If you ever feel like there isn’t enough time in the day, here’s some good news. Days on Earth are actually getting slightly longer.
Well, it’s probably not good news at all. Because the change is imperceptible in daily life, and the cause — is climate change.
To learn how current sea-level rise is slowing down Earth’s rotation, I spoke with Walter Munk, a 98-year-old scientist The New York Times recently called "the Einstein of the oceans."
Munk's work on the link between sea levels and the speed of Earth's rotation has inspired other scientists to dig deeper into the topic. Recently, a team of researchers built on Munk's work to prove that climate change is in fact driving a tiny, but measurable, change in the length of day.
The 'Einstein of the oceans'
When I showed up to Munk's seaside home in La Jolla for an interview, I found him at his desk overlooking the Pacific Ocean, busy working on new scientific papers.
"Yeah, I’m working on a problem that has been puzzling for a hundred years," he said.
The Austrian-born scientist began his career at the Scripps Institution of Oceanography at the dawn of World War II. As a budding scientist, he tackled the challenge of wave prediction. That early work ended up helping to guide amphibious landings for the invasion of Normandy.
By 1960, Munk had expanded his area of expertise, co-writing a book called "The Rotation of the Earth."
"It was not a best seller," he recalled.
The academic text may not have earned him much money. But Munk said he wrote it after becoming fascinated with the Earth’s rotation and the clues it provides about so many other things happening on the planet.
"I think that the Earth is telling us a story that we should be able to interpret," he said.
How climate change makes days longer
One story told by Earth’s rotation is the story of sea-level rise. The speed of Earth’s rotation is actually not fixed at 24 hours. It can change, and has changed in the past, depending on where Earth holds its mass.
When ice melts at the poles, Earth's mass gets redistributed. Water moves toward the equator when it melts, and that outward expansion slows down Earth's rotation, kind of like how a ballerina’s pirouette slows down when she extends her arms.
“The Earth adjusts itself," Munk said.
When the Earth adjusts to sea-level rise, its spin rate gets slower. It may sound surprising, but it’s true. Sea-level rise caused by man-made global warming is actually making days on Earth longer.
But only slightly longer — it all adds up to about about 6 millionths of a second each year.
Obviously, this minute change is not the most concerning thing about sea-level rise. Shrinking coastlines in heavily populated areas will present a much bigger problem for humanity.
But Munk said scientists pay attention to the speed of Earth's rotation because it's an objective way to confirm their understanding of current sea-level rise.
"It doesn’t bother you in your daily life, but it’s a nice way to learn something," Munk said.
It turns out lots of things affect Earth’s rotation. Like energy released by ocean waves crashing on shorelines, and the ongoing mass redistribution caused by the end of the last ice age.
Munk knew to account for these additional factors. But when he tried to add everything up back in 2002, he noticed something strange. Earth didn’t appear to be slowing down as much as it should.
Munk knew climate change had caused sea levels to rise during the 20th century. But that wasn't that showing up in current measurements of the Earth's rotation.
Munk's argument was convincing, scientists say, because he supported his claims with a highly accurate model for understanding past changes in the speed of Earth's rotation.
Munk could predict exactly when and where ancient civilizations had recorded solar eclipses, predictions that could only work with a detailed understanding of how the speed of Earth's rotation has changed over time.
Looking backward in time, Munk's model worked perfectly. But he couldn't get it to agree with contemporary data on Earth's rotation. The contribution of 20th century sea-level rise was nowhere to be found.
Fourteen years ago, he published a paper about this problem, which became known as Munk’s Enigma.
"Sea level is important as a metric for climate change as well as in its own right," Munk wrote at the end of the paper, published when he was 84. "We are in the uncomfortable position of extrapolating into the next century without understanding the last.
Resolving Munk's Enigma
“Munk’s paper could not have been written by anyone else but Munk," said Harvard University geophysicist Jerry Mitrovica. Munk was issuing a challenge to the scientific community, Mitrovica says — a challenge he decided to take up.
“He didn’t say you’re all wrong, ice sheets aren’t melting," Mitrovica said. "He’s saying there are big problems here and these need to be resolved."
Recently, Mitrovica published a paper about resolving Munk’s Enigma. He and his colleagues say Munk’s calculations were a bit off due to three highly technical limitations.
First, Munk relied on estimates of 20th century sea level rise that have been revised in recent years. Sea levels did rise in the 20th century, but not as much as scientists once thought.
Second, the ice age model Munk used also had some shortcomings. It too has been tweaked since Munk put forward his enigma.
And, third, Mitrovica said Munk’s original calculations didn’t account for some complicated physics involving the Earth's core and mantle that also affect Earth's rotation.
With these three changes, Mitrovica said the numbers now do add up — recent sea-level rise has indeed slowed down the Earth’s rotation in an observable way.
“To resolve this enigma required a decade of work and progress in three very different parts of Earth science," Mitrovica said.
Mitrovica said Munk was right to point out these problems when he did.
"Munk is no climate change denier," he said. Instead, Munk was raising issues that forced scientists to refine their models, ultimately giving them a more precise understanding of the effects of climate change.
"If there’s a problem in Earth rotation you better solve it," Mitrovica said. "Because that produces a really fundamental inconsistency in your argument that you’re melting glaciers.”
Munk said he’s glad to see scientists build on his work and prove that days are in fact getting measurably longer due to climate change.
“It’s very satisfying that work like this, when you find things that did not add up, inspired people in subsequent years to do some additional work, to try to understand it," Munk said. "And in some instances, to succeed. I mean, that’s the way science should work.”
At 98, Munk is still trying to sort out other tough issues in oceanography. He says there are plenty of other problems scientists in his field have yet to solve.
