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Mission: Planet Earth

Our World and Its Climate-and How Humans are Changing Them

Roaring Brook Press

MISSION, PLANET EARTH

When I was an astronaut, I spent hours gazing down at the Earth below. Our planet is beautiful. It's home to everything we know and hold dear.

When I looked out the window, I could see winding rivers emptying into blue oceans, mountainsides of a tropical rain forest, and muddy waters of river deltas. I could see city lights twinkle at night and contrails of airplanes crisscross the sky.

More than anything, though, I could see how fragile Earth is. When I looked toward the horizon, I could see a thin, fuzzy blue line outlining the planet. At first, I didn't know what I was seeing. Then I realized it was Earth's atmosphere. It looked so thin and so fragile, like a strong gust of interplanetary wind could blow it all away. And I realized that this air is our planet's spacesuit — it's all that separates every bird, fish, and person on Earth from the blackness of space.

In the last few decades we've started to change that atmosphere. Some of the changes, like the smog hovering over Los Angeles, are even visible to astronauts in space. Others are invisible to the eye but are now easy to measure. The most dangerous — the one that will affect everything on our planet — is the warming that we now know we humans are causing.

Our warming climate is not visible to astronauts, but its effects will be. The next generation of astronauts could look down and see deserts where we now have lakes, meadows where we now have glaciers, and oceans where we now have beaches. Future astronauts may even have to launch into space from a new launch pad — Cape Canaveral could be underwater. They may look down and say, "That's where Washington, D.C., used to be," or "Did you know farmers used to grow wheat in Kansas?"

To a person standing on the ground, our air seems to go on forever. The sky looks so big, and people haven't worried about what they put into the air. From space, though, it's obvious how little air there really is. Nothing vanishes "into thin air." The gases that we're sending into the air are piling up in our atmosphere. And that's changing Earth's life-support system in ways that could change our planet forever.

EARTH: IT'S ALL CONNECTED

Earth is getting warmer. And that's changing its climate.

What is "climate"? A lot of people think that climate a weather are the same thing. Yes, they are related to each other. But they're not the same. "Weather" is what you see when you step outside. You might see bright sunny skies and scattered clouds one day, then black clouds and heavy rain the next. Weather describes the conditions, such as temperature, clouds, rain, and wind, on a particular day and in a particular place.

Climate is the sum total of weather. It's determined over many years, and describes what the weather is usually like and what extremes to expect. If you live in Phoenix, you live in a hot, dry climate. That doesn't mean it never rains. But it does mean that there is less rain in a year than in most other places. If you live in Seattle, you live in a cool, damp climate. That doesn't mean you'll never go sunbathing, but on average it will be cooler and damper than, say, in Phoenix. You can talk about the climate of whatever size area you want. You can talk about the climate of the whole planet, or the climate of a country, a city, or even the climate of a particular meadow or mountainside.

Climates all around the world are changing. Why do we care about this? We, and all the other living things on the planet, have adapted to — and built lives around — the climates in our parts of the world. Polar bears in the Arctic, rattlesnakes in the desert, corals in the tropics, pine trees on cool mountaintops, and wheat farmers in Kansas all live where they do because the conditions in those places suit them. Now it's getting warmer everywhere. Snow and ice are melting, the oceans are getting warmer, and the deserts are getting drier. Spring is coming earlier, winter's not as cold, and the temperature doesn't fall as much at night. Environments everywhere are changing. And the plants, animals, and people who live in those environments have to adapt or move.

Earth is a little like your body. Both have systems that work together to keep them healthy. Take a deep breath. Your respiratory system takes in oxygen and gets rid of carbon dioxide. Earth's respiratory system? Plants take in carbon dioxide from the air and release oxygen, while other living things — like people — breathe in oxygen and breathe out carbon dioxide.

Feel your heart beat. Your circulatory system moves your blood around, carrying oxygen to billions of cells in your body. Winds and ocean currents make up Earth's circulatory system. They carry heat and chemicals around our planet.

From the time our planet's land, oceans, and atmosphere formed, they have been connected to each other. The motion of air and water moves heat around the planet. The water cycle carries moisture from the oceans, through the air, to the land, and back to the oceans. Other cycles transport nutrients around the planet. Once life evolved, plants and animals became a part of these cycles, too. Today, these cycles maintain our planet's livable conditions. They moderate extreme temperatures and keep the composition of the air and oceans about the same from year to year. They move chemical elements like carbon, nitrogen, and oxygen back and forth between the environment and the living things that depend on them.

IN THE BEGINNING

Today, Earth is an oasis. Breathable air hugs our planet, deep oceans wrap most of its surface, and seven continents cover the rest. And life is everywhere! But it wasn't always that way.

4.6 billion years ago, Earth was a giant ball of molten rock. It had no oceans and no atmosphere. Red-hot lava erupted from volcanoes and cracks in the crust. Eventually, the surface cooled. But Earth's insides were still boiling hot. Steam and other gases escaped from the molten interior, and our early atmosphere was formed. That air was thick and made mostly of nitrogen, carbon dioxide, and water vapor — nothing like the oxygen-packed air we breathe today. As the atmosphere cooled, water vapor condensed to form dark clouds. Then torrential rain flooded the low basins and formed Earth's first oceans.

At first, there was no life on our planet. But primitive microscopic organisms evolved in the oceans. They were tiny, single cells that looked like today's bacteria. They stayed alive by taking in simple molecules from the water and using the energy released in chemical reactions. The tiny microbes divided again and again, and their numbers multiplied. Eventually, their microscopic descendants evolved the ability to make their own food through a process called photosynthesis. More than the wheel, the light bulb, the Internet, or any other human invention, this development changed the world.

Photosynthesis is the process by which some living things use the energy in sunlight to combine carbon dioxide and water molecules into sugar for food. As part of this process, oxygen is released. One by one, oxygen molecules began bubbling out of the ancient oceans and collecting in the air. Over the next two billion years, Earth's once unbreathable atmosphere slowly filled with oxygen.

Those tiny microbes were alone on the planet for two billion years. But they were about to have lots of company. All sorts of new life evolved — living things that breathed oxygen. And to this day, life on Earth depends on photosynthesis. The sugar molecules created by photosynthesis start almost all Earth's food chains, and the oxygen molecules released by photosynthesis are what we need when we breathe.

Today, phytoplankton in the ocean and their plant relatives on land carry out most photosynthesis — just like their ancient ancestors. It doesn't matter if you're a microbe or a mongoose, a clam or a college student — all living things need energy. But only plants and some microbes can turn the Sun's energy into an energy source the rest of us can use — food! All the rest of us depend on them for our survival.

THE GREENHOUSE

The sun powers our planet. It provides the light and heat that we and other living things depend on. Earth would be a frozen, lifeless rock without it. But the Sun is not the only thing that determines our climate. The amount of sunlight we receive is important — but so are the interactions of the oceans, atmosphere, land, and life. They determine how much of the Sun's heat is absorbed, and distribute it around the planet.

The Sun is constantly emitting energy in all directions. A tiny fraction of it falls on Earth. Some of the sunlight that does hit Earth is reflected right back out to space and doesn't warm our planet at all. It's reflected by clouds in the atmosphere and by ice and snow on the ground. But some sunlight is absorbed by our vast oceans and rocky land. That's the sunlight that warms the planet.

The warm ground cools off by radiating the heat back toward space. If this heat could make it back out through the atmosphere as easily as the sunlight makes it in, our planet would be much colder than it is.

But the heat doesn't escape so easily. Certain gases in the atmosphere absorb it before it makes it to space. Those gases re-radiate that heat in all directions. Some of it heads out toward space, but some heads back toward Earth. So some of the heat is trapped. This is called the greenhouse effect. Like the glass walls of a greenhouse, which let sunlight in but don't let all the heat out, the greenhouse effect in our atmosphere makes our planet warmer than it otherwise would be. It's almost like there is a thin blanket covering Earth.

The gases in the air that absorb heat in this way are called greenhouse gases. Not all the gases in the atmosphere are greenhouses gases; in fact, most of our air is made of gases that are not greenhouse gases. Oxygen and nitrogen make up approximately 99 percent of our atmosphere. If they made up 100 percent, instead of 99 percent, there would be no greenhouse effect on our planet.

But Earth's air contains small amounts of other gases that are greenhouse gases. Most of these occur naturally, and have been a part of our planet's air for billions of years. One is water vapor. Most of Earth's water is in oceans, rivers, lakes, and ice. But a small part of it is carried for a while through the air. It isn't much — but it's important! This water comes down as rain and nourishes everything on the land. It also accounts for much of the Earth's natural greenhouse effect.

Carbon dioxide and methane are two other important greenhouse gases. Like water vapor, they make up only a tiny fraction of the air. Those few molecules of water vapor, carbon dioxide, and methane provide a greenhouse effect that has warmed Earth for billions of years — long before people began to have an impact on its atmosphere. In fact, if there were no water vapor or carbon dioxide in Earth's atmosphere, our planet would be about 33°C (59°F) colder than it is! Earth would be paved in ice — and we wouldn't be here.

By the way, Earth is not the only planet with a greenhouse effect. The air on Mars is very thin, but it is mostly carbon dioxide. The Martian greenhouse effect warms that planet by a few degrees. Venus is a different story. It has a very thick atmosphere that's almost all carbon dioxide. Without its greenhouse effect, Venus would be hot, but livable — about like the Sahara desert. But the greenhouse effect adds almost 370°C (700°F) to its temperature, and makes it the hottest place in our solar system.

WHAT ELSE WARMS US?

Greenhouse gases have a big effect on Earth's climate. So do the clouds in the sky, the ice on the ground, and even the plants that grow on the land.

Clouds play a huge role in both warming and cooling the Earth. Some cool the planet like giant sun hats by reflecting the Sun's energy and providing shade. But others warm the planet like floating blankets by trapping heat that rises from the ground. High, wispy cirrus clouds don't cast much of a shadow since sunlight passes right through them. But they do a good job of trapping the heat that's trying to escape back to space. So they warm the planet. But down lower to the ground, thick, fluffy cumulus clouds help keep things cool by reflecting sunlight back into space. In today's world, the mix of clouds keeps the Earth a little cooler than it would be if there were no clouds in the sky.

How does the surface of the Earth affect our climate? The sea ice covering the north pole, the huge ice sheets of Antarctica and Greenland, and icy glaciers around the world cover about 10 percent of our planet. White, reflective ice and snow reflect more energy than dark, absorbent land, so the more ice there is, the more of the Sun's energy is reflected back out to space. All this ice keeps Earth cooler than it would be without it.

What about the land? Some things absorb more sunlight than others. You feel this yourself anytime you walk barefoot on a hot summer day. That black parking lot is much hotter on your feet than the white sidewalk. Rocks, dirt, plants, and grass absorb different amounts of sunlight, and reflect the rest.

RIVERS OF AIR

The air doesn't just hover above us. Leaves rustle and flags flutter. Clouds blow across the sky. Our air is constantly flowing around the planet. And it's important that it does. The moving air carries heat from the equator to the poles, and it carries water (in the form of clouds and rain) all around the globe. Where does the energy come from to pump the air around the planet? That energy comes from the Sun. Sunlight warms the oceans and the land. Then the warm surface heats the air just above it. That means the air near the ground is warm — and hot air rises so the air starts to move.

Think of a hot air balloon. You climb into the basket, then turn on a heater that warms the air that fills the balloon. The warm air expands, so it's not as dense as the surrounding air. That's why it starts to rise, lifting you and the balloon with it.

The air near the equator is warmed by heat from the water below, and begins to rise, like the hot air balloon. Cooler air blows in to replace it, and creates a surface wind. Meanwhile, as the warm air rises, it begins to cool. Several kilometers up it hits a natural lid on the lower atmosphere, and becomes a high-altitude wind. Once the air has cooled, it sinks back toward the ground — but now it's almost 2,000 kilometers from the equator where it started.

Because Earth is shaped like a giant ball, different parts of the planet receive different amounts of sunlight. The Sun shines directly on the part around the equator, but its rays only graze the poles. That's why they're not as warm as the tropics. These temperature differences drive winds in rivers of air that circle the globe. Their directions are influenced by Earth's rotation, and they follow regular patterns that vary with the seasons. Air that's over Japan today will be over the United States in a week; if a huge dust storm kicks dust into the air over Africa, the winds will carry it across the Atlantic Ocean to South America. If a volcano throws ash into the air over the Philippines, winds will carry it across the Pacific to the United States. Most importantly, the moving air redistributes heat around the planet, carrying it from the warm tropics to the chilly poles.

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Excerpted from Mission: Planet Earth by Sally Ride, Tam O'Shaughnessy, Jan Adkins. Copyright © 2009 Sally Ride and Tam O'Shaughnessy. Excerpted by permission of Roaring Brook Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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