Why melting ice sheets are making our days longer

Think humans play a relatively small role in how Earth moves in space? It turns out we’re changing how our very planet rotates - and it’s affecting the length of our days.

For billions of years, Earth’s movements - how it spins and at what speed - have been primarily determined by forces beyond human influence, such as the pull of the moon and processes at our core and mantle. Now, the melting of Earth’s ice sheets, accelerated by human-driven warming, is influencing those motions.

The melting of Earth’s large ice masses has made days ever so slightly longer in recent decades and shifted Earth’s axis of rotation, according to two recent studies. The lengthening and shifts may be imperceptible to humans, but they can affect the accuracy of navigation systems or throw off calculations for satellite launches and landings. This extra slowing of Earth’s rotation is likely to stay for at least the next few decades, even if humans’ effect on the climate slows.

“Climate change is melting so much ice that we can see a huge impact on the very way how the planet is spinning,” said Surendra Adhikari, a geophysicist at NASA Jet Propulsion Laboratory and an author of the two papers.

Days are getting longer

As temperatures rise globally, Earth’s polar regions have felt the brunt of the heat added since the 20th century. The melting of the Greenland and Antarctic ice sheets account for nearly one-third of global sea level rise since the early 1990s. But the melting is not affecting just sea levels.

As the polar ice melts, the water moves from the poles toward the equator - making our Earth rotate slower as it gets bulkier. Think of a figure skater who spins slower when her arms are stretched out compared to tucked into her body. The same applies to Earth’s rotation, said Benedikt Soja, a co-author and professor at ETH Zürich.

Soja and the team showed in findings published in the Proceedings of the National Academy of Sciences journal on Monday that recent, rapid melting of Greenland and Antarctic ice has increased the length of our days. Using past observations and projection models, they found the ice loss added time to Earth’s day between 0.3 to 1 milliseconds per century through the 20th century. But since 2000, the rate has accelerated to 1.33 milliseconds per century.

For billions of years, the speed of our planet’s rotation has dominantly been influenced by our moon. The moon yanks on the planet’s oceans and causes the tides to bulge, creating drag and slowing down Earth’s spin. Earth’s rotation has been predictably and consistently slowing down because of the moon’s gravitational forces - around 2.40 milliseconds per century, according to the study’s authors.

But the study “shows what we as humans can really impact in terms of changing Earth’s behavior and dynamics.”

Some scientists were not surprised by the study’s link to climate change. Richard Peltier, a physicist at the University of Toronto, published a study more than a decade ago stating “the changes in Earth rotation documented were caused by the global warming process.” Another recent study showed how this climate induced-day lengthening is affecting our timekeeping and delaying the leap second.

But the new research shows climate change is a bigger influence than previously thought and projects how Earth’s rotation may continue to change, if global warming worsens. By 2100, the scientists involved in the study estimated that the melting ice could lengthen days to 2.62 milliseconds per century, if greenhouse gas emissions are not drastically reduced. Even if emissions are curbed, Earth’s length of day is projected to lengthen by about 1.00 milliseconds per century for the next several decades.

The slowdown from melting ice “could become the new dominant factor, surpassing the moon, which for billions of years shaped the Earth’s rotation,” Soja said.

A few milliseconds here or there might not sound like a big deal, but it can add hours over billions of years. Perhaps more relevant to our lifetimes: These millisecond changes can also affect current technologies.

Our GPS and navigation systems rely on this precision “to the level of milliseconds, otherwise we would make very big errors in positioning and navigation,” Soja said.

Earth’s axis of rotation is shifting

Our planet isn’t just changing how fast it’s spinning, but also where it rotates.

Earth spins on an imaginary line from the North and South poles, but the line isn’t fixed. The points where the axis of rotation meets Earth’s surface drifts and wobbles a few inches per year and several meters every century.

Now, Soja, Adhikari and their team, were able to determine what factors have influenced these “polar motions” in unprecedented detail over the past 120 years. They found that the ice mass loss from the North and South poles play a role in these wiggles, according to another recent study published in Nature Geoscience.

“We could see that climate change, what happens on the surface, basically can also have a small impact on the dynamics that happen in the very interior of the Earth, like the Earth’s core,” Soja said.

Scientists have long known some of the natural processes that can induce mass changes on and within the planet, and thus affecting its axis of rotation. For instance, convection currents deep within the mantle can move tectonic plates on the surface. Heat flows in Earth’s outer core, which help generate the Earth’s magnetic field, also can shift mass.

Now, add climate change.

As ice is lost from the poles, the mass loss can throw off Earth’s geometry and have a bigger impact on shifting Earth’s spin axis. In fact, the new study found that such mass redistribution on the Earth’s surface, including ice melting and global changes in water storage, accounted for 90 percent of variations between years and decades.

Mostafa Kiani Shahvandi, lead author of both the Earth axis and day of length studies, said these surface processes, like the melting of ice, can move the axis more than processes inside Earth.

Adhikari said people haven’t thoroughly investigated the influence of climate change on polar motion, and even this is a starting point, adding: “We are opening questions for future research.”