich and the white.
The good news is that tree inequality is a solvable problem, if our society dedicates time and resources to expanding tree canopy. We estimate that the US would have to increase the size of our urban forests by 62 million trees across our study cities to bring low-income neighborhoods up to the tree cover in comparable high-income neighborhoods.
We calculate this would cost roughly $18 billion in total in additional planting or maintenance costs, likely spread out over a decade or more of investment.
That is a daunting price tag, but such societal investment to close the urban tree cover disparity between rich and poor would save hundreds or perhaps thousands of lives annually, as more people lived through heat waves. A more just and climate resilient city is possible if we choose to create a city with nature at its heart.