Hot in the City: Urban temperatures are rising faster than in rural areas

Joel Oliphint

School is out. Pools are open. Summer is coming. You can literally feel it in the air: Five out of the first six days in June saw temperatures reach 80 degrees or higher.

But if you live in rural Ohio, those temps were probably lower. For decades there has been a localized warming effect going on in many cities, including Columbus, that makes urban areas hotter than their nearby rural counterparts. A 2014 study by independent research organization Climate Central found summers have been getting hotter in the U.S. every year since 1970, but cities are even hotter thanks to an effect called the urban heat island. Of the 60 American cities in Climate Central's study, Columbus ranked No. 8 in a list of cities with the biggest difference between urban and rural temperatures. The study found that every year Columbus averages 16 more days above 90 degrees than rural areas, and that, on average, it's 4.4 degrees hotter in the city.

Maybe right now that doesn't sound so bad. Summer is supposed to be hot, right? It's a fact immortalized in countless summertime anthems. ("Hot town, summer in the city," the Lovin' Spoonful presciently sang.) Hotter temps in the city than in the boonies might seem only slightly inconvenient. Maybe the urban heat island (UHI) could just be rebranded as artisanal, 100 percent locally sourced global warming.

In fact, though, piling the urban heat island effect on top of already rising global temperatures is creating a form of hyper-local climate change, and the cascading side effects can be disastrous. The higher the temperature, the more ozone is created at ground level. Ozone - a colorless, odorless gas produced when emissions from gas-powered engines and industrial facilities react in the presence of heat and sunlight - can lead to respiratory problems, especially in children and older populations.

Higher temperatures also lead to more air-conditioning usage, which not only creates more pollution by requiring more coal-generated electricity, but also stresses the electric grid and can lead to outages, said Susan Ashbrook, assistant director of sustainability with the city's public utilities. A power outage in the summer isn't merely inconvenient. "It can be life threatening for the more vulnerable populations," Ashbrook said.

Part of the reason for higher temperatures in urban areas is that cities tend to have more heat-absorbing materials than rural areas. While Downtown's concrete jungle is probably the first thing that comes to mind when you think of hard materials baking in the sun, the urban heat island also disproportionately affects low-income neighborhoods, which tend to have fewer trees and less green space, Ashbrook said.

Just a few hours south of Columbus, city leaders in Louisville, Kentucky, are boldly and aggressively attacking their UHI problem. Though Columbus is addressing the issue by planting more trees, the city doesn't have the type of neighborhood-specific, multi-tiered approach to UHI that Louisville does. With another hot summer looming, and likely many more in the years to come, is a tree-planting campaign enough?

The need for green spaces

Green space is often spoken of as an amenity that looks nice and improves residents' quality of life - parks for strolling and picnicking, trees for housing pretty birds. And it certainly does those things. But Jean-Michel Guldmann, professor emeritus of city and regional planning at Ohio State's Knowlton School, has found adding green space to a city can quite literally reduce temperatures in urban areas. Hard, impervious materials like concrete store heat, while vegetation blocks radiation and prevents the accumulation of heat by releasing it as water vapor into the atmosphere.

In two studies published in 2014, Guldmann used satellite data to accurately estimate land-surface temperatures in Columbus, which he then analyzed and ran through statistical models. "We tested what would happen if we were to add vegetation," Guldmann said, "and the maps show clearly how the temperature would go down. The more vegetation, the more it goes down."

What surprised Guldmann was how much the vegetation cooled areas around it - the neighborhood effect. "Let's say you have a little parking lot, and you put green space on that," Guldmann said. "Of course it's going to cool that specific parking lot, but it will [also] have a cooling effect within a certain area around it."

Once Guldmann and his research partners understood the relationship between vegetation and temperature through the model, they could use the model to test different approaches and scenarios, like adding trees, grass and green roofs. "Planners need a better understanding of the relationship between the UHI and land-use patterns," Guldmann wrote in the interdisciplinary academic journalEnvironment and Planning, "in order to reduce the UHI and promote more sustainable urban development."

Unfortunately no one in Columbus seemed to notice Guldmann's studies. "I think the gown and town don't have good relationships," Guldmann said. "I've never been able to attract interest from the state or the city for my research and kind of gave up quite a while ago."

That's not to say the city hasn't looked into the issue. Last spring, the Recreation & Parks Department's Forestry Section partnered with Colorado-based Plan-It Geo to release the "Urban Tree Canopy Assessment," a report that aimed to provide a "top-down view of Columbus' urban forest." According to the report, the urban tree canopy covers 22 percent of Columbus, and the goal is to increase the canopy 1 percent each year for five years, as recommended by the Columbus Green Community Plan. To do that, the city launched the Branch Out campaign with the goal of planting 300,000 trees by 2020. So far, 15,863 trees have been planted, according to a running tally on the Branch Out website, and residents can get up to a $100 rebate for planting their own trees.

The canopy report also recommends planting sites based on certain goals, and listed as a "very high priority" in the report is urban heat island mitigation. Ryan Pilewski of Franklin Soil and Water Conservation District and chairman of the Green Team's Branch Out working group said the public health issues associated with UHI, particularly the fact that it disproportionately affects lower-income populations, weighed heavily in the discussions of how to prioritize the tree-planting sites.

"Now we are in that phase where we're looking at what was prioritized," Pilewski said. "Those areas are pretty large, and it's a daunting task to get all those trees in the ground. So now where do we concentrate our efforts? We're in the process of doing that."

In a statement toAlive, Mayor Ginther said reducing the urban heat island effect also tackles issues of social equity in the city. "Our Urban Tree Canopy study showed there is a correlation between low-income areas and low tree canopies," Ginther said. "It is important that Columbus make shading our neighborhoods a cornerstone of our environmental policies, not only to reduce energy usage, but to make healthier environments for residents in all of our neighborhoods."

In addition to the Branch Out tree-planting campaign, other green space improvements the city has made in recent years have likely helped reduce the UHI effect. Columbus Commons replaced a giant slab of Downtown concrete with green space in 2011, and grassy areas have recently been added along Scioto Mile. The Lazarus Building gained a green roof in 2007. But the efforts haven't been coordinated as part of a larger plan to mitigate the UHI.

Contrast that with Louisville, which recently used grant money to commission a study by the Urban Climate Lab at Georgia Tech to determine the extent of Louisville's heat problem and to figure out what the city can do about it. Louisville's campaign is called Cool502, and city officials are encouraging businesses to do their part by installing green or reflective, white roofs (also called "cool roofs") and asking residents to plant more trees and grass (and then tweet about it using the hashtag #cool502). The city has also committed to increasing its tree canopy and is looking into lighter paving products to make streets more reflective and less heat absorbing.

As far as a tree canopy, Louisville is actually in a better spot than Columbus. American Forests recommends a tree canopy of 40 percent, and Louisville is sitting at 37 percent, though the city has a goal of hitting 45 percent.

Maria Koetter, Louisville's director of sustainability, said the UHI study is specific down to the neighborhood level in terms of heat-reduction strategies, like how many trees to plant where, how many green or white roofs to install, and so on.

"We don't want to wait until we have an extreme heat event to do something. It takes steps," Koetter said. "So now we know what we can do to reduce the heat island. As we see the warmest summers on record coming year after year, we know that hotter days in the summer amplify the urban heat island effect, so it's just going to continue to get hotter in our city if we don't do anything about it."

Even though Louisville's tree canopy is stronger than Columbus', the heat island effect is still a problem, which means combatting it effectively has to involve more than planting trees.

"I think it'll be a factor in future planning work," Guldmann says of cities' responses to the UHI. "I think there's more awareness of it. It's probably going to become more and more important."

Kevin Wheeler, planning administrator for Columbus, said the urban heat island "hasn't been something our office has dealt with directly." That could change in the future.

"I was just talking to the city yesterday, and we were unfamiliar with what [OSU prof Guldmann's] work entailed," Pilewski said. "I think we realize that he would be a good resource to talk to in the future. I think we're going to read through that and decipher what his thinking is and how can we use that in helping us prioritize where to go."

"We certainly could have provided them a heat map using updated satellite data," Guldmann said.

Even with urban summer temperatures on the rise, exacerbated by global climate change, there's some good news. Air-quality alerts issued for ground-level ozone pollution in the Columbus area from 2006 to 2015 peaked at 21 alerts in 2012, but from 2013 to 2015, the Mid-Ohio Regional Planning Commission issued only four total alerts, according to data provided by MORPC.

Wheeler also mentioned a significant 2010 revision to the city's parking requirements, which updated the code to require less parking overall and more landscaping. Similarly, the Downtown District Design Guidelines, adopted by City Council in 2013, include a section that encourages green roofs.

Meanwhile, down in Louisville, the UHI campaign is in the public-comment period, and Koetter said most people are latching onto the heat island concept. "A big part of what I'm doing is educating citizens on how to use the data as a tool. It's great data," she said. "Everything we do as a city … will be implementing the strategies in the heat island study."