Climate change is changing the character of the hottest periods in the West, making them more frequent, persistent and deadly.
For almost all of human history, heat waves have been caused by natural variability - or the tendency of weather patterns to occasionally deviate from their typical patterns. Now, however, the accumulation of greenhouse gases due to the burning of fossil fuels increases the likelihood and severity of such extreme heat events.
Although California and the American West will continue to experience cool days and periods of heavy snow, scientists say the long-term trend is for the planet to become hotter due to the continued burning of fossil fuels. Since 1880, the average global temperature has risen by about 2 degrees.
How does climate change affect the length and duration of heat waves? How will rising temperatures affect people and ecosystems? How much hotter is it expected to get if current emissions continue unabated?
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This is what the experts say:
How do we know the planet is warming?
Temperature measurements in the Earth's atmosphere and oceans are monitored by thousands of weather stations, buoys and ships around the world. Scientists use this data to calculate the average temperature on Earth.
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"We know the planet is warming because all of these groups are independently documenting a clear long-term increase in our global average temperature," said Kristina Dahl, chief climate scientist at the Union of Concerned Scientists, a national nonprofit.
"It is a trend that cannot be explained by natural causes, such as changes in volcanic eruptions or solar radiation," she said. "Human emissions of carbon dioxide and methane, two well-known, powerful heat-trapping gases, very clearly explain the trend we've seen."
Nineteen of the 20 warmest years on record have occurred since 2000, according to the National Oceanic and Atmospheric Administration.
The year 2023 was the hottest year on Earth to date. In July, Phoenix recorded 31 consecutive days of temperatures of 110 degrees or higher - the hottest month on record for any U.S. city.
In 2021, an abnormal and extreme heat wave in the Pacific Northwest killed hundreds of people and an estimated 1 billion marine animals off the coast of British Columbia. A study into that event found that such heat waves could become 20 times more likely if current CO2 emissions continue.
Read more: The LA Times investigation into the deadly toll of extreme heat
How much hotter is it expected to get if current emissions continue unabated?
Scientists use a range of potential future emissions scenarios to try to discern how emissions choices will affect all aspects of our climate, Dahl said. Here's an example of what these scenarios show:
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If emissions remain at current levels until about 2050, and then begin to decline, global temperatures would warm by almost 5 degrees by the end of the century. This is what the Intergovernmental Panel on Climate Change considers a 'middle-of-the-road scenario'.
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Under current policies, we would experience a similar amount of warming - between 4.3 and 5 degrees - unless those policies are significantly strengthened. California aims to achieve carbon neutrality by 2045, while the nation as a whole aims for 2050. Other countries have longer goals, such as China, which aims for 2060.
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In a worst-case scenario, in which our heat-trapping emissions triple around 2075, the planet would experience a warming of about 8 degrees. This is unlikely, as it would involve higher emissions than the path we are currently following.
How does climate change affect the length and duration of heat waves?
In much of the US, extreme heat events have increased in frequency since the mid-1960s, and the number of high-temperature records has exceeded the number of low-temperature records since the mid-1980s.
"While there is no single definition of what constitutes a 'heat wave,' we know that cities in the U.S. and around the world have experienced more intense and longer-lasting heat waves over the past 60 years," Dahl said. "Looking globally, the number of days with heat waves has almost doubled since the 1980s. During that time, heat waves have also increased in duration."
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What are the possible consequences of rising temperatures for people and ecosystems?
Extreme heat is one of the deadliest consequences of climate change. Every year, extreme heat kills more Americans than any other climate-related hazard, including hurricanes, floods and wildfires, but it receives far less attention because it kills so quietly.
A 2021 Times investigation found that California has chronically underestimated the death toll from extreme heat, disproportionately harming poor people, the elderly and others who are vulnerable.
High temperatures can affect the human body in many ways. Heat can cause dehydration, dizziness and headaches and can worsen underlying health problems such as cardiovascular disease. Health trackers typically show spikes in deaths from heart problems during and in the days immediately following heat waves.
A sweltering heat wave in California in September 2022 killed 395 people, according to health officials.
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During record heat in Phoenix in the summer of 2023, emergency departments also saw an increase in the number of people suffering from sidewalk burns, because concrete can become 170 degrees or hotter at high air temperatures. Officials said many burn patients may have fainted on the sidewalk due to dehydration, intoxication or other factors that prolonged their exposure and complicated their treatment.
People who work outdoors or otherwise lack air conditioning are at particular risk for heat-related illness and death during extreme heat. California has established heat standards for outdoor workers, but has not yet done the same for indoor workers.
Beyond the health risks, "more frequent, more severe extreme heat is also shaping the way we live and experience the world around us, from whether we can enjoy a visit to a national park to whether it is safe to to walk a few blocks for an ice cream cone," Dahl said.
This also includes effects on ecosystems.
For example, global warming has allowed pests such as bark beetles to survive the winter and expand their range, decimating western forests. Avery Hill, a postdoctoral researcher at the California Academy of Sciences, noted that for every additional 1.8 degrees of global warming, up to 40% more trees could die from beetle infestations.
Warming temperatures are also involved in drying out vegetation, which can contribute to larger, faster and more frequent wildfires.
Furthermore, forests that do not recover from severe fires can completely transition into different ecosystems, which not only affects the range of plants and animals in the area but can also affect the larger food web, Dahl said.
Read more: Earth had just recorded its hottest August - and summer - on record, prompting dire warnings
It's been hot before. Isn't the planet always changing?
Earth's climate has always changed and will continue to change due to things like changes in the shape of our orbit around the sun, Dahl said. However, the changes documented over the past 150 years are unprecedented.
There is more carbon dioxide in our atmosphere now than at any time in the past two million years, she said. Sea levels have risen faster in the past century than in any previous century in the past 3,000 years. Glaciers - and the crucial freshwater they contain - are retreating faster than at any time in more than 2,000 years.
"The source of these changes is very clear: it is us. We are changing our climate because of our hunger for fossil fuels and the energy they provide," Dahl said.
From a geological perspective, you could argue that because humans are part of the planet, these changes are natural, or that we shouldn't do anything to solve the changing climate.
"But the reality is that humans have never experienced these kinds of changes," Dahl said, "and if we want to alleviate the suffering of the people, plants and animals that experience these changes most acutely, we will have to wean ourselves off. away from fossil fuels, and sooner rather than later."
This story originally appeared in the Los Angeles Times.
