Earth’s rotation is slowing - and longer days may have helped oxygenate the atmosphere

Since Earth formed roughly 4.5 billion years ago, its spin has been easing off. The consequence is simple: over geological time, the length of a day has gradually increased.

On everyday human timescales, that slowdown is imperceptible. Over eons, however, it is large enough to help drive profound shifts. One such shift may be especially important for life on this planet: research published in 2021 links lengthening days to the oxygenation of Earth’s atmosphere.

The study proposes that when blue-green algae - cyanobacteria - appeared and spread in enormous numbers about 2.4 billion years ago, they could generate more oxygen (as a metabolic by-product) because the planet’s days were getting longer.

A short video summary of the work accompanied the research.

Why Earth’s day length changes (Earth’s rotation and the Moon)

At first glance, two parts of this story seem unrelated. The first is the steady slowing of Earth’s rotation.

That deceleration occurs because the Moon’s gravity tugs on Earth. As the Moon very gradually moves farther away, its pull contributes to a braking effect on Earth’s spin.

Clues from fossils indicate that 1.4 billion years ago, a day lasted only 18 hours. More recently, 70 million years ago, a day was around half an hour shorter than it is today. Overall, the evidence suggests Earth gains about 1.8 milliseconds per century.

The Great Oxidation Event and why timing matters

The second element is the Great Oxidation Event: a period when cyanobacteria became abundant enough that oxygen in Earth’s atmosphere rose sharply and substantially.

Scientists consider this oxygenation pivotal. Without it, life as we recognise it may never have developed - meaning that even if cyanobacteria sometimes attract sceptical looks today, our existence may depend on their ancient activity.

Yet the Great Oxidation Event still leaves major questions open, including why it unfolded when it did rather than earlier in Earth’s history.

Microbial mats in Lake Huron: a living analogue for early Earth

Researchers turned to modern cyanobacterial communities to help bridge that gap. At the Middle Island Sinkhole in Lake Huron, microbial mats exist that are thought to resemble - in key ways - the cyanobacteria involved in the Great Oxidation Event.

On the lakebed, purple cyanobacteria that release oxygen through photosynthesis share the mat with white microbes that process sulphur, and the two groups compete for position.

At night, the white microbes move up to the surface layer to carry out their sulphur-based metabolism. When daylight arrives and the Sun climbs high enough, they withdraw and the purple cyanobacteria take over the top layer.

"Now they can start to photosynthesize and produce oxygen," said geomicrobiologist Judith Klatt of the Max Planck Institute for Marine Microbiology in Germany.

"However, it takes a few hours before they really get going, there is a long lag in the morning. The cyanobacteria are rather late risers than morning persons, it seems."

That delay matters: it means the part of the day when cyanobacteria can actively produce oxygen is narrower than you might assume. This observation caught the interest of oceanographer Brian Arbic of the University of Michigan, who questioned whether changes in day length across Earth’s history could have influenced photosynthesis-driven oxygen output.

"It's possible that a similar type of competition between microbes contributed to the delay in oxygen production on the early Earth," Klatt explained.

Testing the hypothesis: experiments, field measurements and modelling

To explore the idea, the team combined multiple approaches. They took measurements and ran experiments on the microbes in both their natural setting and in the laboratory. Using those results, they carried out detailed modelling designed to connect incoming sunlight to microbial oxygen production - and then relate microbial oxygen production to Earth’s deep-time history.

"An enduring question in Earth sciences has been how did Earth's atmosphere get its oxygen, and what factors controlled when this oxygenation took place," microbiologist Gregory Dick of the University of Michigan explained in 2021.

"Our research suggests that the rate at which Earth is spinning – in other words, its day length – may have had an important effect on the pattern and timing of Earth's oxygenation."

One might expect day length not to matter much: split the same total daylight into shorter chunks, and the outcome should be similar. But the researchers found a critical complication in how oxygen escapes from bacterial mats.

"Intuition suggests that two 12-hour days should be similar to one 24-hour day. The sunlight rises and falls twice as fast, and the oxygen production follows in lockstep," explained marine scientist Arjun Chennu of the Leibniz Centre for Tropical Marine Research in Germany.

"But the release of oxygen from bacterial mats does not, because it is limited by the speed of molecular diffusion. This subtle uncoupling of oxygen release from sunlight is at the heart of the mechanism."

Longer days and oxygen rise - beyond the Great Oxidation Event

After folding these findings into global oxygen models, the team concluded that increasing day length aligns with rising oxygen levels in Earth’s atmosphere. The association was not limited to the Great Oxidation Event; it also appeared relevant to a later increase known as the Neoproterozoic Oxygenation Event, dated to roughly 550 to 800 million years ago.

"We tie together laws of physics operating at vastly different scales, from molecular diffusion to planetary mechanics. We show that there is a fundamental link between day length and how much oxygen can be released by ground-dwelling microbes," Chennu said.

"It's pretty exciting. This way we link the dance of the molecules in the microbial mat to the dance of our planet and its Moon."

The research was published in Nature Geoscience.

An earlier version of this article appeared in August 2021.