How is the record-breaking US heatwave linked to the climate crisis?

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More than 50 million people in the US are sweltering in a historic heatwave that has sent daily temperature records tumbling across much of the country’s western region.

Heat warnings have been issued in California, Nevada, Arizona and Utah, while officials in Texas urged residents to cut back on power usage over fears that the grid would struggle to cope with surges in demand amid the extreme temperatures.

A growing body of evidence suggests that, across the world, heatwaves are becoming both more frequent and more intense as a result of the climate crisis. A number of high-profile scientific studies have linked human-caused warming to recent heatwaves in regions from the Arctic circle to Australia.

The western US has warmed by around 2C since the start of the industrial era – loading the dice in favour of episodes of extreme heat, as well as severe wildfires and drought, a climate scientist tells The Independent.

Below, The Independent assesses what is known about the impact of the climate crisis on US heatwaves – and how episodes of extreme heat are likely to change in the future.

1. What is happening in the US?

   Much of the western US is currently gripped by a long-lasting heatwave, which saw new daily temperature records set in parts of Arizona, California, New Mexico, Montana, Wyoming and Utah this week.

   On Tuesday, Salt Lake City recorded temperatures of 107F (42C) – a joint all-time high for the Utah capital. Such high temperatures have only been recorded twice before in the city, in July 2002 and July 1960.

   And on Friday, temperatures in Death Valley outside of Las Vegas – one of the hottest places on Earth – hit 128F (53C). Temperatures have not been this high on 18 June in the national park since 1917, according to the National Weather Service.

   The recent heat is particularly remarkable given that, in many parts of the western US, such high daily temperatures have not been recorded since the early 20th century, says Dr Friederike Otto, associate director of the Environmental Change Institute at the University of Oxford and an extreme weather expert.

   “It’s definitely set temperature records. In some western US states, the old records are still from the 1930s – so I think that is pretty remarkable,” she tells The Independent.

   The brutal heat comes amid ongoing historic drought conditions. Around three-quarters of the western US is currently experiencing severe drought, according to the US Drought Monitor.

   The combined severe hot and dry conditions have raised concerns that the region is heading towards another brutal wildlife season.

   Last year’s record-breaking fire season in the western US saw dozens of large and intense fires break out across the states of California, Oregon and Washington. This included the country’s first “gigafire” – a blaze that burned through more than one million acres of land.

2. How is the heatwave linked to the climate crisis?

   Global temperatures have risen by an average of around 1.2C since pre-industrial times. However, temperatures over land are heating up more quickly than over oceans.

   Because of this, the western US has experienced around 2C of warming since the start of the fossil-fuel era – providing the backdrop for more extreme heat events, explains Dr Zeke Hausfather, a climate scientist and director of climate and energy at the Breakthrough Institute in California.

   “The rising temperatures we’ve experienced in the western US over the last century make it a lot more likely that we’ll have extreme heatwaves today,” he tells The Independent.

   “We’ve seen more records fall in the western US in the last decade than any other decade in US history. Part of the reason for that is that the region has already warmed by around 2C since pre-industrial times, with around three quarters of that warming occurring since 1970.”

   As temperatures rise, heatwaves are becoming more frequent, intense and long-lasting, according to a growing body of research.

   The study of how human-caused warming is influencing heatwaves and other types of extreme weather is known as “event attribution”. Scientists in this field have previously found that human-caused climate change made a 2020 heatwave in the Arctic more than 600 times more likely and a 2019 Europe-wide heatwave up to 100 times more likely.

   No formal assessment of the influence of the climate crisis on the current US heatwave has yet been carried out.

   However, previous research suggests that US heatwaves are now 3 to 5C hotter than they would have been without the influence of the climate crisis, according to an estimate from Dr Michael Wehner, a senior scientist at the Lawrence Berkeley National Laboratory.

   A study published in 2015 found that an exceptionally warm summer in Texas in 2011 was made around 10 times more likely by human-caused climate change.

   In addition, a second study published in 2019 found that a 2018 heatwave affecting the entire northern hemisphere – including the US, Europe and parts of Asia – would have been “impossible” without the climate crisis.

   Research also suggests that combined heatwave and drought events – known as “dry-hot extremes” – are also on the rise in the US as a result of the climate crisis. Such events provide the ideal conditions for the spread of wildfires, scientists have warned.

3. How are US heatwaves likely to change in the future?

   As temperatures climb, US heatwaves will continue to become more intense and severe, scientists say.

   A study published in 2019 found that if efforts aren’t taken to put the world on track to meeting its climate goals, the number of days hotter than 105F (40.6C) could triple in the US by the mid-21st century, when compared to the period 1971 to 2000.

   And increases in US heatwaves could be potentially more “dramatic” than in other mid-latitude regions such as Europe, says Dr Otto.

   “Heatwaves are definitely going to increase. In the US, it might be even more dramatically so than in Europe, if you compare it with today,” she tells The Independent.

   The reason for this is that the impact of rising temperatures on heatwaves in the US has so far been “dampened” by other factors including large-scale irrigation, which has had a cooling effect, she says. (This is the one of the reasons why some US states have not seen new record temperatures in recent years despite the warming trend, she adds.)

   Without more irrigation, the expected rise in temperatures is likely to overwhelm this cooling effect – leading to a large uptick in extreme heat events, she explains.

   “Heatwaves will become a much bigger problem in the US than they are now,” says Dr Otto.