WeatherImpact Earth


Climate change and urban development leading to warmer nights in Phoenix

Posted at 4:28 PM, May 18, 2021

PHOENIX — We know that greenhouse gasses contribute to global warming, causing climate change around the world, but we can’t talk about the warming trend without also acknowledging that there is another factor at play.

The ABC15 Impact Earth team is tracking the trend of rising temperatures in the desert Southwest and how urban development plays a role.

Nearly five million people live in the Phoenix metro area, and according to the United States Census Bureau, from 2010 to 2020 we led the nation in population growth.

According to Climate Central, Phoenix is also among the fastest-warming cities.

Ariane Middel is a professor at Arizona State University who studies the urban heat island.

“The urban heat island exists because we bring all these artificial materials into the city," explains Middel. "We build roads, we build buildings, and all of these materials are really great at storing the heat. During the day the sun heats those surfaces, and then at night they slowly release the heat."

According to city records, Phoenix occupied just 17 square miles in 1950. That figure was up to nearly 517 square miles by 2010.

A growing city on top of an already hot desert has amplified the heat, so it just does not cool off at night like it used to.

ASU Associate Professor Matei Georgescu studies the heat island using regional climate models. He says there is clear evidence that urban development is having an impact, with minimum temperatures increasing more rapidly than maximum temperatures.

A study conducted in 2016 by former ASU PhD student Chuyuan (Carter) Wang, found that places like Gilbert, Chandler, and parts of the West Valley saw a significant increase in the intensity of the heat island from 2000 to 2015. The same areas where we have seen some of the most recent development.

Even within older developed areas, neighborhood and landscape design make a huge difference in how the heat island impacts local communities.

For example, in the Historic Encanto District, there are tree-lined streets that block the sun, providing cooler spots in the shade. There is also a large park and golf courses nearby. As water evaporates off the grass in those areas, that cools the surrounding air, making the neighborhood generally cooler.

By contrast, less than three miles away on the west side of I-17 along Thomas Road, there is little grass and few trees.

Instead, you have a lot of pavement, parking lots, and large industrial areas. Those surfaces absorb the heat of the sun during the day and are much slower to cool off at night, making that part of town warmer.

However, it is about more than just landscaping. The heat from our cars, even our air conditioners is also making things warmer.

Research shows that if development continues at the current rate, the warming effect from the urban heat island could be similar to the warming effect of greenhouse gas-induced climate change.

“The night is probably, I mean it’s difficult to say within any precision, where the nighttime minimum temperature will not go below triple-digits,” says Georgescu. He believes that could happen within the decade.

Those warmer nights have a direct effect on how we experience the heat.

“As nights stay warmer, people just don’t get the relief from the heat because there’s just no break. So you have to run your air conditioning at night much more than you would if there was no urban heat island,” says Middel.

So, it is important to find ways to mitigate the heat island even as our city grows.

“When we develop new areas or when we convert naturally vegetative areas into built-up areas like residential areas, we should include vegetation to lower the surface temperature,” says Wang.

The key is to also make thoughtful choices in our own homes.

Georgescu says that even adjusting the air conditioner by one degree, could make a difference.

“What we do in one individual household is not going to have a large effect and that’s precisely the point. There’s unity in the final outcome, then that one degree Celsius or two degrees Fahrenheit is going to have a discernible impact for you specifically in terms of energy savings, for the region specifically in terms of greenhouse gas emissions, and then if this is sufficiently scaled up across the United States and globally, then a very large impact.”