Earth Tilted 31.5 Inches in Less Than 20 Years. Here’s What That Really Means for Us

Caroline Delbert
Wed, August 23, 2023 

Earth Has Tilted 31.5 Inches—But Should We Care?Getty Images

Earth’s tilt recently made headlines when scientists revealed it had deviated 31.5 inches. That data came from a June study, published in the journal Geophysical Research Letters, that found human groundwater pumping is primarily to blame for the jaunty new tilt. And in that study, the increase in tilt was linked with a 0.24-inch rise in global sea level. That’s already a lot to take in. How does the change in tilt affect global sea level? Why does groundwater pumping affect the tilt in the first place? And is a 31.5-inch difference in Earth’s tilt really that big of a deal?

But the truth about Earth’s tilt is far more complex than that, and it involves an unsettling amount of wiggle room across many variables, which affect almost every condition on Earth’s surface. Here’s what you need to know about Earth’s tilt, and why it matters that it keeps shifting.

Earth’s tilt is iconic. It’s why we have the specific range of seasons that we do, and why the North and South Poles each have times with no sunlight at all or no darkness at all. If you think about it, it’s easy to understand why. If Earth’s tilt was perpendicular to its orbit around the sun, then the one hemisphere exposed to the sun would remain the same at all times of year. That hemisphere would always include the very edges of both the North and South Poles—it’s like the Tilt-o-Whirl ride before it, you know, tilts. But with Earth’s tilt, just like on the ride, you are sometimes much closer (and sometimes much farther) from the sun or the ground.

Getty Images

Where most of us live, Earth feels very solid, which can be a bit deceptive. The crust, or the outermost layer made primarily of solid rock, is around 25 miles deep in many places. Just one square foot of rock that’s 25 miles deep weighs nearly 11,000 tons on average; that’s the same weight as the entire retractable roof of Toronto’s Rogers Centre where the Blue Jays play. It was enough to right the Costa Concordia, a 114,000-ton cruise ship that hit land and sank onto its side ten years ago.

But 25 miles thick is only about one-third of one percent of Earth’s diameter, and 11,000 tons is a negligible, far-out decimal of its total mass of 13,170 billion trillion pounds—it’s the paper-thin M&M candy shell on the solar system’s densest planet. On top of the crust are the oceans, and underneath its surface are vast subterranean freshwater areas. Below that, the mantle has a small amount of liquid molten rock, and below that, the outer core is liquid. (Earth’s inner core is believed to be solid.)

Water flows from an underground well to irrigate an orchard on Tuesday, October 12, 2021 in Visalia, California.Getty Images

The recent paper on groundwater explores a specific phenomenon. When people seeking fresh water punch holes into the water reserves below or inside Earth’s crust, they affect how the entire planet is balanced in the simplest way: suddenly, one area of Earth’s outer shell weighs a great deal less than it did before. (What if you hollowed out one part of the outside of a bowling ball or a spinning top? It might still be fine, but it wouldn’t spin the same as it did before.) And because Earth has so much water and molten metal, any new spin reverberates through these other liquids, and can reflect back in a way that makes the new spin even more different.

Earth’s tilt, which is scientifically known as obliquity, is known to vary between 22.1 and 24.5 degrees on a cycle that lasts about 41,000 years. Each degree of Earth’s circumference is about 69 miles, which means 31.5 inches is truly an amount so tiny that it almost doesn’t matter. But this is a specific paper about one isolated factor, and how much it is believed to be affecting Earth’s tilt. And more importantly, this is a human-caused factor, not part of the natural fluctuation of Earth’s tilt. Humans existed 41,000 years ago, but they weren’t drilling into Earth’s crust to drain the groundwater.

 What Is Groundwater Pumping?

According to the U.S. Geological Survey (USGS), there is over 1,000 times more water in aquifers below Earth’s surface than in all of the world’s rivers and lakes. (An aquifer is a body of rock or sediment that holds groundwater). This groundwater can be found virtually everywhere on Earth, even in deserts, but it’s often inaccessible or requires treatment for human use. The water may be near the surface, where it’s just a few hours old, or at great depths, where it may be several thousands of years old.

We’ve become reliant on groundwater pumping as fresh water in lakes and rivers has dried up and disappeared; we use that groundwater for a myriad of purposes, from drinking water to irrigation to mining, according to Issues in Science and Technology, a quarterly journal from the National Academies of Sciences, Engineering, and Medicine and Arizona State University. But that comes with consequences. Excessive groundwater pumping wreaks havoc on our natural bodies of water and wetlands, drying them up; leads to ground collapses; and disrupts, and even kills off, wildlife, fish, and trees. And now, apparently, groundwater pumping has even affected Earth’s tilt.

Earth doesn’t need to be almost perfectly balanced like a top or a bowling ball. In fact, scientists believe a giant impact from a celestial body, called Theia, created Earth’s tilted spin in the first place. This knocked an entire chunk out of Earth’s mass, which scientists believe, in turn, became the moon. The rest of Earth left behind, with a big Swiss cheese crater knocked out of one side, just spun differently. Thankfully, for the living things on Earth, nothing has happened to knock it back into a truer orbit.

And because of the way Earth and other planets spin, they end up smoothing their own shapes over time into something closer to spheres, a concept called hydrostatic equilibrium. In fact, being nearly spherical is a criterion for planethood in the first place, which means the chunky post-Theia Earth might not have qualified until it got its rotational act back together. Earth’s tilt has not affected its ability to maintain a spherical shape, because hydrostatic equilibrium is a phenomenon of each planet’s individual rotation regardless of its pitch.

In 2018, NASA shared news that it had isolated three major causes of the changes to Earth’s tilt in the 20th century. These causes are the melting of Greenland, land “bouncing back” after glaciers move or melt, and mantle convection. In mantle convection, the portions of liquefied rock beneath Earth’s crust are constantly roiling upward and being pushed back down. The density of these different temperatures of rock are different, and throw off the center of mass.

Scientists have known that Earth’s tilt varies based on a huge number of factors, but the ability to study them one at a time is pretty new. “As the Earth undergoes continuous changes at a large variety of time scales, from its inner to outer components, all of those dynamical parameters do not have steady values, but vary with time. Their variations are so small compared to the constant reference values that they could not be observed until recent years,” scientists wrote in a 2020 paper.

That means we’re likely to see more news of more drift of Earth’s tilt attributed to specific things in the years to come.