Is Daylight Saving Time good for the climate?

The clocks have gone back once again, ending Daylight Saving Time (DST), or British Summer Time, for 2024 – an annual practice long criticised by politicians, doctors and commuters.

Scientists are also divided over whether the time change could have the power to counter climate change. A study published in Environmental Research Letters last year found that DST decreases the energy needed to cool office buildings in the summer by nearly 6%. When the clocks go forward in spring, workers arrive at the office an hour earlier "in the cool of the morning", said Anthropocene Magazine. They leave earlier too, "which is typically when demand for cooling is greatest".

"Building energy use is a major contributor to carbon emissions, so figuring out how to cut energy demand for heating and cooling is important in fighting climate change," said the independent science magazine published by Future Earth.

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But this study's findings don't necessarily apply worldwide – other inquiries into the environmental impact of DST have been far less conclusive.

Mixed findings

Daylight Saving Time became official when Germany adopted the practice (in part, to conserve coal during the war) in 1916, soon followed by the UK, but its potential energy-related benefits were being considered much earlier.

Benjamin Franklin is credited with the "original concept", said BBC Future. The US founding father analysed candle consumption in 1784 and suggested that people "alter their overall sleep schedules to save money on lighting costs".

Now, with climate change accelerating, scientists are seriously considering whether a yearly time change helps or hurts the environment.

Arizona is one of the few US states to have opted out of DST, and it does so specifically to save energy. The hot desert state implemented it in 1967 but abolished it the same year after "the clock change caused a surge in energy consumption". People had to run their air conditioners for longer into the evening, driving up costs. (The Navajo Nation within Arizona still adjusts clocks to DST, however.) As the globe heats up, more states and countries "may follow suit".

But several studies published in the US and Europe conclude that DST has "very little effect on energy conservation", said Euronews.

A study by university researchers in Prague used data from 2010 to 2017 to estimate that DST in Slovakia had led to energy savings of just 0.8% of annual electricity consumption.

Another study by the US Department of Transportation in 1975 had similar findings. DST cut about 1% of the country's energy usage, because "much of the energy saved by not having the lights on in the evening was offset by having them on in the morning."

When DST doesn't save

Experts have also called the date of the clocks changing into question. In Italy, the Society of Environmental Medicine calculated that €70 million (£58.6 million) could be saved in fuel bills simply by postponing the date by one month.

Natural time zones as close as possible to solar time would likely "align daily light and temperature curves better with our schedules", said Euronews.

"This could lead to energy savings on early-morning industrial and other lighting, early-morning heating during the colder months as people leave for work an hour later and less air conditioning needed in the car on the commute after work and in the evening at home."

'This is slowing down the entire Earth'

Scientists have discussed at length whether DST might impact climate change, but conversations about whether climate change might impact DST are much quieter.

But a study published in Nature last year suggested that in fact, climate change might be affecting timekeeping in general. Our universally agreed measurement of time, Coordinated Universal Time (UTC), is linked to the duration of a complete rotation of the Earth around its axis, and the average taken from 400 atomic clocks.

"Since the 1970s, UTC has added 27 leap seconds at irregular intervals to keep pace with atomic clocks as the Earth's rotation has gradually slowed," said Heatmap. Then in 2016, that rotation started to speed up.

Now, timekeepers have identified a need for negative "leap seconds" to account for that, Duncan Agnew (the geophysicist behind the study) told Heatmap. These are adjustments to timekeeping necessary to account for the increasing speed of Earth's rotation. Climate change has caused ice to melt in large enough quantities to change the rotation rate "of the entire Earth in a way that's visible".

This makes the DST debate even more complicated. "Just the fact that we can say, 'Look, this is slowing down the entire Earth' seems like another way of saying that climate change is unprecedented and important," said Agnew.