• Tue. Jul 23rd, 2024

Why trees will help keep our hot planet cool

Why trees will help keep our hot planet cool

As temperatures soar across the East Coast, Midwest, and Southeast, hundreds of millions of lives depend on air-conditioning powered by a grid that is strained to its breaking point. The fatal toll of recent heat-related blackouts in India demonstrates the stunning risk climate change poses to American cities, which rely on A/C even more than their Indian counterparts. 

Research has demonstrated that dense tree growth can help cool over-heated cities by as much as 15 degrees. UCG/Universal Images Group via Getty Images

The problem is simple: extreme heat — such as this week’s “heat bomb” over much of the US — drives us indoors and we need A/C to survive. And the resulting spike in electricity usage puts us at the greatest risk of losing power. 

Last year, the US power grid narrowly avoided a catastrophic failure, “quietly surviving” its most brutal summer yet. Scientists warn that future summers will not proceed so well. They calculate that a heat-driven blackout in Phoenix could send half of the city to the emergency room and lead to 13,000 deaths. In the weeks before June’s unprecedented heat, leading experts begged federal agencies to make immediate emergency plans before a heat-driven power failure kills millions of Americans. 

If we are to avoid that fate, we need to start by addressing climate change’s deadliest multiplier — the urban heat island effect (UHIE) — through technologies that do not rely on the grid.

The UHIE, in which heat-absorbing and heat-emitting man-made surfaces spike temperatures in our neighborhoods, is often what tips otherwise uncomfortable summer weather into fatal territory. New research shows that it is far more powerful than scientists previously imagined. And that by mitigating it, we can blunt the dangerous threat of extreme heat. 

The secret lies in understanding why the UHIE leaves people on certain blocks so much more vulnerable to heat-related death than their neighbors just a few steps away. 

American rely on air-conditioning to cool their homes even more than the most stifling developing nations. AFP via Getty Images

Several years ago, Professor Vivek Shandas, a climate scientist at Portland State University, sought to explain a troubling trend. When a heat wave swept across his hometown in Oregon, fatalities were not distributed across the city as one might expect but instead highly concentrated in a few disadvantaged neighborhoods. He launched CAPA, an urban climate adaptation consultancy, in part to explain this mystery.

The volunteer-based, community-driven research campaigns that CAPA led produced some of the first high-resolution descriptions of ambient heat at the human level, providing precise, hyper-local data on how extreme heat affected cities around the world.

Research by Portland State University Professor Vivek Shandas detailed how the effects of heat are not experienced uniformly. AGCI

The results were staggering. Cities like Durham, Raleigh, and Oklahoma City, where scientists and city officials had thought temperatures varied by a few degrees at any given time of day, in fact had temperature differences between neighborhoods as high as 15°F. As the geographic distribution of heat-related deaths across the cities showed, these disparities were the difference between a sleepless night in an 80°F bed and dying of a heat stroke. 

Most surprisingly, the main factor that distinguished the coolest blocks from the hottest ones wasn’t the construction materials, such as concrete, that are usually blamed for the UHIE. Instead, it was the amount of trees and other vegetation in those areas.

Urban tree canopies and green spaces are our most potent weapons against the collision of the UHIE and climate change. Unlike air conditioning, which often cuts out when everyone cranks up their units — exactly when it is needed to save lives — vegetation’s cooling effect only grows the hotter an area gets. Large plants like trees and shrubs not only shade our homes on the days when the sun is most powerful, but they also cool our environment through evapotranspiration.

Even a young tree has a net cooling effect equivalent to 10 room-size air conditioners operating for 20 hours a day. Within 15 years, the effect doubles. 

Now, armed with maps like CAPA’s, we can see how dramatically the heat pockets in our communities line up with gaps in our greenery, and we can quantify the lives we could save through strategic interventions such as green walls, rooftop gardens and trees planted along our streets. 

Author Nadina Galle

These solutions are achievable within a short time frame. As we embark on yet another summer that promises to be more blistering — and more deadly — than the last, we need to use what we have learned to redesign our habitat. We should meet our neighbors under the shade of a nearby tree and build a plan to protect the most vulnerable among us. And we must demand that our municipal leaders fortify our homes for both the present and the future.

Nadina Galle, PhD, an ecological engineer and 2024 National Geographic Explorer, is the author of “The Nature of Our Cities: Harnessing the Power of the Natural World to Survive a Changing Planet,” out on June 18, 2024.

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