Warming Is Measurably Changing The Speed Of Earth's Rotation

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Timekeeping has traditionally had an astronomical basis. Earth is a type of a clock. In simpler times, the planet would spin one full revolution on its axis, and everyone would call it a day. Technologists, however, demand excruciating levels of exactitude. Atomic clocks and not sundials now tell us what time it is. In atomic time, a second is defined as 9,192,631,770 oscillations of a Cesium atom. The goal of the people who want to get things exactly right is to make sure that atomic time is perfectly in sync with astronomical time.

For example, GPS satellites need to know exactly where Earth is beneath them — and precisely what time it is — to accurately get you from your house to the nearest Arby’s. But Earth doesn’t spin at a perfectly constant speed. Our planet is in a complicated gravitational dance with the moon, the sun, the oceanic tides, Earth’s own atmosphere and the motion of the planet’s solid inner core.

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The planet’s fluctuating spin rate is carefully tracked by the International Earth Rotation and Reference Systems Service (formerly the only slightly less bombastically named International Earth Rotation Service). In the early 1970s, Earth was clearly slowing down in its rotation, and a gap was forming between atomic time and astronomical time. Thus was born the “leap second” to adjust for the fact that the “day” was getting a bit longer. Twenty-seven leap seconds have been added to Universal Coordinated Time since 1972. The addition of a leap second happens at the last tick of the clock on the night of Dec. 31. But hold on: Earth is not slowing down anymore. It’s actually been speeding up a bit. In fact, there hasn’t been a leap second added since the end of 2016.

Here is where the casual reader’s head might start spinning. The melting of the ice caps in Antarctica and Greenland shifts mass — meltwater — toward the equator. That process increases the equatorial bulge of the planet. Meanwhile, at the poles, the land that had been pressed down by ice rises, and Earth becomes more spherical. These two changes in the planet’s shape have opposite effects on Earth’s rotation, according to Judah Levine, a physicist at the National Institute of Standards and Technology in Boulder, Colo. The new paper by Agnew contends that, although the core is causing the planet to spin faster, the planetary shape changes caused by a warming climate are slowing that process. Absent this effect, Agnew wrote, the overall acceleration of the planet’s rotation might require timekeepers to insert a “negative leap second” at the end of 2026. Because of climate change, that might not be necessary until 2029, he found.

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https://www.washingtonpost.com/science/2024/03/27/leap-second-melting-poles-climate-time/