Humanity Is Slowing Down Earth
4 min read
Any planet, in our own solar system or beyond, is shaped by a certain set of influences: the whirlwind circumstances of its formation, the contents of its deep interiors, other natural phenomena that ebb and flow with time. These forces help determine the speed at which a world spins, the wobble of its axis, and the invisible boundary between it and the rest of the universe, the line where the final wisps of its atmosphere end and space begins.
Jupiter drew in the most primordial gas at its inception, so it spins faster than our solar system’s other planets. Mars’s wobble is influenced by the sloshing of its molten iron core. Our own planet is still wobbling from the effects of the last ice age.
In recent years, scientists have noticed that the effects of climate change—melting ice, rising sea levels, an atmosphere filled with heat-trapping gases—are affecting Earth’s fundamental properties. These changes might be imperceptible to us, but they are measured on a scale so enormous that they have altered the way our planet inhabits its place in the cosmos.
Consider Earth’s rotational speed, which gives us our 24-hour day. The planet has been slowing down for much of its history, lengthening the days ever so slightly, and scientists have long known that the loss of ice at Earth’s poles could contribute to this slowing. “The physics is very, very straightforward,” Duncan Agnew, a geophysicist at UC San Diego, told me. When polar ice melts, the water moves toward the equator, bulking up the planet’s middle and changing the dynamics that keep the Earth going around.
Over the past several millennia, between 0.3 and 1.0 millisecond has been added to each day in the 20th century, according to a recent study. But since 2000, the days have been lengthening at a much faster pace, Surendra Adhikari, a geophysicist at NASA’s Jet Propulsion Laboratory and one of the study’s co-authors, told me. In the paper, the team didn’t specify whether this recent slowdown is caused by greenhouse-gas emissions. But the accelerating loss of Arctic and Antarctic ice is humankind’s doing, and that extra water is flowing into the equatorial oceans. “We are messing with our climate system,” Adhikari said. If emissions continue to rise, in a worst-case scenario the rate that days lengthen per century could nearly double by 2100, the research suggests. In that scenario, climate change would overtake the primary force that has been slowing Earth down for billions of years: the moon’s pull on the planet’s tides.
The effects of climate change are even more apparent in studies of Earth’s axis, which can wander because of geological and atmospheric processes. For example, when Earth entered its post-glacial era and the frigid weight of all that ice melted into the oceans, the planet’s viscous mantle began to shift, and the crust rose like foam. The rebound has occurred unevenly, shifting the planet’s balance. In the early aughts, scientists registered a sudden change: After a century of wobbling toward roughly the same part of Canada, the axis began drifting eastward. Adhikari and other researchers have attributed most of that shift to the melting of polar ice sheets and the resulting sea-level rise. Once again, the movement of water from the poles to the equator is to blame. “It all boils down to the transport of mass from one part of the planet to another part of the planet,” Adhikari said. In fact, the very shape of the planet is changing, turning a flattened sphere into an even flatter sphere.
Climate change has even affected the way Earth interacts with space. Greenhouse gases trap heat more effectively in the thicker atmosphere closer to the ground, but higher up, where the atmosphere is much thinner, that excess heat escapes into space. When that air cools, it contracts, leaving the higher altitudes beyond even less dense than they already were, Ingrid Cnossen, a researcher at the British Antarctic Survey, told me. While the portion of the atmosphere closest to the ground is becoming hotter, the upper layers are cooling dramatically.
All of these planet-size tweaks, for now, have only small implications for the people living here. The shifting nature of Earth’s wobble must be accounted for to keep certain satellite technology working properly. The cooling upper atmosphere also means that space debris, left behind from our decades of exploration, experiences less atmospheric drag and remains in orbit longer, heightening existing worries about the disruptions that a significant crash could have on modern life. A slightly longer day matters most for those in charge of managing precise timekeeping for GPS systems. The rest of Earth’s inhabitants will continue to feel the effects of climate change in much more tangible ways—such as terrifyingly intense hurricanes in the Atlantic Ocean, the deadly extreme heat hovering over India, and wildfires that firefighters in California struggle to contain for weeks.
But this is the first time in history that humanity has been able to witness itself, in real time, reshaping the most basic facts about our planet, which once seemed like they were beyond human influence. Agnew, who is a history buff in addition to a geophysicist, has found an early suggestion of melting ice slowing Earth down in a scientific paper from 1866. The author wrote about it as a pure hypothetical. Decades later, it is an alarming reality. Two centuries of burning fossil fuels has pressured Earth enough that the planet has had to adjust, responding to our actions as if we were the shifting of its mantle or the tug of the moon.
