The Science Behind Record-Breaking Summer Heat

Year after year, temperature records are broken during the summer as extreme heat becomes more prevalent around the world.  The situation unfolding this summer is no different. Almost every part of the country has reported severe heat waves that have impacted millions of people. 

Residents in Phoenix, Arizona have been grappling with a multi-day streak of temperatures more than 110°F that shows no signs of relenting, while places in the Southeast such as Miami have experienced continuous weeks of heat indices above 100°F.

On a more global scale, the Copernicus Climate Change Service has announced that this past June was the hottest June on record, while the U.S. National Centers for Environmental Prediction announced that July 4th and 5th were Earth’s hottest days since record-keeping started in 1979

While the term “extreme heat” and its related counterparts have almost become commonplace in recent weeks, it’s important to understand that extreme heat is truly, as the term implies, extreme.

Hurricanes, tornadoes, and floods often come to mind when one thinks of extreme weather. But The National Weather Service has released data showing that extreme heat has surpassed all of these events in terms of fatalities

Source: National Weather Service

Extreme heat often appears as a heat wave, defined as “sustained temperatures in the 90th to 95th percentile of the average in a given area”. Heat waves begin when high pressure in the atmosphere moves into an area and starts to push hot air downward. As the pressure continues to block the hot air from rising upward, it becomes increasingly hotter, creating a heat wave

Now, in 2023, there are currently over 90 million people under heat alerts, more than ever before. Understanding heat, therefore, has never been more important. In this article, we’ll be diving into the causes of the heat, the effects of summer heat on us and our planet, and what you can do about it. 

Causes of Record Summer Heat

Extreme heat is often the product of a combination of factors at a certain place and time, rather than just one element. However, some factors contribute to the heat more than others. We’ll be covering three of these larger factors: humidity, El Niño, and of course, climate change. 


When meteorologists and other weather experts explain the status of heat, they often use the term “heat index”. The heat index is similar to temperature in that it is also a measure of how hot it is, but it takes into account another factor: humidity. Humidity is a measure of the amount of moisture in the air, but this moisture can make you feel even hotter because it makes it more difficult for sweat from the body to evaporate (sweat helps you cool off). 

Source: EPA
Multiple research studies have found that humidity amplifies the dangers posed by extreme heat.

One research study found that during humid heat waves, the heat index can surpass 55 °C (131 °F) posing a severe danger of heat stroke or even death. Therefore, it’s important to stay updated on local weather conditions and stay in the know about not only the temperature, but the humidity as well. 

El Niño

This summer, another factor has been at play, exacerbating the unrelenting heat: El Niño. 

A naturally-occurring event taking place every two to seven years in the Pacific Ocean, El Niño has been known to affect climate and weather events around the world. In a normal, non-El Niño year, upwelling occurs along the northwestern coast of South America, where colder, more nutrient-rich water from the deep ocean is brought to the surface by trade winds blowing from the east to west of the Pacific Ocean.

The arrival of these nutrient-rich waters is a boon to local fisheries as fish need these nutrients to survive. However, in an El Niño year, these trade winds are not as strong and the cooler waters never come to the surface. As a result, the water is warmer than normal and fisheries struggle with nutrient-poor waters. 

Source: BBC
So what do these effects have to do with weather, and extreme heat? 

The warmer water has far reaching consequences for weather and climate. It changes usual rainfall patterns, contributing to heavy rainfall and flooding to the southern US and northern South America. On the other hand, El Niño is behind the dry conditions in Australia and Indonesia that can potentially cause droughts and wildfires.

Tropical storms are also affected by El Niño, because it affects atmospheric circulation patterns. Areas in the Pacific near the equator will experience more severe storms, but areas in the tropical Atlantic, such as the southern U.S., experience less stormy weather than normal. 

Source: BBC
Despite differences between regions when it comes to El Niño temperature effects, global temperature often increases during an El Niño.
As of now, the hottest year on record, 2016, was an El Niño year. However, 2023 is on track to surpass the temperature highs of 2016 and become the hottest year ever recorded.

Because of the drier, warmer conditions that El Niño brings about, it has also been linked to increased CO2 levels. Dry conditions and droughts decrease the number of plants available to absorb CO2 from the atmosphere, while wildfires cause more CO2 to be released. 

Although it seems that El Niño is the culprit to blame for all of the turbulent weather and the heat this summer, it is important to realize that the return of El Niño’s cooler counterpart, La Niña, will not reverse these changes: the human footprint is a much larger factor than any natural phenomenon. 

The opposite of an El Niño event, a La Niña event introduces stronger trade winds than normal, bringing colder, more nutrient-rich water to the Pacific shores of North and South America through the process of upwelling, described above. The effects of La Niña vary by region: It often leads to greater precipitation in the Ohio valley area and the Pacific Northwest, warmer-than-normal conditions in the Southeast, and colder weather in the Midwest and Plains areas. La Niña events are also associated with more severe hurricane and tornado seasons. 

Impact of a La Niña event (Source: NOAA)
Our planet experienced a relatively rare La Niña phenomenon recently:  a “triple-dip” La Niña event.

Weaker yet unusually long La Niña conditions were observed from 2020-2022, resulting in lower than average temperatures for ocean water. This prolonged La Niña event inadvertently served as a “cushion” against the effects of climate change regarding extreme heat, the full force of which we are feeling this summer. 

Together, El Niño and La Niña events are known as the El Niño Southern Oscillation (ENSO). The completion of one oscillation usually takes 3 to 7 years. However, a recent study has shown that El Niño events are slated to become more frequent as soon as 2040. The researchers also warned that these findings meant that any associated extreme climate events, such as extreme heat,  would be more likely to occur, regardless of any mitigation efforts to reduce carbon dioxide emissions. 

Climate Change

The human impact is felt in every corner of the Earth, and we experience the effects of climate change every day. The oppressive summer heat this year is no exception: climate change has shaped—and will continue to shape—global temperatures. 

Much evidence has been brought to light to prove how much climate change truly increases temperatures, causing unrelenting heat. For instance, the number of record-breaking high temperatures are more than double the number of record lows over the past decade. In the 1950s, the ratio of record highs to record lows was 1:1. Nearly all climate models project more and more warming, with drastic changes projected to occur within the 21st century. Historical records foreshadow these ominous changes: the eight warmest years on record have occurred since 2014

Source: EPA
This alarming data has almost entirely been caused by the human footprint.

Carbon dioxide emissions from fossil fuel combustion, responsible for trapping heat, reached a record high level in 2021. In addition, despite advances in clean energy access, biomass accounted for 31% of global household energy usage and fossil fuels accounted for 26%, translating into essentially stalled progress for achieving decarbonization—and mitigating extreme heat. When human activities such as mining for coal or drilling for oil are performed, they release pollutants into the air, one of the most well known being carbon dioxide.

Carbon dioxide traps the sun's heat, making Earth hotter, where usually the sun’s heat is reflected back into space. This process, known as global warming, is not limited to just carbon dioxide.

Other greenhouse gases such as methane and nitrous oxide are culprits of warming as well. Harmful human activities don’t just stop at mining as well: deforestation, heavy industrial production, and fertilizer use are all contributors to global warming, among others.

As more and more pollutants reside in our atmosphere, more and more of the sun’s heat is absorbed: as of 2022, climate change has led to about 1°C (1.8ºF) of average warming so far.

This means that at the extremes, warming is much greater than 1°C, something that has been clearly felt this summer. If climate change is left unbridled, many experts expect heat waves to occur more frequently, often longer and with more severe conditions. They are also expected to affect millions more people, especially in communities that are unprepared to address the heat.

The advent of increasingly severe heat, however, can be less damaging if communities and governments, and companies work together to develop mitigation strategies for climate change, which often have direct translations to heat wave behavior. Reducing carbon emissions and adopting environmentally friendly practices can help mitigate the effects of extreme heat, which we cover in The Effects of Heat Waves on People and Planet.

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