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How trees can save us

They are the most effective, efficient, and immediate form of urban climate action—provided they’re planted where people need them most

On Dallas’s hottest summer days, Matt Grubisich would dispatch his colleagues at the Texas Trees Foundation to take manual air temperature readings across the city. “You think the number to go by is the weather station out at the airport,” he says, pointing to Dallas’s Love Field, five miles northwest of the city center. “But then we’d go to a parking lot downtown where it was 6 degrees warmer.”

As the director of operations and urban forestry for the Texas Trees Foundation, Grubisich was trying to demonstrate that Dallas needed to make major design changes to fortify itself against epic heat waves. “It’s easy to explain to people why they park under a tree when they drive to the grocery store,” he says. Getting local leaders and the population at large to take action to bring more trees to the city was a tougher sell.

Thanks to the city’s 2015 effort to map its urban forest, Grubisich and his team already knew that the city’s trees were not evenly distributed. Almost half of Dallas’s trees were located within the Great Trinity Forest, a 6,000-acre nature preserve. That didn’t leave a lot of trees for the rest of the city, where some neighborhoods only had tree canopy over 10 percent of their communities.

Infographic illustrating that urban areas without trees can be up to fifteen degrees hotter than rural areas with tree cover.

To make the case for rethinking the city’s approach to trees, Grubisich and the Texas Trees Foundation turned to data on a larger scale. The organization’s comprehensive urban heat study, released in 2017, showed that one-third of the city was suffering from a phenomenon called urban heat island effect. A full 35 percent of the city was covered by impermeable surfaces, like parking lots, roads, and buildings, which absorb sunlight and end up heating up the air around them.

“I knew we would have a rather robust urban heat island,” says Grubisich. “That number, that was the most alarming part. That was the catalyst.”

Dallas’s heat island was more than robust: Parts of the city were up to 15 degrees Fahrenheit warmer than their rural counterparts. The urban heat island was expanding so rapidly that the ninth-largest city in the country was warming faster than any other large U.S. city except Phoenix.

The study also took a closer look at a 2011 heat wave that killed 112 people when the temperature topped 100 degrees for 40 days in a row. Nearly half of those deaths were directly attributed to the urban heat island. A warming Dallas was already claiming lives.

Despite the dire statistics, in some ways, the study’s timing was fortunate. For decades, environmental nonprofit the Nature Conservancy deployed scientists to unpopulated places to study pristine forests and wild rivers. The group had recently begun bringing its expertise to areas where more people live by focusing on different climate-related challenges in 30 U.S. cities, including Dallas, where the organization had just opened a regional office.

As the organization’s Texas director and committee chair for the North America Cities program, Laura Huffman saw a remarkable opportunity to apply a natural cooling solution to Dallas’s urban heat problem, she says. “The built environment is creating unintended consequences that nature can help fix.”

The 2017 study, which was conducted by the Urban Climate Lab of the Georgia Institute of Technology, divided the city into 4,000 sections and created regional climate model data for each one. Researchers then analyzed possible climate scenarios that showed which neighborhoods were in danger of warming most dramatically due to climate change.

The scale of the problem became even more apparent after another city partner, the Trust for Public Land, began overlaying socioeconomic and public health information on top of the heat data, revealing a troubling narrative. The hottest neighborhoods were almost always lower-income neighborhoods, including places that were seeing more cases of childhood asthma, which disproportionately affects the city’s black kids.

James McGuire, the managing director of Dallas’s environmental quality office, remembers sharing the information with other city leaders, and watching their faces register the same shock. “You would see their minds working, and asking, how do we fix this?”

The city’s chosen solution was ambitious, but it was one Grubisich had hoped for: a data-driven targeted initiative to plant more trees. Over the next two years, the Texas Trees Foundation, the Nature Conservancy, and the Trust for Public Land worked with Dallas leaders to focus its plantings in places that would address heat and health outcomes the most.

“A tree is a piece of equitable green infrastructure,” McGuire says. “For some people, having access to trees can be a matter of life and death.”

When American cities were first laid out, planners punctuated urban blocks with trees at regular intervals to create evenly distributed—and largely aesthetic—rows of greenery. Most of the efforts behind the “million trees” campaigns popular in the 1990s and early 2000s simply replaced dead or dying trees in century-old tree wells, which is why many ran out of steam—and space—before they hit their 1,000,000-tree goals.

Dallas is trying something different, says McGuire. “We’re not going to sprinkle trees around the city to make it look good, or meet certain metrics for how many trees to plant,” he says. “We’re putting trees where it will do the most social and environmental good.”

Even before the 2017 report, Dallas was one of a handful of U.S. cities that had been growing its tree canopy. Out of 27 large cities, 23 cities had lost forest cover over the previous decade. Dallas had seen an increase of .9 percent. But that marginal expansion of the tree canopy hadn’t been enough to outpace the heat island’s advance.

Tree-planting can combat heat island effect by swapping hot hardscape surfaces for shaded permeable areas. But that’s not the only way trees cool cities. Water evaporation from leaves also reduces surrounding air temperatures. That’s why planting trees close enough together so they can grow to create a canopy is critical—shading paired with water evaporation can double the cooling benefits.

Trees also offer an additional countermeasure to extreme heat. Higher temperatures impact local air quality, as warm air traps toxic pollutants emitted by the burning of fossil fuels. This includes the fine-particulate matter from vehicles that creates especially dangerous breathing conditions during hot summer days. Trees can filter deadly air pollution, protecting people from chronic respiratory illness.

In Dallas, the data collected for the Texas Trees Foundation concluded that adding trees could not only help cool down the immediate area—trees could change the weather enough to provide significant energy savings and health benefits. Some neighborhoods could see as much as 15 degrees of additional cooling from new trees on hot summer days—enough to close the rural-urban heat island gap.

The study provided a target number of trees the city needed to plant to achieve the necessary cooling benefits: 250,000. But where the trees were planted could make an even greater localized impact.

“You can plant one tree or 100 trees—it will take millions of trees to make a regional change,” says Grubisich. “But if you have good tree canopy cover it can help with all those things exactly where you are. How do we focus our efforts and the money we do have where they’ll have the biggest impact?”

More people die from heat in Dallas than all other extreme weather events combined, a statistic that holds true nationally as well. In 2017, heat killed more people in the U.S. than hurricanes, tornadoes, flooding, and wildfires.

Longer, more frequent heat waves are expected in the future due to climate change, meaning summer’s death toll will rise. An NRDC study estimated that by 2040, 150 Americans will die every summer day due to extreme heat, with almost 30,000 heat-related deaths annually. That’s twice the number of people in the U.S. now killed annually by gun violence. By 2050, Dallas would be seeing 280 additional deaths per year due to heat.

Adding to the danger is the fact that nighttime low temperatures are getting higher. When air temperatures remain above 80 degrees overnight, humans are unable to regulate their own body temperatures without artificial cooling methods. According to the city’s own heat report, the hottest parts of Dallas have nighttime lows that hover around 80 degrees for five months out of the year.

If Dallas could focus its tree-planting efforts in places where the city could improve lives the most, the investments could act as benefit multipliers—saving money on energy costs for households least likely to have air conditioning, protecting children with asthma, and, most critically, preventing heat-related deaths.

To ensure that the city’s planting and preservation efforts were strategic, the partners provided additional critical data. The Trust for Public Land used a new mapping tool to determine where people were most likely to travel to schools, parks, and health clinics without using cars, making them more vulnerable to heat and pollution impacts. Researchers from the Nature Conservancy worked with the school district to isolate which schools had the highest rates of childhood asthma, targeting schools with high absenteeism rates.

Tree-planting events with community members are now scheduled regularly. High-priority neighborhoods like South Oak Cliff have added up to 1,000 trees in two years. Preliminary surveys show that the city’s tree canopy has increased another 2 percent since the project began. A collaborative effort is underway to gather air quality and health data at several at-risk Dallas area schools.

The difference on these sidewalks is palpable. “You start to see relief pretty quickly,” says Huffman. “There are maintenance issues for a few years, then after that you see impact.” Now the initiative is planting an average of three trees per day.

Dallas recently had a chance to test the resilience of its tree-planting efforts. When a powerful storm swept through the city in June, plunging parts of the city into a week-long blackout, the “right tree, right place” strategy seemed to pay off. Healthy trees that were well-pruned, sufficiently watered, and disease-free were less likely to be toppled. And more recent plantings had situated trees further away from power lines, lessening the risk of outages or fires. Extreme weather events are more frequent in a changed climate. Trees need to be strong and appropriately sited to weather those storms. Over 200 trees were lost, says Grubisich, but if not for the action taken over the last few years, the damage would have been much worse.

On the first day of the 2019 Congressional session, Sen. Ed Markey and Rep. Alexandria Ocasio-Cortez stood on the steps of the Capitol to announce the Green New Deal, a proposal to eliminate the use of fossil fuels while radically transforming U.S. society. The 14-page resolution provides a framework for achieving net-zero greenhouse gas emissions by 2050 while creating millions of high-wage jobs that rebuild the country’s infrastructure more sustainably and equitably. Seven of the 2020 presidential candidates currently serving in Congress co-sponsored the resolution.

Even without a federal deal yet in place, cities are moving forward with their own Green New Deal plans. New York City and Los Angeles have already released local strategies to achieve net-zero emissions by 2050. Those plans include massive investments in high-tech tools like solar panels and electric vehicles. But those investments have social and environmental tradeoffs—and there’s growing concern that these types of large-scale interventions will still not be enough.

The community-level urban-centered tree planting that’s proving effective in Dallas could help address the Green New Deal’s stated goals, put people to work, and provide instant benefits so-called “green” infrastructure cannot.

Dallas’s heat assessment concluded that the money a city invests in tree-planting has been found to be 3.5 times more effective in lowering temperatures than implementing cool materials strategies like white roofs and white pavement. When tree-planting is implemented in conjunction with reflective roofing and paving materials, the combined strategies can reduce the number of heat-related deaths in cities by more than 20 percent.

Targeted urban reforestation in cities could also lower average annual particulate concentrations by 10 percent and average maximum daytime summer temperatures by 3.6 degrees—enough to make a meaningful difference to someone’s health. This is why Paris—which experienced brutal heat waves this summer—recently announced an intensive urban reforestation effort that will ensure 50 percent of the city’s land is planted and permeable by 2030.

Trees can be part of an even bigger solution for reducing emissions. Several recent studies, including the Intergovernmental Panel on Climate Change (IPCC) report released in October 2018, have suggested that replanting trees lost to development and deforestation could capture anywhere from one-sixth to two-thirds of the carbon pollution generated in the industrial age. One study claims there are 255 million suitable acres for tree-planting just in the U.S.—an area the size of Texas.

Planting trees is mentioned in the Green New Deal resolution, but as “afforestation,” a practice of planting trees in places where they’ve never grown before, largely in land outside of cities. Adding tree cover anywhere is a good long-term investment in carbon removal—one that needs to be paired with other efforts to reduce emissions. But in the meantime, planting forests near the places people are could start saving energy, money, and lives right away.

Trees can be the most effective, efficient, and immediate form of climate action a city can take—provided they’re planted where people need them most.

That’s why we need to plant more trees at bus stops, outside buildings, throughout downtowns, near schools, by hospitals, around highways, along bike lanes, instead of street parking. And we need to plant them today.


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