Pumpkin Planet

Pumpkin Planet Photo Credit: Norman Kuring, NASA/GSFC/Suomi NPP.

Earth is a watery planet. And that water doesn't stay put—it gets carried by ocean currents, seeps through the ground, evaporates from one place, and then rains down on another halfway around the globe. Now, it turns out that the huge amount of water sloshing around is literally changing the shape of the earth. You'll find out more in this Science Update.


Watching the earth's waistline. I'm Bob Hirshon and this is Science Update.

We think of planet earth as a perfectly round sphere. But earth is actually shaped more like a pumpkin than a ball. Jean Dickey of NASA's Jet Propulsion Lab in Pasadena, California, says the earth is a little thicker around the middle, and over the past five years, it's been getting paunchier.


What's happening is that we know that there's more mass at the equator, and the earth is getting more rotund, more oblate.

In a recent study, Dickey and her colleagues set out to find out why. They looked at many different ways that mass could be moving around the globe—for example, looking at the movement of water in the oceans and in the ground.

Dickey says their analysis shows that the big reason for earth's bulging waistline is melting glacier ice.


The mass that was centered at the high latitudes is now going into the ground and finally reaching the ocean. And this new distribution of mass is centered on the equator. So there's more mass at the equator than there was before.

Dickey says the results illustrate just how fast glacier ice is melting—fast enough to trigger a rise in sea level as well as a change in the very shape of our planet. For the American Association for the Advancement of Science, I'm Bob Hirshon.

Making Sense of the Research

This study found that melting glacier ice is contributing to the planet's bulging middle. When scientists discovered the bulge, in August of 2002, they were surprised. In fact, they expected to see the opposite: that the earth was getting rounder. Why? Because of melting glacier ice.

Confused? Here's the story: Before the last Ice Age ended about 11,500 years ago, much of the planet's land mass was covered with thick, heavy ice sheets. Only the land that was close to the equator was ice-free. When those ice sheets melted away, the land underneath them began to spring back up—just like a sponge would if you put a book on top of it, and then took the book away.

The difference is that the earth is a lot harder than a sponge, so that rebound effect happens very, very slowly. In fact, it's still going on now. That's why scientists expected to see the earth getting more well-proportioned in shape: the land that's away from the equator is still springing back up from its squooshed-under-the-ice days.

But instead, they found that since 1998, the planet is actually getting wider in the middle. As Dickey's team found out, that's partly because of glacier ice that melted very recently. Most of the glaciers around today are fairly close to the poles; when they melt, the water flows into the ocean and gravitates toward the equator. That adds bulk to the earth's midsection, which counteracts the springboard effect that's happening on land.

In addition to the melting glacier ice, a powerful combination of ocean currents is also transferring water mass from higher latitudes to the equator. And it's worth noting that the change in the earth's shape is very, very small—only a few millimeters.

But the shape of the earth itself isn't really a big concern. It's more of an indicator of the real problem: the rapid melting of glaciers and ice caps due to global warming. Melting glaciers can change the flow of rivers, leading to floods and droughts, and can impact the plants and animals that live nearby. Floods from melting glaciers can destroy towns and alter natural habitats. Unfortunately, these sorts of problems will probably become more and more common in the next few decades.

Now try and answer these questions:

  1. How did melting glacier ice from the Ice Age make the earth rounder?
  2. How is melting glacier ice today making the earth fatter?
  3. What is the key difference between these two processes?
  4. What are some potential consequences of melting glaciers? Is there any way to counteract them?

For Educators

All About Glaciers contains information and links related to glaciers and glacier research.

In the New York Times Learning Network's lesson Ice Breakers, students learn about the effects of global warming on the polar ice caps.

In the National Geographic News article Everest Melting? read about receding glaciers on Mt. Everest and the floods that result. Contains links to other relevant articles.

Related Resources

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