Changes in Land Water Storage and Melting Ice Sheets Drive Polar Motion

Before 2000, Earth's spin axis was drifting toward Canada (left globe). Changes in water mass in different regions (center) have been pulling the drift eastward (right). | NASA/ JPL-Caltech

Changes in terrestrial water storage, along with the melting of polar ice sheets, are driving changes in Earth's polar motion, researchers report. The findings, which appear in the 8 April issue of Science Advances, help explain why there has been a dramatic eastward shift in the general drift direction of Earth's spin axis since the year 2000.

"With our new insight into the relationship between land water storage and polar position we can now place new bounds on how much ice sheets and wet or dry epochs changed during the entire 20th century," said Erik Ivins, a senior research scientist at NASA's Jet Propulsion Laboratory and co-author of the study.

The Earth is spinning from west to east (making one cycle each day) and the axis about which Earth spins is called Earth's spin axis. Because the spin axis crosses Earth's surface at two points, or rotational poles, scientists call the motion of Earth's spin axis "polar motion."

In the past one hundred years or so, scientists have observed changes in polar motion related to the loss of ice sheets. As this meltwater drains into the oceans, it redistributes the Earth's mass in a way that can shift the spin axis.

Specifically, it's been well-documented that the North rotational pole has moved toward Hudson Bay, Canada, during the 20th century, and that this long-term motion is related to a mass deficit in the region following the collapse of the Laurentide Ice Sheet in North America.

Patterns of continental water storage are related to an east-west wobble in Earth's spin axis. In Eurasia, for instance, water loss corresponds to an eastward swing (top) while water gain pushes the axis westward (bottom).| NASA/ JPL-Caltech

But since about the year 2000, the North rotational pole has begun heading along the Greenwich meridian, representing a dramatic, 75-degree eastward shift in drift direction. Many scientists argue that melting of polar ice sheets in Greenland and Antarctica are alone responsible for this eastward shift, but the exact cause remains unknown, despite many theoretical and modeling efforts.

Now, Ivins, along with colleague Surendra Adhikari, a postdoctoral program fellow at NASA's Jet Propulsion Laboratory, have analyzed satellite measurements of Earth gravitational fields (which can be used to track changes in how mass is distributed on the planet) from 2003 and 2015. Their results show that the recent dramatic eastward shift in polar motion is a result of both the melting of the Greenland and West Antarctic ice sheets and both losses and gains in global terrestrial water storage (the amount of water held in the continents) in different parts of the world.

The effects of terrestrial water storage on polar motion demonstrated in this study help to resolve another long-standing puzzle, serving as a possible physical mechanism for the decadal-like east-west swings in polar motion that have been documented throughout the modern 115-year long record. These swings may correspond with near-decadal changes in continental rainfall and drought, the authors say.

"During 2003 to 2015, water mass changes — in the form of ice and liquid water — controlled decadal-scale changes in the position of Earth's spin axis. [Axis] changes of this sort have been observed for more than a century, and no one has found a plausible explanation of them until now," said Adhikari.

Analyzing the 20th century polar motion record could therefore potentially reveal new information about the past climate, such as whether the intensity of drought or wetness has amplified over time and in what locations, providing possible data constraints for models of past climate change. Such model quantification will have important ramifications for climate change during the 21st century, the authors say.