Earth Was Mysteriously Thrown Off-Kilter In 2015. Now, Scientists Think They Know Why.

Have you noticed anything out of whack about Earth since 2015? I’m speaking, of course, about how our planet’s wobble started shrinking that year, a mysterious shift that scientists have been puzzling over ever since.

Now, researchers think they might have an explanation for the sudden decrease in the Chandler wobble (CW), a deviation in Earth’s rotational axis relative to its crust that causes a drift of about 20 feet over a cycle of roughly 14 months. The recent off-kilter wobble, known as the CW reduction event, may have been largely sparked by “mass anomalies” after the La Niña of 2010–2011, according to a new study published in Geophysical Research Letters.

That particular La Niña event was one of the strongest on record and it was “followed by significant ocean mass loss due to changed precipitation and evaporation patterns, providing a possible cause of the CW reduction event,” reports the study.

“Polar motion, like the Chandler wobble, reflects changes in Earth’s overall angular momentum,”  said study authors Taehwan Jeon and Ki-Weon Seo, a geophysicist and associate professor at Seoul National University, respectively, in a joint email to 404 Media. “Changes in regional mass, and velocity fields of ocean currents and winds can affect the wobble’s amplitude. Because the wobble sums up effects from all over the globe, it is usually hard to tell exactly which region contributed how much.”

“Still, given the scale of the phenomenon, it makes sense that global climate events such as ENSO (El Niño Southern Oscillation) have a stronger influence than small local changes,” the pair continued. “Not every El Niño or La Niña does this, but as our study shows, the 2010-2011 La Niña produced an unusually strong anomaly when viewed from the perspective of Earth’s polar motion.”

The Chandler wobble is one of many wandering deviations between the axis and crust; for instance, we recently covered the effect of impounded dam water on Earth’s rotation. The wobble has also increased and decreased many times in the past in response to shifting global mass distributions, so the latest anomaly isn’t unprecedented. 

Still, scientists are curious what might be driving the recent CW reduction, which reached its peak intensity between 2015 and 2020. To approach this mystery, Jeon, Seo, and their colleagues broke the wobble up into two components: the forced wobble and the free wobble.

“The Chandler wobble is actually a two-dimensional pendulum-like motion, but for simplicity it can be compared to a one-dimensional swing,” explained Jeon and Seo. “If you let a swing move without pushing it, it will eventually slow down and stop. That ‘natural’ motion without any external force is what we call the ‘free wobble.’”

“Now imagine giving the swing a push at just the right timing,” they continued. “The swing will keep moving. Depending on how you push, its amplitude can increase or decrease. On Earth, all moving masses (such as air, oceans and water on land) act like those pushes. The part of the wobble driven by these pushes is called the ‘forced wobble.’”

Because the Chandler wobble is the sum of these two parts, the reduction is caused by the free and forced phases cancelling each other out, according to the team’s models. In other words, the study showed that the strong 2010-2011 La Niña event drove the forced wobble out of phase, allowing it to interfere with the free wobble and reduce the overall CW amplitude. 

Jeon and Seo said their results “were partly expected and partly surprising.” On the one hand, they noted that oscillations naturally decay over time, so they were expecting to see only recent changes reflected in the Chandler wobble, with ENSO events of the past few decades playing an outsized role in those shifts.

“What surprised us was that not every ENSO event seems to matter, and in particular, the 2010-2011 La Niña turned out to be the strongest contributor,” the pair said. “That was not something we had fully predicted before doing the analysis.”

With that in mind, it’s possible that the Chandler wobble will continue to go haywire in the coming decades, as ENSO events are being amplified by human-driven climate change. 

“Large-scale and systematic shifts in Earth’s mass and motion can strongly affect the Chandler wobble, especially when they show a cycle close to the Chandler period (about 433 days), which can resonate more strongly,” Jeon and Seo said. 

In particular, the pair pointed to how much ENSO events can disrupt rainfall patterns, which ended up being a main factor in how the 2010–2011 La Niña anomalies impacted on the CW reduction. Changes in global ice mass, however, have more influence over the long-term drift of Earth’s rotation axis, and don’t influence the short-term Chandler wobble as much.

“Since 2010-2011, mass and velocity field changes have continued around the globe, and separating their individual effects remains very challenging,” Jeon and Seo said. “Still, because the Chandler wobble’s amplitude has been increasing again since late 2020, we expect that it may soon return to levels comparable to those observed before 2010.”

“Although the amplitude drop during 2015-2020 was unusual, there were also earlier periods when the Chandler wobble was decreased or increased in amplitude,” the pair concluded. “We suspect that major ENSO events may have played a similar role during those times as well. A next step would be to investigate the broader patterns—what kinds of ENSO events tend to leave a mark on Earth’s polar motion, and what features make them most influential.”