When Japan shut down its nuclear power reactors in the wake of the 2011 Fukushima disaster, it asked its citizens to conserve scarce energy, such as by using fans instead of air conditioning during summer heat.
But that public-spirited conservation push - the kind of call being made across Europe this winter in response to gas shortages following Russia's invasion of Ukraine - is estimated to have caused 7,710 premature heat-related deaths each year, most among Japan's elderly, a new study has found.
The study covered the years 2011 to 2015, the period energy conservation measures in response to the nuclear shutdown remained in effect.
Japan's 2011 Fukushima disaster led to energy-saving drive
Yet study found huge annual rise in heat-related deaths
Researchers say investing in renewable energy now is key
The data suggests that well-intended public policy aimed at curbing people's energy use to limit climate change or tackle other threats could have unintended health consequences - with swift investment in renewable energy the best way to avoid them.
"People usually think energy saving is a good thing. It helps mitigate climate change and helps people save money," said Guojun He, a study co-author and an associate professor at the University of Hong Kong.
But the data from Japan suggests "it's probably a bad idea to restrict individuals' energy consumption," he explained.
"The policy objective should be substituting dirty, non-renewable energy with renewable energy (so) it doesn't matter how much energy you consume."
FUTURE PROTECTION, CURRENT COST
The study - set to be published in the American Economic Journal: Applied Economics - found that pleas by Japan's government to reduce air conditioner use after Fukushima led to the public quickly slashing electricity use by 15%, in a country where air conditioners accounted for nearly half of summer electricity consumption in homes.
But an analysis of health data suggests an additional approximately 7,710 people died annually over the years of the energy conservation push, many on particularly hot summer days, said He, who is also China research director at the University of Chicago's Energy Policy Institute.
Most who perished were over 65 - an age group more vulnerable to heat-related stress - although younger people also experienced a spike in non-fatal heatstroke, the study found.
Understanding such risks is crucial as policymakers seek to balance a push to curb fossil fuel use and climate change to protect future generations while also ensuring the safety of people living today, He said in an interview.
"Policymakers should be aware of this trade-off when they design and implement climate change policies. It's a pretty bad idea to ask people to reduce energy consumption" when they most need it - though encouraging the purchase of more energy-efficient appliances is smart, he said.
Globally, a surge in heatwaves driven by climate change is putting many more people at risk, in both wealthy and poorer countries and communities and in places not previously seen as at risk.
More than 100 people died, for instance, during an unprecedented 2021 heatwave in the U.S. state of Washington, which saw normally cool cities such as Seattle hit with 42 degree Celsius (108 degree Fahrenheit) temperatures.
Wealthier countries with more stable power systems and populations better able to afford cooling are usually better able to deal with heatwaves, heat experts say.
But even there, huge demand for cooling during heatwaves can overwhelm the grid, or power can be lost during climate-fuelled disasters such as storms or floods, leaving people unprotected.
Florida, for instance, saw a 25% surge in deaths among 28,000 nursing home residents left without power in the week after Hurricane Irma hit the U.S. state in 2017, according to the Journal of the American Medical Association.
Researchers said recognising the need to invest now in renewable energy, to provide cheap and low-carbon future power to run air conditioners, was especially crucial in already hot countries like India, to keep people safe as heatwaves worsen.
"Poor countries should think about this question before they encounter it," research He urged, noting heat mortality is already 20 to 30 times higher in India than the United States in part because "people don't have basic access to electricity".
Adding vast new renewable capacity could provide effective protection from heat extremes without making them worse, he said.
"As long as the electricity price from clean energy is cheap enough in coming years, there will be no restriction on how much people can consume - so they can mitigate climate change at the same time they adapt to it," he noted.
India is currently working to ramp up renewables, particularly solar power, with $14.5 billion invested in the 2021-22 financial year, according to the Institute for Energy Economics and Financial Analysis.
Other policy measures can also help countries avoid heat deaths when power runs short, He noted.
In China, for instance, when hydropower dams have run short of power as a result of drought, the country has restricted industrial use of electricity rather than household use - hurting companies but protecting people, he said.
What is clear, the researcher said, is that curbing the use of fossil-fuel power to limit climate change is crucial to protect future generations - but it has a "non-negligible cost" for people threatened by climate extremes today.
"Climate change is already upon us and encouraging less use of air conditioning or other means of adapting to extreme temperatures can kill people living right now," he said.
"A better approach ... is to speed up the transition to clean energy and encourage people to use more clean energy to protect themselves."