NASA
Earth's rotation will soon be affected more by global warming than by the Moon's gravity
Climate – Imagine a boiled egg and spin it on the table: this is the Earth until the 20th century. Replace it with a raw egg, and try to spin it: this is our planet today. And with a (slight) hint of summer exaggeration, the picture will help you understand why the rotation of the blue planet is slowing down more and more.
This surprising conclusion is the calculation of an American team, led by the Jet Propulsion Laboratory, the research arm of the US space agency (NASA), Published in PNAS On July 15, 2024. To understand how the team got here, you really need to know two things:
- First, the length of the day here is not the length of a sunny day (which obviously varies with the seasons). This is the time it takes for the sun to reach its zenith twice in our sky, or the time it takes for our Earth to rotate completely on its axis..
- The second thing to know: This period has been increasing for billions of years, but For 20,000 years, it was balanced by an opposite effect. Rotation accelerator.
Earth is slowing down (but slowing down for 20,000 years)
As you know, the Moon exerts a gravitational pull on the Earth, causing tides. This movement of ocean water slows the Earth's rotation, ever so slightly, lengthening days by about 2.4 milliseconds per century.
But over the course of 20,000 years, this slowdown has been very slightly slowed down. The end of the Paleolithic period marks the end of the last glacial maximum. Since then, the Earth has been slowly warming up: a boon for Homo sapiens, but also for the poles, which have regained their splendor. As the weather cools, the ice at the equator melts, and the water cycle combined with the movement of the Earth’s plates does the rest, redistributing the water towards Antarctica and the Arctic, where it freezes again: the poles regain their strength, and the mass of the Earth moves there again.
As a result of this new line, the Earth rotates a little (very) little faster from year to year, and from century to century. Researchers call this “isostatic glacial adjustment.” As a result, a day lasts a little shorter, by 0.8 milliseconds (milliseconds) per month per century, according to our astrophysicists’ calculations. Think of the hard-boiled egg and the raw egg: fluids, their motions and especially their inertia, affect the body they occupy.
Thus, the end of the glacial maximum (by -0.8 milliseconds per century) slowed the moon-induced lengthening of days for billions of years (by +2.4 milliseconds per century), meaning that over the past 20,000 years, days have lengthened by about 1.6 milliseconds per century (the difference between 2.4 and 0.8).
The Earth's temperature has risen
You follow? Once you've done that, the rest is much simpler. Let's just say it plainly: global warming is shaking everything up. When we understand the previous paragraphs, it makes a lot of sense to imagine that rising temperatures are affecting the Earth's rotation.
Because by melting, the ice of the poles (and Greenland) produces the opposite effect of the glacial isostatic adjustment, and thus tends to slow the rotation of the Earth. But by how much? To find out, the JPL team used all available climate data from 1800 to the present day.
The result is clear. In the first half of the twentieth centuryexplains to HuffPost Geologist Surendra Adhikari Things are starting to get a little erratic, due to El Nino. […] But in the past 20 years, global warming's contribution to longer days has peaked.
The human mark is stronger than billions of years
In the 20th century, global warming also began to affect the length of the days (between +0.3 and +1.0 milliseconds/century, the study tells us), but in the 21st century everything is coming to a head. The effect of melting ice, by creating massive water movements, amounts to +1.33 milliseconds/century of day lengthening: a figure that, according to the team of researchers, should increase significantly.
The study therefore estimates that if the concentration of carbon dioxide in the atmosphere continues to increase, causing the poles to melt ever more, the warming effect alone could reach and exceed +2.4 milliseconds per century, exceeding the greenhouse effect. The moon’s gravity and the resulting tidal force. This, apart from the complex calculations to measure the equivalent of the blink of an eye every century, is what justifies this study and its conclusions.
Indeed, even if you have only followed the adventures of the Earth’s rotation with one eye, this is what you should remember: for the first time in the geological history of our planet, the tides will be the first brake on the Earth’s rotation, but on the melting of the poles. This is nothing less than an incalculable symbol of the way humans are redesigning their environment, often unwillingly. Even if it means disrupting a dynamic that has been at work for several billion years.
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