A recent study by researchers at the University of Wisconsin in Madison offers one of the most detailed records of the variation in temperature between cities and the surrounding rural areas, known as the urban heat island effect.
Jason Schatz and Christopher Kucharik of the Nelson Institute wanted a data set that would accurately reflect how temperatures varied in Madison at the neighborhood level. They mounted 151 temperature sensors on telephone poles in areas with varying levels of building density. Each sensor took measurements every 15 minutes for 18 months between March 2012 and October 2013. The researchers then looked at what environmental factors could be influencing the variation in temperature from urban and rural areas.
"Our study is a step forward in terms of method—we describe the urban environment in much more detail," said Schatz, lead author of the study.
Understanding the phenomenon is becoming more important as a growing number of people are moving to cities and climate change is driving overall temperatures higher, according to the study's authors.
By 2030, about 87 percent of North America's population is expected to live in urban areas, up from 80 percent of the current population, according to 2010 data from the World Health Organization. According to the study, a lot of that urban growth is likely to happen in small- to medium-sized cities like Madison, since half the people in urban areas live in cities that have populations between 100,000 and 500,000.
They found that the density of urban development and lack of vegetation had the greatest impact on temperature variation. Warming within Madison was more pronounced during the summer, and particularly at night, under calm, clear conditions.
Higher heat retention
Cities tend to retain heat more than rural areas because dark surfaces absorb heat in the day and concrete is much slower than vegetation to release heat at night. In areas with more vegetation, plant evapotranspiration helps to cool the air.
The researchers hypothesized that the greater amount of foliage in the summer could explain the seasonal differences of the urban heat island effect.
Other variable environmental factors like wind speed, cloud cover, humidity, soil moisture and snow cover also played a part in the warming effect. Land elevation and lake proximity also altered temperatures in specific areas.
"Having better data allows us to look at a lot more variability across the metro area, not just at a period of weather," said Jason Vargo, a researcher at the Nelson Institute Center for Sustainability and the Global Environment.
Vargo was not involved in the original research but is working with Schatz on a follow-up study. They are using the neighborhood-specific temperature data to study how the urban heat island effect may relate to hospital admissions and crime in specific parts of the city.
Using the collected temperature data, along with publicly available foliage data from the National Land Cover Database, the researchers created an equation that calculated the heat island effect in different areas.
"As long as land coverage is well represented, you'll get a strong sense of what the climate is like," Schatz said.
Although the exact equation couldn't be used in other cities, the study could be easily replicated if other municipalities invested in an equivalent number of temperature sensors and used the national database for foliage coverage in their area, the researchers wrote.
They published their findings in the journal American Meteorological Society last month.
City planners undervalue tree benefits
The study did not focus on ways to help cities become cooler, but plenty of other researchers are working on tackling the problem. One method that municipalities are trying is urban forestry, a tactic Madison would be well positioned to use, said Kathleen Wolf, a research social scientist at the University of Washington.
"Madison, Wis., is one of the premier cities in the U.S. in regards to urban forestry; they've had a system in place for decades," said Wolf, who studies humans' interactions with trees in urban environments. "Other cities in the U.S. are just coming into it."
Although the field has been around for a couple of decades, broader acceptance of functional rather than decorative tree planting is fairly recent, said Wolf, who has worked in her field 25 years. About a decade ago, she started to see broadening interest in urban forestry, and in the last five years, "it's just exploded," she said.
Cities like Washington, D.C.; Chicago; Portland, Ore.; and Seattle have developed good urban forestry programs, according to Wolf.
There is still plenty of room for improvement across the country. Based on her personal experience, people who study urban planning don't get much exposure to environmental planning. "Many city planners don't think of urban forestry as a substantial solution or element they need to address," she said.
Wolf emphasized the return on investment for tree-planting cities.
"City officials are always thinking about depreciation of infrastructure. With trees, immediately when they start to grow, they appreciate," said Wolf.
It can take decades for trees to reach full maturity, so tree planting can't be a quick fix. Even so, the appeal of taking local action against warming temperatures is inspiring more collaboration. Urban planners, engineers and urban foresters have started to interact more closely with each other and think in larger systems, she said.
"Cities are a sort of laboratory of climate change effects in the future," said Wolf.
Increasing foliage cover is not the only way to help keep cities cooler. Cities can lower temperatures by incorporating reflective surfaces on roofs to deflect heat absorption. Another larger-scale approach could be to make cities more compact. According to a 2010 study from the Georgia Institute of Technology in Atlanta, urban sprawl, even in cities with relatively small populations, leads to a faster increase in the rate of extreme heat events than in compact cities.
"Here in Madison, we can grow up or we can grow out. It's worth knowing what effects that could have" on temperature, Schatz said.
A super-hot summer gets amplified
To Schatz, the summer of 2012 exemplified temperatures Madison and the rest of southern Wisconsin could see more frequently in July and August by the middle of the century.
That summer, Madison had 39 days above 90 degrees Fahrenheit. The annual average is usually 12 days over 90, according to National Oceanic and Atmospheric Administration data.
Schatz cited a 2011 report by the Wisconsin Initiative on Climate Change Impacts that predicted the southern part of the state will likely experience about 25 days above 90 F per year by 2050. Northern Wisconsin, which usually experiences about five days over 90 degrees, would likely exceed 90 F 12 times per year by midcentury.
That research did not take into account the heat island effect, so temperatures within Madison could be several degrees hotter than predicted in the report, Schatz said.
"It's really important to understand [urban heat island's] nature and its consequences, especially in light of climate change," he said.
Although the summer of 2012 was the second hottest since 1939, according to NOAA, it was an "isolated warming incident." The summer was the only time in the 21st century that Madison experienced temperatures exceeding 100 F. The three hottest days happened in a single heat wave between July 4 and 6, with highs reaching 102 and 104.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500