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Extreme Temperature

As more energy is trapped within our atmosphere due to the burning of fossil fuels and resultant greenhouse gas emissions, the average global temperature is increasing. Temperature trends over decades are generally measured in fractions of degrees, but within these trends there are larger, more abrupt changes lasting just days or weeks that can be measured in tens of degrees, called heat waves. Heat waves are defined by U.S. NOAA as a period when daily mean temperatures for a region go above a threshold temperature–usually the 95th or 99th percentile–for two or more consecutive days. Extreme heat has both direct and indirect effects on human health.

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How does climate change contribute to the increasing prevalence of extreme temperatures?


Climate change is the result of shifts in the balance between incoming solar energy entering the earth’s atmosphere and outgoing energy radiated from it. The burning of fossil fuels–and the accumulation of greenhouse gasses such as carbon dioxide, methane, and nitrous oxide–in addition to other anthropogenic processes like deforestation and other land use practices, has thrown off this energy balance. The rise in heat-trapping greenhouse gasses effectively acts as a blanket, causing the earth’s temperature to increase. 


Not only is climate change increasing the average global temperature, but it is also increasing the frequency of extreme heat events, causing shorter winters, earlier springs, and longer “heatwave seasons.” The variability in both average temperature and extreme heat events is not uniform, and can differ dramatically depending on region. For example, temperatures in the contiguous United States have risen 2.5 degrees Fahrenheit since 1970 compared to a global average of 1.7 degrees (5th National Climate Assessment). Additionally, extreme heat events are happening more frequently in the western United States compared to the rest of the country. Furthermore, cities and metropolitan areas can be many degrees hotter than their surrounding areas–a phenomenon known as Urban Heat Island–increasing the magnitude and duration of extreme heat events. Not only are extreme heat events becoming more frequent, longer-lasting, and more severe, but the average nightly temperatures are increasing as well, disrupting a natural period of cooling for people, crops, as well as both the natural and built environment, further contributing to the damaging effects of rising temperatures. 


While the trends of extreme temperatures are largely defined by warming and heatwaves, there is mixed data suggesting that the process of Arctic warming may contribute to disruptions in stratospheric circulation and extreme cold events as well.

What happens when the body gets too hot?

The human body maintains a temperature of around 98.6 degrees Fahrenheit with little variation from person to person. The body creates heat in many ways, from cellular metabolism, to breathing, to exercising. Usually it’s quite good at getting rid of this heat via sweating or other mechanisms, but when the body’s ability to dissipate heat is exceeded, its core temperature can increase, a process called hyperthermia. When this happens, protein function and chemical processes necessary for life can be negatively affected, or even stop altogether.  This can lead to functional impairment or even failure of vitals organs such as the kidney, liver, heart and brain.

What happens when the body gets too cold?

The body’s temperature drops when heat loss exceeds heat generation. This process is known as hypothermia. Initially, the body tries to adapt by shivering, constricting blood vessels, and conserving heat, but these processes can become overwhelmed and ineffective as heat loss increases. A variety of negative consequences ensue, including confusion, poor judgment, autonomic dysfunction, and the dysfunction of vital physiological processes.

What are the health harms of the extreme temperatures?

Heat-related illness can come in many different forms, from minor conditions such as heat rash and heat cramps, to life threatening processes like heat stroke, heart attacks, ischemic strokes, and respiratory failure. Higher temperatures also have negative effects on pregnancy, increasing the risk of preterm delivery, low birthweight, and stillbirth. Cold-related illnesses include frostbite and frostnip, gangrene, cardiac arrhythmias, acid-base disturbances, and coagulopathies, to name a few. 


Nobody is immune to the detrimental effects of extreme temperatures but the very young and the very old are at particular risk since their bodies are not able to regulate temperature as well. Additionally, populations with pre-existing medical conditions, people belonging to racial/ethnic minorities, and those who are houseless or in poorer housing conditions are at increased risk of heat-related illnesses.

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How is this affecting Arizona?

Arizona is one of the hottest states in the country. Temperatures have risen nearly 2.5 degrees fahrenheit since the beginning of the 21st century, and the first 21 years of this century have been the warmest period on record. Maricopa County, which includes Phoenix, is the hottest metropolitan area in the US and in 2023, with more than 425 heat-related deaths, beat its own record for the highest heat-related death rate across the entire country. During the summer of 2023, Phoenix experienced the hottest three months since record keeping began in 1895, and it experienced a record 31-day streak of temperatures at or above 110 degrees F. Nighttime low temperatures are also decreasing, which could contribute to more harmful and intense future heat waves.

Climate change, extreme temperatures, and extreme weather events increasingly affect infrastructure such as electricity and power grids. Power grids are particularly stressed during summer months when demand is highest. Models suggest that, should a power grid failure and heat wave occur simultaneously, the heat-related death in Phoenix could reach as high as 1% of the population, which is more than 13 thousand people. While only a model, this highlights the disproportionate risk of increasing extreme heat events for millions of Arizonans as these concerning climate trends continue.

What can we do to protect ourselves?

During extreme heat events there are things that can be done to help reduce the risk of heat-related illness. These include limiting outdoor activities to the coolest time of day, using appropriate sun protection, drinking plenty of water, utilizing AC in your home or going to buildings with AC, checking on those who are elderly or more vulnerable, and seeking medical care immediately if you or someone you know starts to experience symptoms of heat-related illness such as headache, confusion, passing out, extreme weakness or exhaustion.


Other measures that can be done to mitigate heat-related illness include increasing urban trees and green spaces, selectively utilizing cooling building materials, establishing early warning systems, creating public cooling facilities and building resilient infrastructure particularly for vital services such as power and healthcare. 


Beyond these immediate measures, there are steps that you can take to reduce the risk of future extreme heat events and heat related illnesses. These measures focus on reducing greenhouse gas emissions. They include supporting the transition away from fossil fuels, promoting solar and other renewable energy sources, pushing for the use of electric vehicles in both private and public sectors, educating family and friends, supporting policies and measures that aim to curb emissions, and advocating locally within your organization or community for changes that will help reduce the upward trend in rising temperatures, extreme heat events, and the negative health consequences associated with them. 

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