Large Parts Of The US Set To Experience ‘dangerous’ Heat

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The July extreme heatwave cast over tens of millions of Americans is expected to continue into next week, raising fresh concerns over the increasing health risks posed by soaring temperatures and humidity.

The US’s national weather service has warned that “dangerous” heat would develop over the Pacific Northwest over the weekend, along with parts of California and the south-western states such as Nevada and Arizona.

Heat advisories, warning people of dangerously hot weather, have already been in place across several states, including the US east coast.

Temperatures are forecast to reach as high as 51.6C (125F) in the Colorado River Valley and Death Valley over the weekend, according to the NWS. Death Valley holds the world temperature record of 57C (134F) reached on July 10 1913, although what is regarded as a more reliable modern measurement at 54C (130F) was struck in July 2021.

This week in Washington DC, the temperature hit 40C (104F), the highest temperature since 40.5C (105F) was recorded over a decade ago in 2012.

The high temperatures come as US state officials lobby the government for greater access to funding for both extreme heat and wildfire smoke effects.

Fourteen state attorneys-general wrote to the Federal Emergency Management Agency this week to urge them to allow extreme heat and smoke to be declared “major disasters”, and outlining the high numbers of heat-related deaths.

In 2023, Associated Press analysis of Centers for Disease Control and Prevention data showed a record number of 2,300 death certificates produced in the US in 2023 mention the effects of “excessive heat”.

The attorneys-general argued that the official numbers likely “understate the true extent of heat-driven mortality across the country”.

Scientists are increasingly focused on so-called wet bulb temperatures, which factors in the effect of humidity on the human body and gives a more sophisticated portrait of weather conditions than temperature alone.

As air temperatures rise, the warm air holds more moisture, limiting the ability of the human body to cool itself down through sweating.

The US’s National Weather Service describes Wet Bulb Globe Temperature as an “experimental forecast tool” designed to indicate the expected level of heat stress on the human body, by taking into account all the factors of air temperature, humidity, wind speed and solar radiation.

Meteorologists measure it by covering a thermometer with a wet cloth, allowing the evaporation of the water to cool the thermometer. This evaporation mirrors how the human body cools itself by sweating.

Scientists combine this reading with a standard air temperature reading, and a black globe thermometer designed to measure solar radiation, and the cooling effects of wind.

Some of the highest wet bulb temperatures over May to July were experienced in the US capital of Washington, along with big cities in Texas and Florida.

While Phoenix and Tucson, both in Arizona, had some of the highest dry temperatures recorded over that time period, their relative humidity levels were lower.

While scientists have theorised that a wet bulb temperature of 35C is the upper limit of what the human body can adapt to, research published in 2022 found that the threshold was variable but fell “significantly below 35C”, with some falling at 25C.

A study published in May 2020 found that the combination of heat and humidity in some parts of the world — including in India and Pakistan — have already experienced weather “nearing or beyond prolonged human physiological tolerance”.

In June, Selwin Hart, the UN’s special adviser on climate change, told the Financial Times that countries urgently needed to take action to protect people from extreme heat, which he labelled a “silent killer”.

Hart warned that the recent spate of high temperatures was “just a sign of things to come”.

The warning comes as scientists from Europe’s Copernicus Climate Change Service forecast that this year is on track to become the warmest on record.

Global surface air temperatures breached the threshold of 1.5C for each of the past 12 months, Copernicus found, while seas have reached their warmest temperatures for 15 months in a row.