Global warming is slowing the earth’s rotation.

As rising global temperatures melt Earth’s polar ice sheets, the shifting water is creating such a huge redistribution of our planet’s mass that its rotation speed is dropping. This unusual result of climate change is interacting with other forces that affect the planet’s rotational speed in ways that could ultimately even alter the way we keep time. In just a few years, we may have to make the first-ever deletion of a “leap second”—according to a new study published on Wednesday in Nature.

“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.

The discovery comes with some startling implications for timekeeping. Most of the world uses Coordinated Universal Time (UTC) to regulate clocks and time. Because measuring techniques have grown vastly more precise in the past few decades, a leap second has occasionally been added to UTC to compensate for the slowing of Earth’s rotation, which is linked to various other factors. For example, the gravitational pull of both the sun and moon create a tidal bulge in the oceans that acts to slow the planet’s rotation.

When a leap second is added, the last minute of a designated day extends to 61 seconds, with the additional second labeled as 23:59:60. This maintains alignment between civil time, based on Earth’s rotation relative to the sun and the standard time used for daily life, and the much more precise atomic time.

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. 

Source: www.scientificamerican.com

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