There are parts of Louisville, Kentucky, that are enveloped in green, where towering trees arc over broad avenues and walkers, joggers and bikers enjoy beautiful parks designed by Frederick Law Olmsted, the man who drew up plans for Manhattan’s Central Park.

Even on the hottest days of summer, these neighbourhoods feel comparatively refreshing next to the more sun-baked quarters of the city, where shade is often an unavailable commodity on the street.

It isn’t just that heat is uncomfortable – it kills people Brian Stone Jr, Georgia Tech

Cities are their own climates, often hotter than their surroundings due to the way surfaces like asphalt trap heat even as cars and buildings exude it. When a city is markedly warmer than surrounding rural areas, it is called an urban heat island – and Louisville ranks among the worst heat islands in the US, according to a 2014 study, with an average temperature difference of 2.7C (4.8F). Worse still, a 2012 study by Georgia Tech’s Urban Climate Lab found that Louisville was the fastest-warming urban heat island in the nation.

Part of the reason for Louisville’s temperature extremes is geography. But a lot of it comes down to trees.

Q&A Why is heat an issue in US cities? Show Donald Trump is pulling the US out of the Paris climate accord - meanwhile sweltering US cities are facing ever greater challenges on coping with heat and the climate crisis. This year saw the hottest July on record and many US cities have endured heatwaves - including those as far north as Alaska, where thermometers hit 90F (32C) for the first time. This week Guardian Cities is examining the growing challenges for US cities and the best ideas for reducing the impact of the climate crisis on communities. Mark Oliver, special projects editor, Guardian US

A study commissioned by Louisville in 2015 found that the city had lost 54,000 trees a year between 2004 and 2012, reducing the city’s canopy cover from 40% to 37% over the period. Today, canopy cover is likely to be around 27%, according to Cindi Sullivan, executive director and president of the nonprofit TreesLouisville.

Trees provide shade while also lowering the temperature of their surroundings through evaporative cooling. Without action, it is feared the tree canopy will continue to decline as trees fall due to storms, pests and age – a scenario that could see the city’s rapid warming continue, alongside a number of other deleterious effects.

Louisville’s Eastern Parkway is one of the city’s greener areas, with towering trees providing shade. Photograph: Josh Wood/The Guardian

“Without a robust tree canopy,” said Sullivan, “our air quality is going to continue to decrease, stormwater and flooding from these extreme weather events is going to increase, the effects of draught are going to increase. There will be more health problems.”

Five or six decades ago you wouldn’t move into a house without planting a tree, because that tree was your air conditioning Cindi Sullivan, TreesLouisville

And while Louisville may be among the worst heat islands, the problems seen here are replicated nationwide – the city was only fifth-worst US heat island in the 2014 study, behind Las Vegas, Albuquerque, Denver and Portland, Oregon.

“Most cities by the middle of the century [will be becoming] increasingly dangerous places to be outside,” said Brian Stone Jr, who wrote the Georgia Tech report on urban heat islands. “So if no steps are taken, that will just be amplified.”

He added: “I think this is a non-trivial health-related issue for all large cities in the US.

“It isn’t just that heat is uncomfortable – it kills people and is only set to get worse as the climate crisis continues.”

While individual cities have little ability to impact the planet-wide climate crisis on their own, they do have the ability to temper the urban heat island effect within their own borders.

Trees were recently planted in the Phoenix Hill neighbourhood, which has some of the lowest tree canopy coverage in the city. Photograph: Josh Wood/The Guardian

Louisville is hoping to do just that. When it was named the fastest-warming US city in 2012, it did not take its new superlative lightly. The city created an office of sustainability, hired an urban forester and tapped Stone to conduct an urban heat management study, the first of its kind for a major American city.

The city’s main approach to cooling itself has been twofold: promoting the use of more “cool surfaces” for paving and roofs to reflect radiation and heat away, but, more importantly, encouraging the planting of trees.

“You need to do both. It’s not one silver bullet,” said Maria Koetter, formerly head of the city’s office of sustainability.

The link between trees and health

TreesLouisville offers free trees to Louisville’s residents that can either be delivered to them or picked up. The group, in conjunction with the city, also offers a rebate to residents of up to 40% of a tree’s price – up to $80 – for trees they purchased themselves.

Sullivan says that with 70% of the land available to plant in Louisville privately owned, getting individual families and businesses to plant trees is the only way the city can preserve and ultimately grow its tree canopy.

Thermal image showing the impact of urban heat islands and the cooling effect of greenery in Melbourne, Australia. Photograph: City of Melbourne

“Five or six decades ago you wouldn’t move into a house without planting a tree, because that tree was your air conditioning,” she said. “So the idea that trees are a valuable asset is something that is not front of mind anymore for many people.”

The Depave Project aims to plant trees in parking lots, and the city is offering rebates to residents and businesses that use cool roofs.

Meanwhile, the University of Louisville’s Green Heart Project has just started to plant the first of its 10,000 trees in south Louisville neighbourhoods as part of a study looking into the impact on greenery on health that it says is the first of its kind.

Aruni Bhatnagar, the professor of medicine leading the project, calls it “a community-based clinical trial in which instead of pills, we have trees.”

A woman tries to cool off in a mist of water in Las Vegas – the worst urban heat island in the US. Photograph: John Locher/AP

The main hypothesis of the estimated project, which is expected to cost $20m, is that planting trees will reduce air pollution and thus improve cardiovascular health. But Bhatnagar says they will also be looking closely at secondary outcomes: when it’s greener and cooler, will residents be more inspired to exercise outside, lowering rates of diabetes and obesity? Will stress and anxiety levels drop? Will trees shield enough noise that sleep quality and duration improves?

Tree cover: an inequality issue

In cities like Louisville, tree canopy cover often corresponds directly with things like race and income, bringing heat, lack of shade and the associated heat-related health problems to areas already suffering other inequalities. An NPR analysis of 97 major US cities this summer found that poorer neighbourhoods were more likely to be hotter in more than three-quarters of the cities.

Sullivan said a TreesLouisville analysis had found that Louisville neighbourhoods that were red-lined in the 1930s – that is, where loans and other services were refused based on racial discrimination – have 22% canopy cover today, while those that weren’t red-lined have 49%.

In announcing the city’s study of its heat island problem in 2016, Louisville mayor Greg Fischer said: “We know that too often the zip code where you are born can correlate with negative health outcomes. That’s unacceptable.”

But getting Louisville to a tree canopy cover to the city’s goal of between 40% and 45% may prove difficult.

Louisville needs to plant about 100,000 trees a year for the first 10 years of a canopy building program in order to reach its goal of 40-45% coverage. Photograph: Daniel Dempster Photography / Al/Alamy

Reaching those goals could take decades and cost north of $1bn, according to the city’s canopy study. Money is tight and the city, despite its commitment to the heat island issue, recently folded its office of sustainability into another department and laid off Koetter, the office’s director.

“I wouldn’t say we are necessarily on the path” to addressing the heat island effect, says Koetter. “I would say we are aware of what needs to happen.”

She added that to meet its tree canopy goals, Louisville needed to plant about 100,000 trees a year for the first 10 years of a canopy building program, but that she did not see the city as being able to generate enough revenue to do that.

Stone, the Urban Climate Lab director, pointed out that New York City was able to plant 1 million trees over the course of eight years, but said he was less optimistic that Louisville could reach its tree canopy goals.

In 2017, Louisville passed an ordinance requiring that trees removed from public rights of way must be replaced. A new tree-protecting ordinance that would require developments to maintain certain levels of tree canopy cover is currently being considered by the city’s metro council.

“We’re in a situation right now where many tree advocates and environmentalists think it doesn’t go far enough, that the percentage of trees that need to be saved in new developments needs to be increased,” said Bill Hollander, the councilman who introduced the proposal. “We also have some members of the development community saying it goes too far.”

Ebony Pryor hopes newly-planted trees will help reduce the relentless summer heat and smells from nearby chemical plants. Photograph: Josh Wood/The Guardian

Sullivan of TreesLouisville said the proposed ordinance is a step in the right direction, but that more needs to be done – and fast.

“There’s an old eastern adage that says the best time to plant a tree is 20 years ago. The second-best time is now,” she said. “We’re way behind, I’d say decades behind other communities that have already figured some of this out.”

Ebony Pryor, 42, hopes the planting being done now will make a difference. She works as a business manager at New Birth Church in Rubbertown, a largely industrial area of the city known for its chemical plants. In the summer, eggy, sulfury smells waft into the church on some days and the heat is relentless.

“It’s just straight sun and it’s 10 degrees or more hotter down here than other parts of the city,” she said. “That’s where the trees come in. The trees will be able to take some of that smell out – and definitely shade the area.”

TreesLouisville recently planted young trees at New Birth Church, the first of 10,000 trees the group plans to put in Rubbertown in the coming years.

“While you don’t seen an immediate effect, over time it will improve and help the quality of life down here,” said Pryor. In 10 or 15 years, “I think it’s going to be beautiful – and smell good.”

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