Can Our Cities Survive the Heat?

Key Concepts

• Human climate niche (11-15 degrees Celsius average temperature)
• Extreme heat waves and their increasing frequency
• Heat dome formation and its impact
• Urban heat island effect
• Transpiration and its cooling effect
• Green corridors and their role in temperature reduction
• Reflective surfaces (white roofs, cool pavement) and their effects
• Mean radiant temperature (MaRTy)
• Permaculture and rainwater harvesting

The Threat of Extreme Heat

The video begins by establishing that human populations have historically concentrated in areas with average temperatures between 11 and 15 degrees Celsius, the "human climate niche." However, rising global temperatures are threatening even these hospitable regions. The 2021 Pacific Northwest heat wave, which some scientists believe wouldn't have been possible without global warming, serves as a stark example. Cities like Houston and Miami are projected to experience several months of dangerous heat annually by 2100.

The Tragic Case of Ashlyn Maddox

The story of Ashlyn Maddox, who died of extreme heat exposure in Portland during the 2021 heat wave, highlights the deadly consequences of urban heat. Despite being only 50 feet from her air-conditioned home, she succumbed to the 116-degree heat. The McDonald's bag found with her suggests she walked a long distance in extreme heat with little shade, illustrating the dangers of urban design that lacks adequate cooling infrastructure.

Heat Domes and Their Formation

The video explains how heat domes contribute to extreme heat waves. Normally, the jet stream pushes weather systems from west to east. However, when the jet stream develops large meanders, it can trap high-pressure systems in place. These systems act like a lid, evaporating clouds and depleting soil moisture. With less moisture to moderate the heat, the sun's energy directly heats the air, causing temperatures to skyrocket. There's growing evidence that climate change is slowing the jet stream and causing more meanders, leading to more frequent and intense heat waves.

The Urban Heat Island Effect

The urban heat island effect amplifies warming in cities, increasing temperatures by 2-5 degrees Celsius during the day and up to 22 degrees Fahrenheit at night. This effect is caused by heat-trapping infrastructure like asphalt and concrete, making cities particularly vulnerable to extreme heat.

Vivek Shandas' Research on Urban Heat

Researcher Vivek Shandas' work reveals significant temperature differences between neighborhoods within cities. In 2021, he recorded a 25-degree Fahrenheit difference between a wealthy, tree-filled neighborhood and a lower-income area with little tree cover in Portland. He emphasizes that heat kills more people than any other natural hazard and projects an eightfold to tenfold increase in heat-related illnesses by 2040. Shandas advocates for increasing tree cover as a primary solution to cool cities.

The Cooling Power of Trees

Trees provide a dramatic cooling effect through shade and transpiration. Shade blocks solar radiation, preventing sidewalks and roadways from absorbing heat. Transpiration, the process of water evaporating from leaves, consumes heat energy, reducing air temperatures by up to eight degrees Celsius.

Declining Tree Cover in US Cities

Despite the proven benefits of trees, Vivek Shandas' research shows a consistent decline in tree cover in US cities, particularly in low-income neighborhoods and communities of color. A study of 33 cities revealed that the hottest cities and neighborhoods are losing the most trees.

Medellin, Colombia: A Model for Urban Cooling

Medellin, Colombia, is presented as an example of a city that is successfully addressing urban heat through its Green Corridors program. Started in 2016 to combat air pollution, the program connects green spaces with tree-lined streets and shaded avenues. The city claims that the program has cooled their average temperature by two degrees Celsius.

Investigating Medellin's Green Corridors

Maiya and Vivek visit Medellin to investigate the effectiveness of the Green Corridors program. They observe the integration of multimodal transportation with green space, including shaded bike lanes and walkways. They also visit lower-income neighborhoods and find that the city has invested in revitalizing green spaces, even in dense, unplanned areas.

Temperature Measurements in Medellin

Temperature measurements taken along green corridors and non-green corridors in Medellin revealed a 7- to 8-degree Fahrenheit difference, even on a relatively mild day. This data supports the city's claim that the Green Corridors program is effectively cooling temperatures.

Heat Adaptation Strategies in Phoenix, Arizona

The video shifts focus to Phoenix, Arizona, a hot and dry city, to explore heat adaptation strategies in arid environments. Reflective surfaces, such as white roofs and cool pavement, are being used to redirect heat away from street level.

The Catch with Reflective Pavement

Researchers using a robot called MaRTy discovered that reflective pavement can have unintended consequences. While it can reduce surface temperatures, it can also reflect heat back into the atmosphere at human height, potentially increasing the heat load on people.

The Importance of Urban Forests in Phoenix

Measurements taken with MaRTy revealed a 66-degree Fahrenheit difference in heat load between exposed asphalt and a greener, shaded area in Phoenix. This highlights the critical role of urban forests in providing relief from extreme heat, even in desert environments.

Permaculture and Rainwater Harvesting in Tucson, Arizona

Brad Lancaster in Tucson, Arizona, demonstrates how permaculture and rainwater harvesting can create thriving urban forests in dry climates. He explains that the average neighborhood street in Tucson drains over a million gallons of rainfall per mile per year, which can be directed to plantings instead of storm drains. By selecting native species adapted to the local climate, these plantings can thrive on rainwater and street runoff alone.

Conclusion

The video concludes that trees are a crucial part of the solution to combat urban heat, regardless of the climate. Examples from Medellin, Phoenix, and Tucson demonstrate various strategies for increasing tree cover and creating cooler, more livable cities. The next episode will explore the relationship between climate change, flooding, and water scarcity in the Southwest.