Understanding the Climate Clock

  • By Caitlin Saks & Ari Daniel
  • Posted 04.18.18
  • NOVA

The climate has changed on a schedule for millennia. Have humans broken the climate clock?

Running Time: 05:06


Understanding the Climate Clock

Published April 18, 2018

Caitlin Saks: This is our climate. What do I mean? Most of us have heard of the ice ages from the movies. 20,000 years ago, places like New York City were covered in ice. But that wasn't the only time. There were a bunch of ice ages, going back millions of years.

Here's the crazy part: by looking at cores of mud from the bottom of the ocean, we can tell these ice ages actually came and went on a schedule, like clockwork. In fact, the entire world got hotter and colder with the ice ages. What caused these huge swings in Earth's climate?

Actually, it was clockwork, corresponding to slight shifts in Earth's orbit, now called Milanković cycles, after the Serbian scientist who started to figure this out. We know that the gravitational pull of the sun, moon, and other planets makes the Earth's axis wobble like a top, on a cycle of about 20,000 years. The tilt of the Earth also shifts about every 40,000 years. The shape of Earth's orbit around the sun goes from more circular to more like an oval on a schedule of about 100,000 years.

All these changes affect how heat from the sun warms the Northern Hemisphere where ice sheets can grow. The same types of planetary movements that create our days, weeks, and years actually seem to create something like super-seasons: our climate.

But that's only part of the story, because with the orbital cycles, when the Northern Hemisphere warms, the whole planet warms. When the Northern Hemisphere cools, the whole planet cools. Changes in Earth's position set the tempo of the climate clock. However, they don't explain why the shifts in temperature are so extreme, and why they are global, going from times when New York City could be covered in a thick layer of ice to times like today when it hits 100 plus degrees in the summer. What else is going on?

A major clue came when scientists extracted cores of ice from Antarctica dating back hundreds of thousands of years. Trapped inside were little air bubbles of ancient atmosphere. They revealed the amounts of carbon dioxide in the atmosphere going back 800,000 years. Remarkably, the rhythm of this record matches that of the ice cores recorded in the ocean muds, almost perfectly.

We know that CO2 is a greenhouse gas, a gas that traps heat on the planet. This could explain the big global shifts! But where does this CO2 come from? Why would gas in our atmosphere match the tilt and wobble of the Earth?

This is where that can of soda comes in. These are carbon dioxide bubbles, now visible because I've just depressurized the can of soda. One theory is that as the ice sheets melt, they release pressure on the world's volcanoes, setting them off like a freshly opened can of soda. But that can only account a little bit for what was going on.

Another theory is that the oceans act like a sea of soda. How do I get a ton of bubbles to come out? By stirring it up! The same thing happens with the world's oceans. They also contain carbon dioxide. Our planet—the blue marble—is 70% ocean. Even though a can of seawater doesn't have quite as much carbon dioxide as a can of soda, that's still a lot of carbon dioxide in all of the world's ocean!

When ice sheets start to grow, that changes everything. The wind patterns change, which shifts ocean currents, and that changes how the ocean mixes. Scientists believe that the biggest reason CO2 tracked the orbital cycles is because when the planet warmed just a touch, that enhanced ocean mixing. Then just like our stirred can of soda, the oceans lost some of their fizz to the atmosphere. More CO2 in the atmosphere meant more heat was trapped on the planet.

But today there's another source of carbon dioxide. Every year, humans dump over 30 billion tons of carbon dioxide into the atmosphere. That's well over 50 times the amount that comes from volcanoes. As a result, we're warming the planet ten times faster than anything we have ever seen before. The tight link between the Earth's orbital cycles and the atmosphere is now gone. We have actually broken the climate clock.



Caitlin Saks
Digital Producers
Caitlin Saks & Ari Daniel
Production Assistance
Yasmeen Fakhro & Annie Kim
Editorial Review
Julia Cort
Science Advisors
Richard Alley & Julie Brigham-Grette
Written, Produced, and Directed by
Doug Hamilton
Caitlin Saks
© WGBH Educational Foundation 2018


Additional Visuals


(main image: whoopee cushion on stool)
© WGBH Educational Foundation 2018

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