研究：抽取地下水改变了地球自转方式

A recent study says that pumping massive amounts of groundwater to the surface can affect the position of Earth's poles.

The poles are at either end of the imaginary line running through the Earth. The turning motion of Earth around this line causes day and night. The tilt of the line, in relation to the sun, causes the seasons.

Scientists have known for more than 100 years that the Earth's poles change position a little over time with what is called polar motion.

A group of international scientists recently published their study of polar motion. Earlier studies linked the way water is spread out over the Earth to polar motion. The scientists wanted to see if the movement of groundwater could help explain differences between observed and predicted polar motion.

The researchers used a climate computer model to estimate that humans pumped a total of 2,150 gigatons of groundwater to the Earth's surface from 1993 to 2010. One gigaton is equal to about 1 billion metric tons, the American space agency NASA notes. The model estimated that world sea levels rose six millimeters because of the additional groundwater.

Groundwater pumped to the surface is used for crops and drinking water. It is an important resource to support human life. The recent study suggests that the movement of groundwater to the oceans can, in a way, affect Earth's rotation and might also add to sea level rise.

A 2016 study first showed evidence that ocean water flows can affect polar motion. But the latest study is the first to explore in depth how groundwater brought to the surface might change Earth's tilt.

The researchers created their climate model to study polar motion. Described another way, polar motion is when the position of the planet's rotational axis moves in relation to Earth's crust.

Water movements on Earth can cause the planet to rotate "a little differently as water is moved around," the researchers said in a statement.

Ki-Weon Seo is a geophysicist at Seoul National University who led the study. It recently appeared in the publication Geophysical Research Letters.

Seo said, "Earth's rotational pole actually changes a lot." He added that the study shows "that among climate-related causes, the redistribution of groundwater actually has the largest impact on the drift of the rotational pole."

The researchers depended on data collected by satellites over nearly 20 years. They used their computer model to estimate what kinds of water redistribution caused a notable change in the Earth's tilt. The team's main finding was that the Earth tilted nearly 80 centimeters towards the 64 degrees east longitude line between 1993 and 2010.

The study notes that where the groundwater is pumped from affects how much polar motion happens. The biggest changes were recorded in mid-latitude areas: areas not too close to the North Pole and not too near the equator. During the study period, the most water was redistributed in western North America and northwestern India. Both of these are in mid-latitude areas.

Seo noted that the rotational pole changes caused by groundwater are too small to affect Earth's seasons. He said that, in theory, making changes to slow the rate of groundwater removal could affect Earth's polar motion. But major changes would likely only be seen if such efforts were highly organized and continued for many years.

"I'm very glad to find the unexplained cause of the rotation pole drift," Seo said. "On the other hand, as a resident of Earth and a father, I'm concerned and surprised to see that pumping groundwater is another source of sea-level rise."

Seo noted that the latest study could mark the start of additional research into how water flow changes affected different, specific areas of the world. He said the same rotational pole data could be used to learn about possible causes of redistributed water over the past 100 years. "Were there any...changes resulting from the warming climate? Polar motion could hold the answer," he added.

Surendra Adhikari is a research scientist at NASA's Jet Propulsion Laboratory. He was not involved in this study. In a statement, Adhikari said the research was "a nice contribution and an important documentation for sure."

He published the 2016 paper on water redistribution's effects on rotational drift. Adhikari added that the study clearly demonstrates "the role of groundwater pumping on polar motion, and it's pretty significant."

I'm Bryan Lynn.

Bryan Lynn wrote this story for VOA Learning English, based on reports from Geophysical Research Letters, the American Geophysical Union and the U.S. Geological Survey.

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tilt – n. a state of having one side higher than the other; leaning to one side

rotation – n. the act of turning around a central point

axis –n. a line around which something turns, such as the Earth

crust – n. the outer, solid part of the Earth

redistribute – v. to take from one area and put in another area

impact – n. effect

longitude –n. the system of lines going north to south that help travelers know where they are on the Earth

drift –n. the slow movement of something away from where it was before to a new position that is hard to predict

contribute – v. to help cause something to happen

role – n. the part something plays in an event or situation