Climate change is slowing Earth's rotation

And could play havoc with our timekeeping
28 March 2024

Interview with 

Duncan Agnew, University of California, San Diego


A floating iceberg.


The news is awash with the worsening effects of global climate change. But the stories reported are usually centred around the human and ecological side of the ongoing crisis. Now, however, a new study published in the journal Nature has suggested that the melting poles might cause a shift in our timekeeping too, by making Earth fatter around the middle. Duncan Agnew is from the University of California, San Diego…

Duncan - The leap second seems very peculiar. The problem is that the timescale that we all use is doing two things. It is matching atomic clocks, which tell very accurate time, and it is also trying to match the rotation of the Earth. And those don't give you exactly the same thing. A day on rotation of the Earth is not the same as a day from atomic clocks. And so just as if one clock runs slower than another, the difference adds up. The difference between these two measures of time adds up and every so often it gets to be as much as a second, which is a lot for some people. And then an additional second is added to a minute in order to bring the two times back into agreement.

Chris - Why does the world worry about such a tiny amount of time as a second every so often? Why does it matter?

Duncan - The difficulty with leap seconds is that all the computers in the world need to know about them or they're not synchronised. It's exactly the problem you have when you go to summertime and you've forgotten that everybody's gone to summertime and then you discover you're late to everything. Now a second doesn't sound like much, but for example, the standard for giving times of stock trades is a millisecond, a thousandth of a second. So that kind of very precise timing is actually very common. We just don't notice it.

Chris - And so if it goes off whack, we could end up with fraudulent transactions, errors in our transactions, people taking out money from ATMs when they didn't, and so on. So it would amount to a financial disaster and all kinds of problems.

Duncan - Right. And the financial system, I think, knows about this and deals with it well, but you never know what could go wrong somewhere else if somebody hasn't synchronised their computer and every time there's a leap second, some people don't do it when they should. Some people actually put a second in the wrong direction. There's plenty of scope for human error.

Chris - So your project started as a consideration of time and this concept of a leap second, but where did you take it next? Where was your thinking going?

Duncan - The reason for leap seconds, as I said, is the difference between atomic clocks and the time as viewed as the rotation of the earth goes around once a day. And so I came at this as a geophysicist who studies how the rate of Earth rotation varies. It does not change very much, but over time it changes a lot. In geologic time, hundreds of millions of years ago, the day was much shorter. There were 400 days in a year. Currently there are changes. They are small, but they're the thing that gives you leap seconds. So I looked at causes of changes in Earth rotation

Chris - And way back in history, then, why was the Earth spinning faster? Where has the energy gone?

Duncan - Basically, this is because of the tides. So the Moon and the Sun, gravitational attraction causes tides in the ocean. The simplest way to put it is that the motion of the water has some friction in it and exerts a frictional force on the solid Earth. And so it's like putting a brake on something. And so it's called tidal friction and it slows the Earth down. It's been slowing the Earth down since the earth had oceans. And over the long run, that's the biggest change in rotation.

Chris - But as anyone who's ever ridden a fairground ride knows, that also the rate at which something rotates is down to where you put the mass, the weights, on it. So will the redistribution of things like water around the Earth also have an impact? Because of course, that has changed over the time that the Earth's been around.

Duncan - Yes, that's exactly right. If you have something spinning and it changes shape, then it will spin faster or slower. The usual example, and there are plenty of YouTube videos of this, is ice skaters who start to spin, pull their legs and arms in and start spinning faster in the case of the Earth. What's been going on particularly recently is a lot of melting of ice in Greenland and Alaska. That ice flows into the ocean and is spread out over the entire Earth. And so effectively that's the opposite thing where the skater moves her arms out, the water has moved from near the pole too far away from the pole, and that's slowing the Earth down. So it's acting in the same direction as tidal friction.

Chris - How did you actually do this to be able to say to me what you just said?

Duncan - The way we know about this is partly from studies of melting ice and sea level, but this change of mass, of water moving from being frozen at the poles to all over the ocean, changes the Earth's gravitational field, and that's measured by satellites. That's been measured since 1976. And there was a long term trend in it that people understood. And starting around 1990, that trend slowed down and now it's reversed. So I used the gravitational field measurements from satellites as a way to get at what water distribution had occurred and how that would change rotation.

Chris - And how much of a difference is this making already and how much of a difference might it make if we see climate change play out the way that we think it will?

Duncan - The difference already is not large, it's now comparable to tidal friction. This didn't exist 30 years ago. The amount of slowing from tidal friction was what it was. It's still the same. In the meantime, ice melting is now slowing the Earth as much as tidal friction does, and we expect that to go up. Now, this is a relatively small change. It's not the catastrophic part of global warming. There are plenty of other things that are catastrophic about it, but it's a change we could see. And to me, one of the impressive results of this paper is the realisation that global warming is causing the entire Earth to slow down. It's not just some local effect that we could actually see in our very precise measurements, another sign of the fact that global warming is causing unprecedented things to happen.


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