Author(s): Caroline Hasler

Grand Canyon heat may become more dangerous

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Every year, millions of visitors flock to U.S. national parks. They hike in Yosemite, watch geysers gush at Yellowstone, and admire the iconic vistas of the Grand Canyon. But some park visitors’ heat-related health risks are on the rise.

In a recent study, scientists estimated that the rate of heat-related illness among visitors to Grand Canyon National Park may more than double in the coming decades as climate change continues to bring the heat.

The study provides a useful example of the risks that climate change poses to public health.

“The study provides a useful example of the risks that climate change poses to public health,” said Jennifer Marlon, a climate change researcher at the Yale School of the Environment and the Yale Program on Climate Change Communication, who was not involved in the study.

The Grand Canyon is one of America’s most popular national parks, attracting more than 4.7 million visitors in 2022. Many people hike the trails or admire spectacular views. But in hot weather, spending time outdoors and engaging in physical exercise increase the risk of heat-related illness.

High temperatures can cause a person’s body to overheat. The effects can be mild, manifesting as temporary cramps, nausea, or tiredness. In extreme cases, however, symptoms can be life-threatening. The risk of serious illness is especially high among pregnant people, the elderly, and those with underlying heart or respiratory conditions.

In the southwestern United States, climate change is driving increasingly frequent and severe heat waves and droughts that are projected to worsen in the coming decades. “In the U.S., heat waves used to only occur during a roughly 2-week period in the summer a few decades ago; now they occur repeatedly over more than 2 months each summer,” Marlon said.

Because the region contains some of the country’s most popular national parks, millions of people will pursue outdoor recreation in hot, dry areas that are expected to become hotter and drier—conditions that may create hot spots for heat-related illness.

Temperature and humidity matter, but so does timing

In the new study, researchers predicted how climate change could affect the risk of heat-related illness in Grand Canyon National Park. They used data on visitation, heat-related illness, temperature, and humidity from 2004 to 2009 to define a baseline risk. During that time, the National Park Service recorded 483 heat-related illness events in the Grand Canyon, including six deaths.

The researchers used 14 climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) to project temperature and humidity during each year’s peak visitation season out to the end of the century under two scenarios: an intermediate one in which climate-altering greenhouse gas emissions peak in 2040, then decrease, and another scenario in which emissions continue to rise. They then determined how many more cases of heat-related illness would be caused by either change in conditions.

According to the results, the number of cases could increase by 29%–137% by 2100. However, these estimates don’t account for observed and projected increases in the number of annual visitors to the park. Considering recent visitor counts, the worst-case model predicts more than 250 annual cases of heat-related illness by the end of the century, compared with 81 average yearly cases in 2004–2009.

The researchers also found it’s not just temperature that matters—it’s timing: Although Grand Canyon National Park is hottest in July and August, the projections showed that the risk of heat-related illness is highest in the so-called “shoulder season” months of April and May, when visitors aren’t as prepared for the heat.

“Many visitors to national parks are already inadequately prepared for the strenuous exertion and increased exposure to the elements,” Marlon said. As heat waves in the shoulder season become more common due to climate change, hikers who aren’t expecting high temperatures will be especially at risk.

Fighting the heat

The new study could guide efforts to increase patrols during the shoulder season.

The National Park Service (NPS) has taken measures at the Grand Canyon to monitor and manage heat-related illness, including preventative search and rescue patrols during the peak season. The new study could guide efforts to increase patrols during the shoulder season, explained Danielle Buttke, an epidemiologist and health coordinator at the NPS and lead author of the study. The projected increase in heat-related illness could also help the NPS plan ahead and build more shade structures and water stations along popular trails.

Visitor preparedness is crucial to preventing heat-related illness, but to prepare, visitors first must be aware of the risks associated with extreme heat. To increase awareness, “communicating with our many audiences is a key tenet of action,” said Larry Perez, communications coordinator at the NPS’s Climate Change Response Program. The study’s results suggested that informing visitors about extreme heat in the shoulder season, which is typically assumed to be cooler, could mitigate the rising risk under climate change.

The NPS conducts outreach to inform visitors about the heat and its potential health effects, as well as prevention strategies. Park visitors can guard against heat-related illness by drinking enough water, limiting their outdoor exposure, and being alert to their body’s heat responses. If they feel any symptoms outlined by the U.S. Centers for Disease Control and Prevention (CDC), they should seek shade and rest. If symptoms persist or worsen, people should seek medical attention, according to the CDC.

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