Tuesday, July 16, 2019 | 2 a.m.
Las Vegas could feel like at least 105 degrees Fahrenheit on more than one in six days by late-century, according to a report released today by the nonprofit scientific and advocacy group the Union of Concerned Scientists.
“Killer Heat in the United States” and an accompanying peer-reviewed study published in Open Access Journal analyze and predict climate change-fueled trends in heat indexes nationwide under different emissions scenarios. The heat index is defined as the “real feel” temperature for people when factoring in relative humidity along with air temperature.
Globally, average temperatures have increased by 1.8 degrees Fahrenheit as of 2018 compared to the pre-industrial area, according to NASA. Ninety-seven percent of scientists agree that rising average temperatures and other impacts of climate change have been caused by human activity, especially the burning of fossil fuels.
Las Vegas has historically seen 18 days per year with a heat index above 100 degrees Fahrenheit and three days per year with a heat index above 105 degrees Fahrenheit, the Union of Concerned Scientists reports. Historic numbers were based on data from 1971 to 2000, said Kristina Dahl, a senior climate scientist at the Union of Concerned Scientists and the lead author of the study.
Absent reductions in global greenhouse gas emissions, the number of days in Las Vegas that feel like 100 degrees or more could jump to 71 by mid-century and 96 by late century, warns the report. The metropolitan area could also experience 60 days per year that feel like at least 105 degrees Fahrenheit by the end of the century, compared to just three days historically.
“While people in the Las Vegas region may be acclimated to a certain amount of heat today, what we could be seeing mid-century is much more severe and will put many more people at risk,” Dahl said.
Temperatures in Las Vegas have already risen since the “historic” 30-year timespan used in the report, Dahl noted. In April, the nonprofit news outlet Climate Central reported that average temperatures here rose by 5.76 degrees Fahrenheit between 1970 and 2018 alone.
The report presented estimates for heat indexes under three different emission reduction scenarios: no action, slow action and rapid action.
If “slow action” is taken worldwide to reduce emissions starting in the mid-century, Las Vegas could see an average of 27 days per year with a heat index above 105 degrees by late-century. If “rapid action” is taken to reduce greenhouse gas emissions, that number could drop to 20 days per year.
In general, high relative humidity yields a heat index greater than the actual air temperature, while low relative humidity typically corresponds with a heat index equal to or slightly lower than the air temperature, Dahl said. Because Las Vegas and the rest of the southwest have relatively low humidity, the heat index tends to be very close to the air temperature.
By contrast, the difference between the two measurements can be stark in other parts of the country, such as the southeast and the southern Great Plains, both of which the report predicts will experience the most extreme warming under any emissions scenario.
“What we wanted to do here, since this was a nationwide study, was to use a metric that combined temperature and humidity, because we noted that there were no studies that covered the whole U.S. at a really fine resolution that also included a humidity component,” Dahl said.
Las Vegas National Weather Service meteorologist Trevor Voucher questioned the use of the heat index as a reliable way to show warming trends.
“If you were to do something to comment on the warming over time, then I would use strictly temperature rather than heat index,” Voucher said. “The heat index, unfortunately, is a bit misleading to the public as far as what it implies.”
But Dahl argued that the heat index is significant because it affects the risks to human health associated with heat. For example, if the air temperature is 100 degrees and humidity is 60%, it’s more difficult to sweat and cool down than on a 100 degree-day with 5% humidity.
“When it’s more humid, that (sweat) evaporation doesn’t happen as effectively, so it’s much harder for you to cool yourself,” she said.
The report concludes that under a rapid action scenario, “many places” could avoid prolonged and potentially deadly heat. This scenario assumes an immediate reduction of emissions in order to keep average global temperatures to no more than 3.6 degrees Fahrenheit above pre-industrial temperatures, corresponding to the goals set by the Paris Climate Agreement.
“When we look out at the long-term and how this could play out over the next 70 years, we can see that if we work hard to reduce global carbon emissions, we can really limit the magnitude of that expansion in extreme heat that Las Vegas would experience,” Dahl said.