Earth’s Rotation Slowing: Climate Change’s Unseen Impact on Day Length
It might feel like the days are dragging on more than usual, and according to a new study, there’s a scientific reason behind it. Researchers from the University of Vienna and ETH Zurich have discovered that the length of our days is increasing at a rate that scientists are calling “unprecedented.”
The Earth’s rotation is slowing down, causing days to lengthen by approximately 1.33 milliseconds per century. This rate of change is faster than anything observed in the last 3.6 million years of our planet’s history. The culprit? Climate change.
The Figure Skater Effect: How Melting Ice Slows Earth’s Spin
The phenomenon can be understood by analogy to a figure skater. When a skater extends their arms, they slow down their spin. Similarly, as global warming accelerates the melting of polar ice sheets and glaciers, the resulting rise in sea levels is effectively pushing mass further away from Earth’s axis of rotation.
Professor Benedikt Soja of the University of Vienna explained that while natural cycles have always caused variations in Earth’s rotation throughout history, the current rate of change, driven by human activity, is exceptionally rapid. “Only one time – around 2 million years ago – the rate of change in length of day was nearly comparable, but never before or after that has the planetary ‘figure skater’ raised her arms and sea–levels so quickly as in 2000 to 2020,” he stated.
It’s important to note that Earth’s days have never been precisely 24 hours long. Various natural factors, including the Moon’s gravitational pull, processes within the Earth’s mantle, and atmospheric changes, have always caused minute fluctuations in rotation speed. These are generally considered part of the planet’s natural cycles.
However, previous research, including work by Professor Soja and his colleagues, has indicated that human actions are now influencing Earth’s rotation as much, if not more, than these natural factors. By redistributing mass across the globe through melting ice and rising sea levels, we are increasing the planet’s moment of inertia. This increased inertia directly leads to a slower spin and, consequently, longer days.
Subtle Changes, Significant Consequences
While these changes are measured in milliseconds and are not noticeable to the naked eye, they could pose serious challenges for systems that rely on extremely precise timekeeping.
Professor Soja highlighted the potential impact: “Even though the changes are only milliseconds, they can disrupt systems that require extremely precise time keeping. This includes space navigation, GPS and satellite navigation systems, and synchronisation of atomic clocks, which makes it so important to precisely monitor the Earth’s rotation.”
As climate change continues to intensify, the researchers anticipate that this deceleration of Earth’s rotation will only become more pronounced.
The Science Behind Longer Days
The speed of Earth’s rotation is governed by the law of the conservation of angular momentum. Imagine our planet as a giant spinning top.
- Mass Closer to the Axis: When the Earth’s mass is concentrated closer to its axis of rotation, its moment of inertia is lower, resulting in a faster spin.
- Mass Further from the Axis: Conversely, when mass is distributed further away from the axis, the moment of inertia increases, causing the planet’s spin to slow down.
Currently, significant amounts of mass are stored at the poles in the form of ice sheets and glaciers. Climate change is causing these ice reserves to melt, with the water flowing towards the equator. This redistribution of mass moves it away from the axis of rotation, leading to a slower spin and, therefore, longer days.
Projections suggest that by the end of the 21st century, climate change’s influence on day length could surpass that of the Moon’s gravitational pull. This could mean an increase in day length by as much as 2.62 milliseconds per century, starting from the 2080s.
Unlocking Earth’s Past to Understand Its Future
Scientists have a remarkable tool for understanding these long-term changes: the fossilised remains of ancient marine organisms. By examining the tiny shells of a single-celled organism called benthic foraminifera, researchers can glean insights into past environmental conditions, including sea levels, stretching back millions of years.
Dr. Soja elaborated on this method: “From the chemical composition of the fossilised shells of these marine organisms, it is possible to trace past sea–level fluctuations. Based on this information on past sea level, we developed a physics–informed machine learning model to derive the corresponding changes in day length over the last 3.6 million years.”
This analysis confirmed that natural fluctuations in sea levels have indeed altered Earth’s rotation in the past.
Over geological timescales, the waxing and waning of vast ice sheets would push mass towards and away from the equator, thereby slowing and accelerating Earth’s spin. However, the study reveals that no natural event in the last 3.6 million years has impacted our planet’s rotation as rapidly as human-induced greenhouse gas emissions.
The most significant natural periods of rapid change previously identified occurred around two million years ago, a time when exceptionally high CO2 levels meant Greenland was ice-free and forested. Yet, even during this era of intense natural climate shifts, Earth’s rotation did not slow down as dramatically as it has in just the last 25 years. This underscores the profound and accelerating impact of human activity on our planet’s fundamental rhythms.
