The physics behind this phenomenon is simple, according to Professor Benedikt Soja, who is affiliated with the Federal Institute of Technology in Zurich and specializes in geodetic density. This is the science that deals with measuring the Earth. “Think of a skater’s rotations. If he extends his arms and legs, he will spin slower, and vice versa. The speed of rotation changes depending on the distribution of mass. Angular momentum, which is a measure of the amount of rotational motion, is conserved.”
“The same is true on Earth. A huge amount of water is stored in the ice caps of Greenland and Antarctica, close to the Earth’s axis of rotation. When the ice melts and the liquid water ends up in the oceans, its mass becomes evenly distributed across the Earth. On average, it will then end up farther from the axis of rotation than it is now.
Minimum time saving
Because of this overall distribution of the Earth’s mass, with more mass farther away from the axis of rotation, the speed at which the Earth rotates on its axis will slow down. This makes a day longer, because it coincides with one rotation of the Earth on its axis.
Still, for anyone hoping to get some extra time to get through busy days, the effort is worth it. The difference is only a few milliseconds, says Suga. “Based on data from the past 20 years, we see that the length of the day will increase by about one millisecond per century. We compared different climate scenarios, and in the most pessimistic scenario where most of the polar ice melts, the slowdown in Earth’s rotation could increase to 2 to 3 milliseconds per century.”
The time saving is almost negligible, but it is still significant. Because in this scenario, the influence of climate change on Earth’s rotation outweighs the moon’s gravitational pull. It has always been the dominant factor in Earth’s rotation speed. “The fact that human-induced climate change has so quickly outpaced the moon’s influence is something to think about,” Suga continues.
El Nino
The Earth’s rotation rate—and its related length of day—is a complex phenomenon. Mass distribution plays a role, as does the Moon’s gravity, as do the atmosphere, ocean hydrology, motions in the Earth’s liquid core, and interactions between the core and the mantle.
Pinpointing the exact influence of a single element is no easy task, says astronomer and geophysicist Véronique Dehant. Dehant works at the Royal Observatory of Belgium, specializing in Earth and Mars rotations. “It is very likely that the melting of the ice caps is slowing down the Earth’s rotation. This can also be observed on Mars. Mars has ice caps that are largely made up of frozen carbon dioxide. These polar caps vary with the seasons, causing small fluctuations in Mars’ rotation speed, a few tenths of a millisecond.
The atmosphere certainly has an effect on the speed of the Earth’s rotation. By the friction of the winds, the pressure they exert on the Earth’s surface, and by the gravitational pull that the mass of the atmosphere exerts on the mass of the Earth. “Certain weather phenomena cause minute changes in the speed of the Earth’s rotation, due to the friction of the winds and the movements of the mass in the ocean. Just think of the El Niño phenomenon, where the normally cold sea waters of the Pacific Ocean warm very strongly. Keep in mind that these are very small variations, barely amounting to a tenth of a second.
Fossil coral as a calendar
The Earth’s climate has never been stable. For example, during various ice ages, the poles, and large parts south of them, were covered in thick ice sheets. This must have had an effect on the speed of the Earth’s rotation, explains Dehant. “If the mass of the ice, and therefore the distribution of the mass, changed, that would have had an effect on the speed of rotation,” he says. However, we don’t have detailed data on the Earth’s rotation from that period. Very long baseline interferometry (measurements based on simultaneous observations of objects in space by multiple telescopes) only began in the 1960s, and scientific astronomical observations have been made for the past 200 years. We can go back even further in time by analyzing historical records of eclipses, for example from the Babylonians.
However, there is a way to look back in time. Some corals show small daily growth rings, similar to the annual rings of a tree. These indicate that the coral is growing daily, an increase that is linked to the amount of sunlight. Based on these “day rings” of fossil corals, it is possible to infer the length of days in the past.
These data show a clear slowing down of the Earth’s rotation speed over millions of years, and thus an increase in the length of the day. “For example, in the Devonian period, about 200 million years ago, the year consisted of 400 days, each of 22 hours. There was a clear slowing down of the Earth’s rotation speed over a very long period,” says DeHant.
“The data from the coral do not show small variations due to climate changes. This delay, which has been occurring for millions of years, is due to the tidal influence of the Moon. The gravitational force of this celestial body creates a tidal bulge on the Earth. If the Earth were perfectly elastic, this bulge would be perfectly aligned with the position of the Moon. However, this is not the case. The Earth is not elastic, so this bulge is not in sync with the movement of the Moon. This creates friction, especially in the oceans, which slows down the Earth’s movement.
The wobbly earth
The melting of the ice caps not only affects the speed of the Earth’s rotation, but also the position of the axis of rotation itself, as Benedikt Soja describes in his research. The axis around which the Earth rotates does not exactly coincide with the symmetric axis of the Earth that passes through the poles. The position of the axis of rotation varies with respect to the poles. This so-called polar movement occurs within an area of about twenty square meters. This is partly due to movements in the Earth’s core, or the periodic melting and refreezing of the Greenland ice sheet.
“The reduction in ice cover certainly has consequences for the position of the Earth’s axis of rotation relative to the poles,” explains Suga. “This huge ice sheet is not located well to the North Pole, but much further south.” Antarctica is centrally located around the Earth’s axis. When large masses of ice melt in Greenland, the balance between the poles changes, causing the axis to shift. This phenomenon can also be observed today.
Does it actually matter if the Earth rotates a few fractions of a second slower? Certainly not for time measurements, because they are made with extremely precise atomic clocks. When the number of atomic clocks deviates from the Earth day, an extra second is added to reset the two time scales. The last time such a leap second was included was in December 2016.
But for astronomical research, satellite applications and space travel, it is really important to know the Earth’s rotation speed accurately, according to Suga. “For positioning satellites, for example, or for space observations. Launching rockets or calculating spacecraft trajectories also require such precision. A small deviation on Earth can have serious consequences in deep space.
