Activity 3 How Do Earth's Orbital Variations Affect Climate?

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Activity 3 How Do Earth’s Orbital Variations Affect Climate?

Activity 3 How Do Earth’s Orbital Variations Affect Climate?

Goals Think about It In this activity you will: When it is winter in New York, it is summer in Australia. • Understand that Earth has an axial tilt of about 23 1/2°. • Why are the seasons reversed in the Northern and • Use a globe to model the Southern Hemispheres? seasons on Earth. • Investigate and understand What do you think? Write your thoughts in your EarthComm the cause of the seasons in notebook. Be prepared to discuss your responses with your small relation to the axial tilt of group and the class. the Earth.

• Understand that the shape of the Earth’s orbit around the Sun is an ellipse and that this shape influences climate.

• Understand that insolation to the Earth varies as the inverse square of the distance to the Sun.

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Investigate Part A: What Causes the Seasons? Now, mark off 10° increments An Experiment on Paper starting from the Equator and going to the poles. You should have eight 1. In your notebook, draw a circle about marks between the Equator and pole 10 cm in diameter in the center of your for each quadrant of the Earth. Use a page. This circle represents the Earth. straight edge to draw black lines that Add the Earth’s axis of rotation, the connect the marks opposite one Equator, and lines of latitude, as another on the circle, making lines shown in the diagram and described that are parallel to the Equator. below. Label the Northern and This will give you lines of latitude in Southern Hemispheres. 10° increments so you can locate your latitude fairly accurately. Note 23.5˚ that the lines won’t be evenly spaced from one another because latitude is measured as an angle from the center Equator of the Earth, not a linear distance. Northern Hemisphere plane of 2. Imagine that the Sun is directly on orbit the left in your drawing. Draw horizontal arrows to represent incoming Sun rays from the left side of the paper. rotation axis Southern Hemisphere 3. Assume that it is noon in your community. Draw a dot where your Put a dot in the center of the circle. community’s latitude line intersects Draw a dashed line that goes directly the perimeter of the circle on the left. up and down from the center dot to This dot represents your community. the edge of the circle. Use a protractor to measure 23 1/2° from a) Explain why this represents noon. this vertical dashed line. Use a blue b) At any given latitude, both north pen or pencil to draw a line through and south, are the Sun’s rays the center of the “Earth” at 23 1/2° striking the Northern Hemisphere to your dashed line. This blue line or the Southern Hemisphere at a represents the Earth’s axis of rotation. larger angle relative to the plane Use your protractor to draw a red of orbit? line that is perpendicular to the axis and passes through the center dot. c) Which do you think would be This red line represents the Equator warmer in this drawing—the of the Earth. Label the Northern and Northern Hemisphere or the Southern Hemispheres. Next you Southern Hemisphere? Write need to add lines of latitude. To do down your hypothesis. Be sure to this, line your protractor up with the give a reason for your prediction. dot in the center of your circle so that d) What season do you think this is it is parallel with the Equator. in the Northern Hemisphere? 762 EarthComm CS_Ch12_ClimateChange 3/1/2005 4:56 PM Page 763

Activity 3 How Do Earth’s Orbital Variations Affect Climate?

4. Now consider what happens six 3. Position the globe so that its axis is months later. The Earth is on the tilted 23 1/2° from the vertical, and the opposite side of its orbit, and the North Pole is pointed in the direction sunlight is now coming from the of the light source. right side of the paper. Draw 4. Turn on the light source. horizontal arrows to represent incoming Sun’s rays from the right. a) Record the initial temperature. Then record the temperature on the 5. Again, assume that it is noon. Draw thermometer every minute until the a dot where your community’s temperature stops changing. latitude line intersects the perimeter of the circle on the right. 5. Now position the globe so that the axis is again tilted 23 1/2° from the a) Explain why the dot represents vertical but the North Pole is pointing your community at noon. away from the light source. Make b) Are the Sun’s rays striking the sure the light source is the same Northern Hemisphere or the distance from the globe as it was in Southern Hemisphere more Step 4. Turn on the light source again. directly? a) Record the temperature every c) Which do you think would be minute until the temperature stops warmer in this drawing—the changing. Northern Hemisphere or the 6. Use your observations to answer the Southern Hemisphere? Why? following questions in your d) What season do you think this is notebook: in the Northern Hemisphere? a) What is the difference in the average temperature when the Part B: What Causes the Seasons? An North Pole was pointing toward Experiment with a Globe your “community” and when it 1. Test the hypothesis you made in Part A was pointing away? about which hemisphere would be b) What caused the difference in warmer in which configuration. temperature? Find your city on a globe. Using duct tape, tape a small thermometer on it. The duct tape should cover the thermometer bulb, and the thermometer should be over the city. With a permanent black marking pen, color the duct tape black all over its 2 m surface. 2. Set up a light and a globe as shown.

Use only alcohol thermometers. Place a soft cloth on the table under the thermometer in case it falls off. Be careful not to touch the hot lamp.

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Part C: What Would Happen if the 2. Repeat the experiment you did in Earth’s Axial Tilt Changed? An Part A, except this time use an axial Experiment on Paper tilt of 35°. 1. Repeat the experiment you did in a) Compared to an axial tilt of Part A, except this time use an axial 23 1/2°, would your hemisphere tilt of 10°. experience a warmer or colder winter? a) Compared to an axial tilt of 23 1/2°, would your hemisphere b) Compared to an axial tilt of experience a warmer or colder 23 1/2°, would your hemisphere winter? experience a hotter or cooler summer? b) Compared to an axial tilt of 23 1/2°, would your hemisphere experience a hotter or cooler summer?

Part D: The Earth’s Elliptical Orbit 3. Stretch out the rope from the dowel around the Sun until it is tight, and hold a piece of chalk at the bend in the rope, as 1. Tie small loops in each end of a rope, shown in the diagram below. While as shown in the diagram above. holding the chalk tight against the 2. Pick a point in about the middle of rope, move the chalk around the the floor, and put the two loops dowel. together over the point. Put a dowel a) What type of figure have you vertically through the loops, and constructed? press the dowel tightly to the floor.

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Activity 3 How Do Earth’s Orbital Variations Affect Climate?

4. Draw a straight line from one edge b) What would be the shape of the of the circle that you just made to curve when the dowel points are the opposite edge, through the spaced a distance apart that is just center of the circle. This line equal to the length of the rope represents the diameter of the circle. between the two loops? Mark two points along the diameter, each a distance of 20 cm from the Part E: How Energy from the Sun Varies center of the circle. Put the loops of with Distance from the Sun the rope over the two points, hold them in place with two dowels, and 1. Using scissors and a ruler, cut out a use the chalk to draw a curve on square 10 cm on a side in the middle one side of the straight line. Move of a poster board. the chalk to the other side of the 2. Hold the poster board vertically, line and draw another curve. parallel to the wall and exactly two a) What type of figure have you meters from it. constructed? 3. Position a light bulb along the imaginary horizontal line that passes dowels from the wall through the center of rope the hole in the poster board. See the 20 cm diagram. center chalk poster board 10 cm x 10 cm

major axis

minor axis 2 m 2 m

5. Using different colors of chalk, make a few more curved figures in the same way. Choose sets of dowel points that are farther and farther away from the center. a) Describe the shapes of the figures you constructed. Make sketches in Avoid contact with the hot light bulb. your EarthComm notebook. Do not look directly into the light.

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4. Turn on the light bulb, and turn off the wall. What is the relationship the lights in the room. If the room is between the two numbers you not dark enough to see the image on obtain? the wall, close any curtains or shades, c) Compute the area of the image on or cover the windows with dark the wall, and compute the area of sheets or blankets. the square hole in the poster board. 5. With the chalk, trace the edge of the Divide the area of the image by the image the hole makes on the wall. area of the hole. Again, divide the distance of the light bulb from the a) Measure and record the length of wall by the distance of the poster the sides of the image you marked board from the wall. What is the with the chalk. relationship between the two b) Divide the length of the image on numbers you obtain? the wall by the length of the sides 6. Repeat Part E for other distances. of the square in the poster board. Now divide the distance of the a) What do you notice about the light bulb from the wall by the relationship between the area of distance of the poster board from the image and the area of the hole?

Reflecting on the Activity and the Challenge In this activity you modeled the tilt of You also modeled the Earth’s elliptical the Earth’s axis to investigate the effect orbit around the Sun. This will help you of the angle of the Sun’s rays. You to understand one of the main theories discovered that the axial tilt of the Earth for explaining why the Earth’s climate explains why there are seasons of the varies over time. You will need to year. You also discovered that if the tilt explain this in your newspaper articles. were to vary, it would affect the seasons.

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Activity 3 How Do Earth’s Orbital Variations Affect Climate?

Digging Deeper THE EARTH’S ORBIT AND THE CLIMATE The Earth’s Axial Tilt and the Seasons The Earth’s axis of rotation is tilted at about 23 1/2° away from a line that is perpendicular to the plane of the Earth’s orbit around the Sun (Figure 1).This tilt explains the seasons on Earth. During the Northern Hemisphere summer, the North Pole is tilted toward the Sun, so the Sun shines at a high angle overhead.That is when the days are warmest, and days are longer than nights. On the summer solstice (on or about June 22) the Northern Hemisphere experiences its longest day and shortest night of the year. During the Northern Hemisphere winter the Earth is on the other side of its orbit.Then the North Pole is tilted away from the Sun, and the Sun shines at a lower angle.Temperatures are lower, and the days are shorter than the nights. On the winter solstice (on or about December 22) the Northern Hemisphere experiences its shortest day and longest night of the year.

N 23.5˚ autumnal equinox axis of N rotation plane of orbit

N autumn summer N winter summer solstice solstice winter spring N

vernal equinox

Figure 1 The tilt of the Earth’s axis explains the seasons. Note that the Earth and Sun are not shown to scale.

How Do Earth’s Orbital Variations Affect Climate? In Activity 2 you saw that paleoclimatologists have developed a good picture of the Earth’s climatic history.The advances and retreats of the continental ice sheets are well documented. Nevertheless, questions remain.

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Why does Earth’s climate sometimes become cold enough for ice sheets to advance? Why does the climate later warm up and cause ice sheets to Geo Words retreat? The answers to these questions are not yet entirely clear. Most eccentricity: the ratio of the climatologists believe that variations in the geometry of the Earth’s orbit distance between the foci and the length of the major axis of around the Sun are the major cause of the large variations in climate.These an ellipse. variations have caused the advance and retreat of ice sheets in the past obliquity: the tilt of the couple of million years. Earth’s rotation axis as measured from the If the Earth and the Sun were the only bodies in the solar system, the perpendicular to the plane of geometry of the Earth’s orbit around the Sun and the tilt of the Earth’s axis the Earth’s orbit around the would stay exactly the same through time. But there are eight other known Sun. The angle of this tilt varies from 22.5° to 24.5° planets in the solar system. Each of those planets exerts forces on the Earth over a 41,000-year period. and the Sun.Those forces cause the Earth’s orbit to vary with time.The Current obliquity is 23.5°. Moon also plays a role.The changes are slight but very important.There are precession: slow motion of three kinds of changes: eccentricity, obliquity, and precession.These the axis of the Earth around a cone, one cycle in about three things are called the Earth’s orbital parameters. 26,000 years, due to Eccentricity gravitational tugs by the Sun, Moon, and major planets. The Earth’s orbit around the Sun is an ellipse.The deviation of an ellipse orbital parameters: any one from being circular is called its eccentricity.A circle is an ellipse with zero of a number of factors that eccentricity.As the ellipse becomes more and more elongated (with a larger describe the orientation and/or movement of an orbiting body major diameter and a smaller minor diameter), the eccentricity increases. or the shape of its orbital The Earth’s orbit has only a slight eccentricity. path. insolation: the direct or Even though the eccentricity of the Earth’s orbit is very small, the distance diffused shortwave solar from the Earth to the Sun varies by about 3.3% through the year.The radiation that is received in the difference in insolation is even greater.The word insolation (nothing to do Earth’s atmosphere or at its surface. with insUlation!) is used for the rate at which the Sun’s energy reaches the Earth, per unit area facing directly at the Sun.The seasonal variation in inverse-square law: a scientific law that states that insolation is because of what is called the inverse-square law.What you the amount of radiation found in Part E of the investigation demonstrates this.The area of the passing through a specific area image on the wall was four times the area of the hole, even though the is inversely proportional to the square of the distance of that distance of the wall from the bulb was only twice the distance of the hole area from the energy source. from the bulb. Because of the inverse-square law, the insolation received by the Earth varies by almost 7° between positions on its orbit farthest from the Sun and positions closest to the Sun. Because of the pull of other planets on the Earth–Sun system, the eccentricity of the Earth’s orbit changes with time.The largest part of the change in eccentricity has a period of about 100,000 years.That means that one full cycle of increase and then decrease in eccentricity takes 100,000 years. During that time, the difference in insolation between the date of

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Activity 3 How Do Earth’s Orbital Variations Affect Climate?

the shortest distance to the Sun and the date of the vernal farthest distance to the Sun ranges from about 2° (less equinox than now) to almost 20° (much greater than now!). The two points you used to make ellipses in Part D of summer the investigation are called the foci of the ellipse. solstice 152,500,000 147,500,000 (Pronounced “FOH-sigh”.The singular is focus.) The aphelion perihelion Sun is located at one of the foci of the Earth’s elliptical km km Sun winter orbit.The Earth is closest to the Sun when it is on the solstice side of that focus, and it is farthest from the Sun when it is on the opposite side of the orbit. (See Figure 2.) Earth Does it surprise you to learn that nowadays the Earth autumnal is closest to the Sun on January 5 (called perihelion) equinox and farthest from the Sun on July 5 (called aphelion)? That tends to make winters less cold and summers less Figure 2 Schematic diagram showing hot, in the Northern Hemisphere. occurrence of the aphelion Obliquity and perihelion. The tilt of the Earth’s axis relative to the plane of the Earth’s orbit is called Geo Words the obliquity.The axis is oblique to the plane rather than perpendicular to it. perihelion: the point In the investigation you discovered that a change in the obliquity would in the Earth’s orbit that cause a change in the nature of the seasons. For example, a smaller obliquity is closest to the Sun. Currently, the Earth would mean warmer winters and cooler summers in the Northern reaches perihelion in Hemisphere.This might result in more moisture in the winter air, which early January. would mean more snow. In cooler summers, less of the snow would melt. aphelion: the point You can see how this might lead to the buildup of glaciers. in the Earth’s orbit that is farthest from the Sun. Currently, the Earth tilt tilt reaches aphelion in 24.5˚ 22˚ early July.

Figure 3 The angle of tilt of the Earth’s axis varies between about 24.5° and 22°, causing climatic variations over time.

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Again, because of the varying pull of the other planets on the Earth–Sun system, the Earth’s obliquity changes over a period of about 40,000 years.The Geo Words maximum angle of tilt is about 24 1/2°, and the minimum angle is about 22°. Milankovitch cycles: the At times of maximum tilt, seasonal differences in temperature are slightly cyclical changes in the geometric relationship greater.At times of minimum tilt angle, seasonal differences are slightly less. between the Earth and the Precession Sun that cause variations in solar radiation received at the Have you ever noticed how the axis of a spinning top sometimes wobbles slowly Earth’s surface. as it is spinning? It happens when the axis of the top is not straight up and down, axial precession: the wobble in the Earth’s polar axis. so that gravity exerts a sideways force on the top.The same thing happens with orbital precession: rotation the Earth.The gravitational pull of the Sun, Moon, and other planets causes a about the Sun of the major slow wobbling of the Earth’s axis.This is called the Earth’s axial precession, and axis of the Earth’s elliptical it has a period of about 26,000 years.That’s the time it takes the Earth’s axis to orbit. make one complete revolution of its wobble. There is also another important kind of precession related to the Earth. It is the precession of the Earth’s orbit, called orbital precession.As the Earth moves around the Sun in its elliptical orbit, the major axis of the Earth’s orbital ellipse is rotating about the Sun. In other words, the orbit itself rotates around the Sun! The importance of the two precession cycles for the Earth’s climate lies in how they interact with the eccentricity of the Earth’s orbit.This interaction controls how far the Earth is from the Sun during the different seasons. Nowadays, the Northern Hemisphere winter solstice is at almost the same time as perihelion. In about 11,000 years, however, the winter solstice will be at about the same time as aphelion.That will make Northern Hemisphere winters even colder, and summers even hotter, than today. Milankovitch Cycles

eccentricity tilt precession combined signal average marine Early in the 20th century a Serbian (%) (degrees) index (radiation oxygen-isotope scientist named Milutin Milankovitch received) ratio 0 2 6 22.0 23.0 24.0 0.04 -0.02-0.07 -2.7 0.0 2.7 2α 0 -2α 0 hypothesized that variations in the Earth’s climate are caused by how 100 insolation varies with time and with 200 latitude. He used what is known 300 ++= about the Earth’s orbital parameters 400 (eccentricity, obliquity, and 500 precession) to compute the 600 variations in insolation. Later 700 scientists have refined the time (thousands of years ago) 800 computations.These insolation cycles are now called Milankovitch Figure 4 Interpretation of Milankovitch cycles cycles. (See Figure 4.) over the last 800,000 years.

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Activity 3 How Do Earth’s Orbital Variations Affect Climate?

Climatologists now generally agree that Milankovitch cycles are closely related to the glacial–interglacial cycles the Earth has experienced in recent geologic time. Figure 3 in Activity 2, based on a long Antarctic ice core, shows how temperature has varied over the past 420,000 years.There is a Check Your clear 100,000-year periodicity, which almost exactly matches the eccentricity Understanding cycle.Temperature variations seem to have been controlled by the 100,000- year eccentricity cycle during some time intervals but by the 41,000-year 1. Explain why the days are longer than the obliquity cycle during other time intervals. nights during the Climatologists are trying to figure out how Milankovitch cycles of insolation summer months. trigger major changes in climate.The Milankovitch cycles (the “driver” of Include a diagram to help you explain. climate) are just the beginning of the climate story. Many important climate 2. What are the three mechanisms must be taken into account.They involve evaporation, factors in Milankovitch precipitation, snowfall, snowmelt, cloud cover, greenhouse gases, vegetation, cycles? and sea level.What makes paleoclimatology difficult (and interesting!) is that 3. Explain how these factors interact with one another in many complicated ways to Milankovitch cycles produce climate. might cause changes in global climate.

Understanding and Applying What You Have Learned 1. You have made a drawing of on the wall to change if the light winter and summer in the source was moved farther away Northern Hemisphere showing the from the wall (but the cardboard tilt of Earth. was left in the same place)? a) Make a drawing showing Earth 3. The tilt of the Earth varies from on or about March 21 (the about 22° to about 24.5° over a vernal equinox). Indicate from period of 41,000 years. Think which direction the Sun’s rays about how the solar radiation are hitting the Earth. would change if the tilt was 24.5°. b) Explain why the daytime and a) What effect would this have on nighttime last the same length people living at the Equator? of time everywhere on the Earth b) What effect would this have on on the vernal equinox and on people living at 30° latitude? the autumnal equinox. c) What effect would this have on 2. In Part E of the Investigate section people living at 45° latitude? you explored the relationship d) What modifications in lifestyle between energy from the Sun and would people have to make at distance to the Sun. How would each latitude? you expect the area of light shining

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Preparing for the Chapter Challenge Use a style of writing appropriate for • How does a variation in this tilt a newspaper to discuss the following affect the nature of the seasons? topics: • How might a variation in the tilt affect global climate? • How does the tilt of the Earth’s • How does the shape of the axis produce seasons? Earth’s orbit influence climate?

Inquiring Further 1. Sunspots and global climate 2. Milutin Milankovitch Do sunspots affect global climate? Write a paper on Milutin There is disagreement over this in Milankovitch, the Serbian scientist the scientific community. Do some who suggested that variations in research to find out what sunspots the Earth’s orbit cause glacial are and how the activity of periods to begin and end. How sunspots has correlated with global were his ideas received when he climate over time. Why do some first published them? scientists think sunspot activity affects global climate? Why do some scientists think that sunspot activity does NOT affect global climate?

False color telescope image of a sunspot.

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