Does global warming affect the earth's crust

Climate change is shifting the earth's axis

We humans even influence the axis of rotation of our planet - at least indirectly. Because the increasing ice melt in the polar regions as well as regional groundwater withdrawals are shifting the earth's mass equilibrium, as a study now suggests. This imbalance has been accelerating the migration of the geographical poles, especially since the 1990s, as researchers report in the specialist magazine "Geophysical Research Letters".

The geographical poles mark the position of the earth's axis of rotation and also indicate its changes. In fact, the earth's axis constantly moves slightly back and forth and performs slight pendulum movements both in the course of the year and in longer periods of time. The cause for this is, on the one hand, mass shifts in the earth's interior, for example due to the convection currents in the earth's mantle or the slow springback of the earth's crust after the last ice age.

On the other hand, processes on the earth's surface contribute to the displacement of the earth's axis. The biggest factor is the distribution of the earth's water masses. The weight and location of ice sheets, glaciers, oceans and underground groundwater resources influence the mass distribution and thus also the “unbalance” of our planet. This is where humans and their interventions in the earth system come into play.

Change of direction of the pole in the mid-1990s

Shanshan Deng from the Chinese Academy of Sciences and her colleagues have now re-examined and quantified what influence humans have on the earth's mass balance and thus the earth's axis. To do this, they first used data from the Earth Rotation and Reference Systems Service (IERS) to determine the pace and extent of polar migration since the 1970s. Using additional data, they then determined the influence of the known geological factors.

It turned out that polar migration has changed significantly since the mid-1990s. Their direction changed from more south to east, at the same time the speed of the pole drift increased 17-fold. "Surprisingly, this change is not caused by the isostatic equalization of the Ice Age or by the mantle convection," report Deng and her team. The influences of the oceans and the atmosphere also do not fully explain the observed changes.

Melting ice drives polar migration

This suggests that the earth's water distribution plays a role. The researchers used data from the GRACE and GRACE-FO satellite missions to determine whether this is the case and what exactly is the effect. Their satellites have been measuring the earth's gravitational field since 2002 and thus also provide information on how, for example, ice masses, groundwater reserves or other hydrological factors have changed.

The result: "The terrestrial water distribution had a slight influence on the polar migration even before the 1990s, but since then it has been driving the pole drift eastwards at an average rate of 3.26 millimeters per year," report Deng and her colleagues. They identify the loss of large ice masses due to global warming as the greatest influencing factor. Because this causes ice to be lost, especially in the Arctic, the earth's water distribution and thus the mass balance has shifted.

"The accelerated ice melt caused by global warming is the most likely reason for the change in direction of polar migration in the 1990s," says Deng. "Alaska, Greenland and the southern Andes are the three areas most severely affected by glacier retreat, and this is also reflected in the hydrological mass distribution," said the researchers. This is also confirmed by an earlier study that the Greenland ice melt affects the pole drift.

Groundwater abstraction as an additional factor

However, ice loss alone cannot explain the full extent of the changes. There is another, more direct human intervention in the water balance, as the team found: the excessive abstraction of groundwater. It causes many underground water reservoirs to gradually empty because more is withdrawn than can flow in.

"The data show that areas with groundwater depletion, such as northern India and the Middle East, are also significantly driving the Poldrift," report Deng and her team. The massive water abstraction around Beijing or the pumping out of groundwater for irrigation in California also affect the distribution of the water masses. "This underlines that even local water management can make itself felt in this way," comments Vincent Humphrey from the University of Zurich.

Human influence exposed

It is true that the currently observed changes in polar migration and the Earth's axis are not strong enough to affect our everyday lives. Nevertheless, they demonstrate that humans can also influence fundamental properties of our planet through their actions. Because the subtle oscillation of the earth's axis is no longer just a result of purely natural processes, but is also shaped by anthropogenic factors.

"The study highlights an interesting fact, because it tells us how big the mass change we have caused is: big enough to shift the axis of the earth," emphasizes Humphrey, who is not involved in the study. (Geophysical Research Letters, 2021; doi: 10.1029 / 2020GL092114)

Source: American Geophysical Union

April 30, 2021

- Nadja Podbregar