"This is another one of those 'this has never happened before' things that we're seeing from global warming: the idea that this effect is large enough to change the rotation of the entire Earth," says study co-author Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography.
The mass of the miles-thick ice sheets that cover Greenland and Antarctica exerts a strong gravitational pull on the oceans. As the ice sheets melt, all that mass shifts away from the poles and toward the equator, reducing that pull and also causing Earth's rotation to slow down. To understand why this happens, picture a figure skater gracefully spinning on the ice with their arms tucked tightly around their head. As they gradually lower their arms and extend those limbs outward, their spin slows down. [...]
What Agnew found is that the slowdown caused by polar ice melt has been effectively masking a speedup of Earth's rotation caused by changes in the rotation of our liquid outer core. Over the past 50 years, a day has become about 0.0025 second shorter. If global warming never happened, we would likely have needed to subtract a leap second sooner. But with the influence of warming, Agnew estimates, we will need to do so by around 2028 or 2029, although he admits his prediction is uncertain. "There's never been a negative leap second before, and leap seconds themselves have always been a problem for people running computer networks," Agnew says, given that many crucial systems rely on precise timekeeping. "Having to include a negative leap second would be a bigger problem because they've never had to do it."
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