Weather Station

World on a Computer

by Stephanie Mabee

John Shaffer keeps a world in his computer. Shaffer is a graduate student at Arizona State University. His world shows how Earth's temperature has changed over hundreds of thousands of years. His computer calculates these changes in weather according to an idea thought up by a Russian scientist 70 years ago.

The Russian scientist's name was Milankovitch. His idea, or theory, uses information about eccentricity, axial tilt, and precession to calculate the differences in insolation and its effect on global climatic conditions.

Say what? Sounds complicated, doesn't it? It is. But Shaffer and ASU geography professor Randy Cerveny are making global weather changes easier to understand.

In 1924, Milankovitch came up with the idea that Earth's orbit affects the amount of radiation it gets from the sun. This incoming radiation is called insolation. The changes in radiation dramatically affect the big ice sheets we call glaciers.

The Milankovitch Theory uses three characteristics about Earth's orbit to find out how much solar radiation the planet receives. These three characteristics are eccentricity, axial tilt, and precession.

Eccentricity describes the shape of Earth's orbit. Earth revolves around the sun, but the shape of the orbit isn't always circular. Over a period of 100,000 years, Earth's orbit slowly changes from circular to egg-shaped and then back to circular.

The second characteristic is called axial tilt. Imagine a straw pushed through the center of an orange. When the straw is straight up, the orange doesn't tilt in any direction. But if you tilt the straw at an angle, the orange tilts in the same direction.

The same thing happens with the Earth. The "straw" is Earth's polar axis—an imaginary line that runs from the North Pole all the way through the planet to the South Pole. As Earth orbits the sun, the axis is never straight up and down.

Earth always tilts at a small angle. Astronomers can measure the degree of Earth's tilt.

Picture a line drawn from the center of the Earth to the center of sun. The polar axis crosses this line. The measure of the angle between the imaginary line and Earth's polar axis is the degree of axial tilt. Currently, Earth's axis tilts at 23 degrees away from this line.

Earth also spins. Think about a top spinning across the floor. Earth's spin looks like the top just before it stops and topples over.

Because Earth rotates on its axis, Earth's spin is wobbly. This wobbly motion is called precession.

There are two effects of precession. Today, the North Pole points toward the North Star (called Polaris). But the North Pole has not always pointed in the same direction. As Earth rotates, the North Pole moves across the sky. In 11,000 years, the North Pole will be pointing away from the North Star. By then it will be pointing toward a star called Vega.

During the same 11,000 years, today's winter season will become the summer season. This is the second effect of precession.

In 1998, the calendar date for the winter solstice is December 21. But 11,000 years from now, in the Year 12,198, the calendar date for the winter solstice will be June 22.

Imagine a ball so big that the universe fits inside it. Astrophysicists call this ball the celestial sphere. Then imagine the center of the ball at the center of the Earth. The celestial sphere has poles and an equator just like the Earth.

From the Earth, we see the planets and the sun move around inside the ball. The sun follows an annual path around the inside of the ball. This path is called the ecliptic.

The sun moves both north and south of the celestial equator. The southern point of the ecliptic is the winter solstice. At the winter solstice, the sun does not rise as high in the sky because the Earth is tilted away from the sun. The winter solstice is the shortest day of the year.

The summer solstice is the longest day of the year. The summer solstice happens at the northern point of the ecliptic.

Researchers measure eccentricity, axial tilt, and precession using the science of astrophysics. Geographers and climatologists use these numbers to determine how much solar radiation Earth receives today. They also study how much solar radiation came to the planet in the past.

When Milankovitch thought up his theory in 1924, he had to do the math by hand. It took days to figure out the total amount of incoming solar radiation. Milankovitch didn't have time to study the entire planet.

Shaffer did his work by studying glaciers in Canada and Greenland. Today's scientists use computers to study the whole planet.

Milankovitch measured solar radiation during the summer. He had good reason. "What happens in the summer represents weather patterns throughout the year," Shaffer says.

During the summer, solar radiation affects how fast ice sheets grow or melt. Low solar radiation during the summer keeps temperatures cool and helps glaciers to grow.

Of course, wet winters also are important to growing glaciers. Increased solar radiation during the winter causes rain and snow. The rain and snow fall on top of the ice sheet. When they freeze, the glaciers grow bigger.

The opposite conditions cause ice sheets to melt. No ice builds up on the glacier if it does not rain or snow during the winter. High solar radiation during the next summer means warmer weather. A warm summer makes ice melt off the glacier.

Thanks to hard work by the ASU scientists, students everywhere will have an easier time understanding global weather patterns. And John Shaffer gets to keep a world inside his computer.