Spotlight on heat waves

How can we protect people’s health from heat waves? What impact will they have on agriculture and power grids? A conference held in early July in New York brought together experts from around the world to discuss these issues and to understand the unusual weather phenomena resulting from climate change.

The mysteries of heat waves

In the summer of 2003, just over 3,000 heat-related deaths were recorded in France. But when epidemiologists looked into the matter, based on the “excess mortality” compared to the norm, the toll climbed to 15,000 deaths.

“No matter how good the death registration system is, the death toll from heat is still five to 10 times lower than the actual death toll,” Stanford University ecologist Christopher Callahan told the Columbia University Heatwave Conference in New York. “It happened with the Chicago heat wave in 1995 and the Washington heat wave in 2021. It’s a mystery that needs to be solved.”

For now, the most likely culprit is an increase in the effects of pollution with heat, perhaps due to chemical or weather reactions. Callahan tested whether other variables associated with heat, including humidity, nighttime temperature and nonconsecutive days of heatwaves, could explain the discrepancy. They didn’t.

The same phenomenon is also observable in Quebec: according to two studies from the National Institute of Scientific Research (INRS) discussed at the New York congress, the heat waves of 2010 and 2018 caused 100 to 150 deaths, but the excess mortality attributable to heat each year is 470 deaths.

Humidity is at the heart of another heatwave mystery. “Physiologists consider high humidity to be very bad for your health, but epidemiologists don’t see a negative effect on mortality from high humidity during heat waves,” says Jane Baldwin, a meteorologist at the University of California, Irvine.

M^me Baldwin has embarked on a research project to solve this puzzle. For now, it appears that part of the difference between mortality predicted by physiologists and that observed by epidemiologists, during heat waves with high humidity, may be attributable to a protective effect of high humidity in the days leading up to the heat wave.

She will now test whether humidity might be harmful only in urban heat islands or inside homes without air conditioning. She also notes that most epidemiological data are collected in wealthy countries that are not in the tropics, where the combined effects of heat and humidity are greatest.

Ambivalence about air conditioning

At the health session of the Columbia University conference, several presenters cited a figure from a 2023 climate health impact assessment published in the Lancet : a nearly five-fold increase in the number of heat-related deaths worldwide.

This figure, however, does not take into account the possibility of increasing the number of homes with air conditioning. The main presenter (keynote) health session, epidemiologist Kristie Ebi of the University of Washington explained why.

“Air conditioning reduces mortality from heat waves, but we can save lives without air conditioning,” said Mr.^me Ebi. Using air conditioning increases greenhouse gas emissions, which will worsen climate change. The solution, she says, is to reduce greenhouse gas (GHG) emissions.

After the presentation of M^me Ebi, a participant asked how to reconcile the urgency of the situation with the prescription to reduce GHG emissions, which will not quickly solve the problem of more frequent heat waves.

M^me Ebi then indicated that it is necessary to prioritize warning systems allowing populations vulnerable to heat, for example the elderly, to take shelter in cooler public places, and also to require the replacement of dark roofs with white or green roofs.

A 2021 study by Lancet reported that air conditioning reduced the number of heat-related deaths in the United States by 68,000 to 21,000 in 2019, but only by 142,000 to 72,000 in China and by 49,000 to 47,000 in India.

Flexible agriculture

Nathan Mueller is one of the leading figures in agricultural adaptation to climate change. This biologist from Colorado State University has been publishing forecasts of its impact on the sector’s productivity for about fifteen years.

But more recently, he’s found that going back in time doesn’t measure that impact of climate change. “That means farmers have adapted by investing in technologies to keep yields up,” Mueller said. “But that’s still a cost.” Future adaptations, however, will be more costly, ranging from precision irrigation to biotechnology to breed plants and animals better adapted to a warmer planet.

Climate failures

Climate change won’t just drive up electricity consumption for air conditioning. It will also reduce the capacity of the power grid in several ways, said Andrea Staid, an environmental engineer at the Electricity Research Institute (ERPI), a U.S. nonprofit.

Among the problems: the loss of battery capacity due to heat, or the need for air conditioning to keep them cool, the reduced efficiency of the power grid’s transformers, the lengthening of electrical wires subjected to currents close to their capacity given the increase in demand, and the difficulty of cooling nuclear power plants. In 2003, half of France’s nuclear power plants were temporarily out of service because of the heat wave.