Earth and the Sun PDF

4.3 Earth and the Sun CAN YOU EXPLAIN IT? Gather Evidence As you explore the lesson, gather evidence to help explain how the amount of solar energy reaching Earth changes over time and how these changes can affect Earth’s global climate. FIGURE 1: Between 850 and 630 million years ago, Earth may hav...

4.3 Earth and the Sun CAN YOU EXPLAIN IT? Gather Evidence As you explore the lesson, gather evidence to help explain how the amount of solar energy reaching Earth changes over time and how these changes can affect Earth’s global climate. FIGURE 1: Between 850 and 630 million years ago, Earth may have been almost completely covered in ice. About 10% of Earth’s surface today is covered in ice. Massive ice sheets cover most of Antarctica and Greenland, while sea ice covers the Arctic Ocean. Glaciers are also found in temperate and even tropical latitudes, but only at very high elevations where the air is significantly cooler than it is at sea level. Hundreds of millions of years ago, however, conditions may have been very different. Evidence for glaciers at sea level near the equator suggests that Earth was much colder than it is today. Why would Earth have been significantly colder in the past? Infer Where do you find ice sheets on Earth today? What are conditions like in these places? Use evidence to make an inference about Earth’s conditions 700 million years ago, when ice sheets and glaciers where found near the equator. 206 Unit 4 Earth in the Solar System Image Credits: (t) ©Louise Murray/Alamy; (b) ©Chris Butler/Science Source Earth and the sun interact through energy. EXPLORATION 1 The Earth-Sun System Think about the last time you went outside. How does the interaction between Earth and the sun affect your life? Most of what we see outside during the daytime is visible because of sunlight. The warm air, wind, and rain that we feel exist because of the sun’s energy. In addition to light, the sun also emits streams of charged particles called the solar wind. Earth’s magnetic field—which originates in Earth’s core—exerts a force on those particles, causing them to deflect toward the poles. There they interact with the gases in Earth’s atmosphere, causing the greenish-purplish glow of the aurora. The aurora is an example of the many interactions in the Earth-sun system. FIGURE 2: An aurora, as seen from the International Space Station in 2012, is the result of the interaction between the solar wind and Earth’s magnetic field and atmosphere. Earth-Sun System Components Analyze Think about your everyday experiences. How are they affected by your interactions with the sun or by Earth’s interactions with the sun? Explore Online Hands-On Lab Image Credits: ©Science Source Suppose you travel back in time to Earth’s past and find yourself on a planet that is covered in ice—snowball Earth! How would you go about understanding this frozen version of Earth? You learned that in science it is useful to think of events or phenomena as occurring within a system. Large-scale changes to Earth may involve many of its systems. A change that affects Earth’s temperature may also involve Earth-sun system interactions. Earth-Sun Motion Design an experiment to measure the movement of Earth. Matter and energy are components of the Earth-sun system. Most of the matter and the energy in the system is concentrated in the sun. Composed mostly of hydrogen and helium atoms, the sun is 330 000 times as massive as Earth and has a volume at least 1 300 000 times as great as Earth’s. Though matter is concentrated in the sun, Earth has a greater average density. Earth has an average density more than five times as great as the average density of the sun. It has a thin atmosphere made up mostly of nitrogen and oxygen, a solid surface that is largely covered in water, rock, ice, and living things, and a dense metallic core made of nickel and iron. Earth is orbited by the moon, a small rocky body without a significant atmosphere. Lesson 3 Earth and the Sun 207 The Earth-sun system includes not only Earth and the sun and the materials they are made of, but also the solar energy emitted from the sun, the gravitational forces keeping the objects close together and moving in their orbits, and the processes that are affected by sunlight and gravity. Every 365 days, Earth completes one orbit around the sun. Earth’s orbital motion is important, because it ensures that Earth stays at approximately the same distance from the sun throughout the year and receives a steady supply of energy from it. Although the total amount of energy that Earth receives does not change significantly throughout the year, the way that energy is distributed on the surface does vary. Approximately once every 24 hours, Earth completes one rotation on its axis. This rotation results in the cycle of day and night. Earth’s axis is not perpendicular to its path around the sun. Instead, it is constantly pointed toward Polaris—the North Star. As Earth orbits the sun, the orientation of its axis stays the same relative to Polaris, but it changes relative to the sun. In January, for example, the North Pole is pointed away from the sun, while in July it is pointed toward the sun. Earth-Sun-Moon System Interactions Interactions between Earth and the sun occur mainly through gravity and energy. The gravitational effects of the sun on Earth’s surface are very difficult to notice. These effects are easier to observe when the moon is included as part of this system. FIGURE 3: The force of gravity and inertia keeps the moon in orbit around Earth and the Earth-moon system in orbit around the sun. FIGURE 4: Although the moon’s gravity is the main reason for tides on Earth, the sun’s gravity also has an effect. Earth sun spring tides FIGURE 5: Extreme low and high tides occur when the effects of the gravitational pull of the sun and the moon are added together. moon (full) moon (new) not to scale neap tides moon (3rd quarter) Earth sun moon (1st quarter) not to scale The gravitational force between Earth and the sun is, in part, responsible for Earth’s motion in space. At any given time, a planet is moving through space in two directions: straight forward and straight toward the center of the sun. Where do these two motions come from? A planet’s forward motion is a result of its inertia, the tendency to keep moving as it has been moving since it formed. The motion toward the sun is a result of the gravitational force between the sun and the planet. The planet continuously accelerates toward the sun. It doesn’t fall into the sun because of its inertia. Explain How does gravity affect the motion of Mercury relative to the sun? How is this effect similar to and different from the effect of gravity on the motion of the Earth-moon system around the sun? Earth’s orbit is not perfectly circular. As a result, the distance between Earth and the sun varies slightly throughout the year. In January, Earth is about 5 million km closer to the sun than it is in July. Because the gravitational pull between two objects increases with decreasing distance, objects orbit faster when they are closer. In January, Earth moves through space slightly faster than it does in July. Image Credits: ©irisphoto1/Fotolia; ©Louise Murray/Alamy Energy Solar energy travels through space in the form of electromagnetic radiation. Of the light emitted by the sun, 41% is visible light, another 9% is ultraviolet light, and 50% is infrared radiation. FIGURE 6: Light is emitted by the sun and then absorbed, reflected, refracted, and radiated by materials on Earth’s surface and in the atmosphere. Lesson 3 Earth and the Sun 209 FIGURE 7: Earth’s surface energy budget The total amount of energy reflected and emitted by Earth is equal to the total amount that reaches Earth from the sun. 16 units absorbed by water, vapor, dust, ozone, and carbon dioxide 6 units scattered by air 100 units incoming solar radiation 30 total units reflected and scattered 20 units reflected by clouds 19 total units absorbed by the atmosphere 4 units reflected by water and land 3 units absorbed by clouds 51 units absorbed by water and land 70 units radiated as longwave radiation When solar energy reaches Earth, it interacts with the atmosphere and surface. Some is reflected off clouds, land, water, and ice. Earth is visible from space because of the sunlight that reflects off it. Light that is not reflected is absorbed by rock and water on the surface and by gases in the atmosphere. Once light is absorbed, it causes the material to heat up. The ground is hotter during the day than at night, because it absorbs sunlight. As a material heats up, it emits energy in the form of invisible infrared radiation. Earth’s surface radiates infrared energy out toward space, but some is absorbed by Earth’s atmosphere. Math connection Calculate The albedo of a surface is a measure of how reflective it is. To calculate the albedo, divide the amount of solar energy reflected by the surface by the total amount of solar energy that reaches Earth. What is Earth’s albedo? Overall, the amount of solar energy that reaches Earth from space is balanced by the amount that is reflected and radiated back to space. However, gases in the atmosphere, known as greenhouse gases, absorb and give off infrared radiation. As a result, Earth’s atmosphere absorbs some of the outgoing radiation and keeps it in the Earth system for a while, which raises Earth’s surface temperature. This process is called the greenhouse effect. Without the greenhouse effect, much of Earth’s heat energy would be lost almost immediately to outer space. Earth’s average surface temperature would be about 33ºC cooler than it is now. The greenhouse effect has helped Earth thrive as a planet. Recently, however, there has been such a significant increase in the levels of carbon dioxide in the atmosphere that Earth’s energy budget may be out of balance. Many scientist warn of the possibilities of global climate change. Predict Think about how light moves through different materials. What are some factors that could change the amount of sunlight absorbed by Earth’s surface and atmosphere? 210 Unit 4 Earth in the Solar System EXPLORATION 2 Solar Energy in Earth’s Systems Energy is continuously moving outward from the sun in all directions. The amount of energy that reaches Earth does not change significantly from hour to hour, day to day, or even month to month. In spite of this, however, we can feel differences from place to place and from season to season. Energy in Systems A diagram of Earth’s energy budget is a model showing the ways that energy moves to and from Earth. What this model does not show is how solar energy flows within Earth’s systems. It does not show all the ways that the flow of solar energy influences Earth’s surface—how it affects the atmosphere, the hydrosphere, lithosphere, or the biosphere. When we study the importance of solar energy to Earth systems, it is useful to examine what happens when the amount of solar energy in a location changes. Image Credits: ©Roman Mikhailiuk/Shutterstock FIGURE 8 : Snow melts and flowers start to bloom in the late winter and early spring. Explain Do you agree or disagree with the following statement? Without the sun, there would be no changes in the weather on Earth. Use reasoning to support your argument. You learned that when solar energy reaches Earth, it interacts with matter on the surface and in the atmosphere. While the amount of energy emitted by the sun is nearly constant, its distribution over Earth’s surface changes. During the spring, the amount of energy reaching a particular part of Earth increases. With more direct sunlight and more hours of daylight, the amount of energy absorbed by the ground increases. The ground heats up more and emits more energy, warming the air above. The warmer air, along with the increase in intensity of light and the switch from snowfall to rain, causes the snow to melt. The meltwater seeps into the ground. With less snow reflecting the sunlight, more light is absorbed by the ground. The warmth and water trigger the growth of spring flowers. Once the plants are above ground, their leaves use the sunlight and water during the process of photosynthesis, converting sunlight into chemical energy within the plant. Animals such as deer and squirrels eat the flowers, using the stored sunlight in them to live and grow. Lesson 3 Earth and the Sun 211 FIGURE 9: Solar energy drives the water cycle and the global wind systems. precipitation 90°N condensation cloud particles 60°N water vapor evaporation 30°N evaporation transpiration 0°N run off 30°S run off 60°S groundwater a b Water changes phases as it moves through the water cycle. These changes are driven by energy from the sun. Collaborate With a partner, explain how each part of the water cycle would be affected if the amount of solar energy reaching the surface in a given place changed. Use reasoning to support your explanations. 90°S Uneven heating of Earth’s surface results in global winds. Rising warm air and sinking cool air currents (red and blue arrows) form patterns that produce surface winds (white arrows). Sunlight has a profound effect on individual Earth’s systems. It is a major factor in Earth’s global cycles and processes. Sunlight provides the energy that causes water to change form as it moves through the water cycle. Solar energy drives local and global wind patterns, which develop as the sun heats up parts of Earth more, or more quickly, than other parts of the planet. Differences in weather from place to place, and changes in weather from day to day, are also a result of differences in the way sunlight interacts with Earth’s different surfaces over distance and time. Because the sun is the main source of energy for living things on Earth, sunlight is also key in the cycling of carbon and oxygen between the atmosphere and living things. Plants, for example, use more oxygen in spring and summer when the length of day and sunlight intensity increases. Distribution of Solar Energy on Earth’s Surface If you live in a region with distinct seasons, you are familiar with the cyclic pattern of temperature, precipitation, and daylight change that occurs during the year. These changes are a result of Earth’s shape, the tilt of its axis, and its orbit around the sun. north pole FIGURE 10: Earth’s shape and axial tilt affect the amount of energy received by different regions of the planet. N 80° N 60° N 40° sunlight sunlight ) 5° N (23. cer n a C ic of Trop N 20° equator (0°) S) 3.5° n (-2 r o c pri N 20° ic of Ca Trop S 40° S 60° S 80° south pole 212 Unit 4 Earth in the Solar System direction of Earth’s rotation Therefore, the total amount of solar energy that tropical regions receive is significantly greater than that of polar regions. This difference in energy distribution affects climate and drives the movement of winds and ocean currents. Because Earth’s tilt relative to the sun changes as it orbits the sun, the concentration of sunlight in any given area changes throughout the year. This results in seasons. During the June solstice, when the North Pole is tilted toward the sun, the sunlight is most intense at 23.5°N of the equator. The Northern Hemisphere experiences the warmer temperatures and longer days of summer, while the Southern Hemisphere experiences winter. During the December solstice, when the North Pole is tilted away from the sun, sunlight falls most directly at 23.5°S of the equator. The Northern Hemisphere experiences lower temperatures and shorter days of winter, while the Southern Hemisphere experiences summer. During the spring and fall equinoxes, in March and September, respectively, Earth’s axis is not tilted away or toward the sun, and sunlight is most intense at the equator. All areas of Earth receive 12 hours of daylight, and there is no difference in the amount of energy received by either hemisphere. Seasons in the Northern Hemisphere FIGURE 11: Seasons in the northern hemisphere spring winter N N June solstice fall days are longer and the weather is warmer in South America than in North America. Explain this difference in terms of interactions between Earth and the sun. N March equinox summer Explain In January, the N December solstice September equinox not to scale not to scale The effect of seasonal changes on temperature and daylight are most dramatic near the poles and least dramatic near the equator. In March and September, both poles experience 12 hours of daylight. But in December, the North Pole is pointing away from the sun at such an angle that the region does not experience any hours of daylight whatsoever. At the same time, the South Pole is pointing toward the sun at such an angle that it experiences a full 24 hours of daylight. This situation is reversed in June. At the equator, however, the difference in intensity of light and number of daylight hours changes much less throughout the year than it does at the poles. Explore Online Hands-On Lab Positions of Sunrise and Sunset Collect and analyze data describing the positions of sunrise and sunset, and then make predictions for future months. Model Make a sketch that shows why the intensity of sunlight differs from place to place because Earth is spherical. Use the sketch to show why areas that are tilted directly toward the sun receive more solar energy than those that are not. Lesson 3 Earth and the Sun 213 EXPLORATION 3 Earth-Sun System and Climate Change Studying the current conditions of the Earth-sun system helps us understand Earth’s daily and seasonal changes in weather, differences in climate from place to place, and Earth’s global climate conditions in general. However, evidence in the rock and fossil record suggests that Earth’s global climate has changed significantly in the past. Earth has experienced much cooler and much warmer periods. The current variations of the Earth-sun system cannot explain these changes. Could solar radiation, Earth’s tilt, and its orbit have changed over time, and if so, could these factors explain changes in Earth’s climate? Solar Variability Since 1978, scientists have used satellites to measure the amount of sunlight that reaches the top of the atmosphere. Measuring sunlight away from Earth’s surface allows us to see patterns that are related to the amount of energy given off by the sun, rather than daily and seasonal patterns related to Earth’s rotation and orbit. For centuries, scientists have known that the amount of energy emitted by the sun changes over time. It fluctuates on a cycle of about 11 years. For example, in 1999 the sun emitted about 0.1% more energy than in 1996, but about the same as it did in 1988. It turns out that the amount of energy that the sun emits is related to sunspot activity. Sunspots are darker spots within the sun’s bright surface. The number of sunspots visible each month varies between almost 0 to nearly 200. In general, the more sunspots there are, the more energy is being emitted by the sun. This relationship between sunspots and solar energy is very useful. We have only a few decades of actual measurements of solar energy, but we have more than 400 years of scientific observations of sunspots, beginning with Galileo in 1610. We can therefore use historical records of sunspots to infer changes in solar energy. relationship between sunspot activity and average change in air temperature between 1860 and 1960. What explains this relationship? Sunspot Activity and Change in Air Temperature, 1860–2000 FIGURE 12: Plots showing sunspot activity and solar energy reaching Earth’s upper atmosphere Sunspot Activity and Earth Surface Temperature 0.4 Temperature anomaly (˚C) Analyze Describe the 0.2 0 –0.2 –0.4 Sunspot number –0.6 100 75 50 25 0 1860 1880 1900 Source: Stanford Solar Center Source: Stanford Solar Center 214 Unit 4 Earth in the Solar System 1920 Year 1940 1960 1980 2000 How do changes due to sunspot activity affect Earth’s climate? By comparing graphs of sunspot cycles and average global temperature change over time, it becomes apparent that even a small change in energy emitted by the sun does affect Earth’s climate. Perhaps the most dramatic example is known as the Maunder Minimum. Between 1645 and 1700, there were very few sunspots. This corresponded to a particularly cold period in Europe, part of a period known as the Little Ice Age. In addition to the 11-year sunspot cycle, the sun appears to go through other longer period cycles as well. As the sun ages, it is becoming hotter and brighter. However, this change is very slow. Predict Suppose the sun went through several decades of very low sunspot activity. How could this affect Earth’s surface? Support your prediction with evidence and reasoning. Changes in Earth’s Motion The amount of energy that reaches Earth depends not only on the amount of energy emitted by the sun, but also on changes in Earth’s motion in space. FIGURE 13: Earth’s orbit can be more or less circular. Its eccentricity, or how much it deviates from a circle, changes over periods of about 100 000 years and 413 000 years. Seasons in the Northern Hemisphere 11 000 years ago 24.5° 21.5° 21.5° 24.5° fall N summer N December solstice spring Earth’s Earth’s axial tiltAxial Tilt 41 000-year 41 000-year cycle cycle N September equinox winter FIGURE 14: Earth’s tilt varies between 21.5° and 24.5° over a period of about 41 000 years. N June solstice Currently 23.5° currently 23.5° March equinox not to scale At its closest point, Earth is 147.1 million km from the sun. At its farthest point, it is 152.1 million km from the sun. As a result, about 7% more solar energy reaches Earth in January than in July. Over time, however, Earth’s orbit becomes more elliptical and less elliptical, or eccentric. As a result, the difference between its closest and farthest approach to the sun changes. When Earth’s orbit is more circular, Earth spends more time closer to the sun. When Earth’s orbit is more eccentric, Earth spends more time farther from the sun. Analyze How might changes in eccentricity affect the amount of solar energy reaching Earth? Changes in the eccentricity of Earth’s orbit can affect the differences between seasons. For example, if Earth is closer to the sun during the summer, summer will be slightly warmer. Eccentricity can also affect the total amount of energy received from the sun. When Earth’s orbit is more circular, it receives slightly more solar energy during the year than when it is more elliptical. This results in slight fluctuations in temperature over tens of thousands of years. Lesson 3 Earth and the Sun 215 Explore Online FIGURE 15: The direction that Earth’s axis points in space changes slowly over time. Earth is currently tilted about 23.5° relative to its orbital plane. However, about 10 000 years ago Earth’s tilt was about 24.5°, and 30 000 years ago its tilt was about 22.2°. The more Earth’s axis is tilted, the greater the differences between seasons—winters are colder and summers are warmer. The smaller the tilt, the less the weather changes from season to season. Scientists think that axial tilt affects global climate—not just seasonal changes. When the tilt is smaller, less winter snow melts during the cooler summers. This can result in the expansion of glaciers and ice sheets. Presently, Earth’s axis points toward Polaris, the North Star. Over a period of about 26 000 years, however, the axis itself rotates. This motion is called precession, and it is similar to the wobble of a spinning top. Because of precession, the direction that Earth’s axis points in space changes, which affects the timing of the seasons. Today, for example, summer in the Southern Hemisphere occurs when Earth is closest to the sun. However, 11 000 years ago, Earth’s axis pointed in the opposite direction, just as it does today, and summer occurred in the Northern Hemisphere when Earth was closest to the sun. Model Make a sketch to show what Earth would look like at different points in its orbit if its axis were pointing to a different star in space. How might this change affect seasons on Earth? Use reasoning to support your claim. Independently, the effect on seasons or global climate of each of these changes in Earth’s motion in space—its eccentricity, tilt, and precession—is clear. But because all of these changes occur at the same time, understanding the combined effect is complicated. Scientists use mathematical and computer models to understand the combined effects of Earth’s motion in space. Explain Earth has gone through many cycles of ice ages followed by periods of warmer climate. How might these cycles be related to patterns of solar output and patterns of change in Earth’s motion in space? Evidence of Past Climate Evidence shows that over thousands of years, Earth has gone through many glacial periods. These periods are separated from each other by interglacial periods—periods of warmer weather, melting of ice sheets, and rises in sea level. But how can we determine the timing of glacial and interglacial periods? Milankovitch Cycles and Temperature from the Vostok Ice Core Source: NOAA data 216 Unit 4 Earth in the Solar System 0 500 490 480 470 460 450 440 430 420 410 400 390 380 Source: NOAA data Image Credits: ©Robert Simmon/NASA Goddard Space Flight Center 3 2 1 0 –1 –2 –3 –4 –5 –6 –7 –8 –9 –250 000 –200 000 –150 000 –100 000 –50 000 Years relative to present Temperature Solar Power— 65º N in July Watts/m2 at 65º N in July FIGURE 16: Geologists compared temperature data to Milankovitch models to test the hypothesis that icePower ages are a result of changes in Earth’s motionininIce space. Solar Model and Surface Temperature Recorded Cores Temperature (˚C) Analyze Describe the relationship between solar energy reaching the Northern Hemisphere in July to average air temperature in Antarctica. What could explain the relationship? In the early 20th century, Serbian scientist and engineer Milutin Milankovitch developed a model to show how a combination of changes in Earth’s motion would affect solar radiation reaching the Northern Hemisphere and, therefore, Earth’s glaciation patterns. When Milkankovitch developed his model, it was difficult to put precise dates on when each glacial and interglacial period occurred. Image Credits: (tl) ©Justin Sullivan/Getty Images North America/Getty Images; (tr) ©Jgz/Fotolia; (bl) ©Karim Agabi/Science Source; (br) ©qualitygurus/iStock/Getty Images Plus/Getty Images In the 1970s, scientists developed new methods for estimating global temperatures using natural recorders of climate variability, such as tree rings, fossils, and glacial ice cores. Scientists study the texture and composition of layers in ice cores to infer the timing and duration of glacial and interglacial periods. The graph in Figure 16 combines data from Antarctic ice cores with calculations of solar radiation to show global changes in Earth’s temperature over time. In order to accurately time interglacial and glacial periods, scientists study the ratio of different types of oxygen in ice cores and in the shells of marine fossils. They also measure the thickness of annual growth rings in old and fossilized trees to determine past climate conditions. V Patterns FIGURE 17: Patterns in tree rings, ice cores, layers of sedimentary rock, and locations of rocks transported by glaciers all reveal that Earth has gone through cycles of cooler and warmer climates. a Tree rings reveal information about climate when the tree was growing. b Rock layers contain fossils that can be used to figure out ocean temperatures. Pattern Evidence In order for an idea to be accepted as scientific theory, it must not only make sense theoretically and mathematically, it must also be supported by an abundance of evidence. Milankovitch’s idea that patterns of changes in Earth’s motion cause patterns of change in Earth’s climate is supported by many observations and measurements from many different places on Earth. Evaluate Which of the c Ice cores can be used to infer snowfall rates and air temperature. d Glacial erratics are evidence that ice once covered an area. types of evidence shown in the images are most useful for constructing a record of climate change over thousands or millions of years? Use reasoning to support your response. Predict How could you use an understanding of Earth’s motion in space to predict changes in global climate over the next few hundred thousand years? How could it be possible to predict future climate with complete accuracy? Lesson 3 Earth and the Sun 217 Continue Your Exploration Data Analysis Cycles of Glaciation In the early 20th century, Milutin Milankovitch developed a model to test his hypothesis that cycles of glacialinterglacial periods in Earth history could have been caused by changes in Earth’s motion in space. The result was a graph similar to the graph at the bottom of Figure 18, which shows a clear pattern of increasing and decreasing amounts of incoming solar radiation—insolation—reaching the Northern Hemisphere in July. These periodic changes in Earth’s orbit are now known as Milankovitch cycles, and they correlate strongly to patterns of glacial and interglacial periods on Earth. 1. Over the past 600 000 years, when was Earth’s orbit most elliptical? When was it least elliptical? Use the graphs, along with what you have already learned in this lesson, to answer the following questions. 3. Over the past 600 000 years, when was Earth’s axis tilted most? When was it tilted least? 4. Assuming Earth’s orbit is the same as it is today, how does tilt affect the amount of energy reaching the Northern Hemisphere in July? Milankovitch Cycles 5. The day of perihelion is the day when Earth is closest to the sun. Describe how the day of perihelion changes over time. 0.06 0.03 0 6. How does the day of perihelion affect climate? 25 23 7. How would you describe the way that eccentricity, tilt, and the time of perihelion together seem to affect the amount of energy reaching the Northern Hemisphere in July? Earth closest to the sun 21 Northern summer insolation (watts/m2) Tilt (degrees) Eccentricity FIGURE 18 Combined changes eccentricity, tilt, and precession affect Earth'sinOrbital Cycles axial and Solar Energy seasonality and the difference between seasons. Sept June March Dec Sept 600 Explain Why is the graph of northern summer insolation not a perfectly regular pattern? 500 400 –600 2. How does ellipticity affect the total amount of solar energy that reaches Earth? –500 –100 –400 –300 –200 Thousands of years before present 0 Source: NOAA data Language Arts Connection Research how variations in Earth’s orbit determine the intensity of sunlight that falls in the far North during the summer months and how weak summer sunlight over a period of years can cause snow accumulation that can lead to ice ages. Present your findings in the form of an infographic, a poster presentation, or a slide-show presentation. MILANKOVITCH CYCLES ON MARS 218 Unit 4 Introduction to Earth and Space CLIMATE FEEDBACK Go online to choose one of these other paths. EVALUATE Lesson Self-Check CAN YOU EXPLAIN IT ? FIGURE 19: Between 850 and 630 million years ago, Earth may have been almost completely covered in ice. Ice sheets currently cover most of Antarctica and Greenland. The North Pole itself is an ocean, which is almost permanently covered in sea ice. Glaciers are also found in temperate and even tropical latitudes. Glaciers like these, including the Furtwängler Glacier located almost on the equator at the summit of Mt. Kilimanjaro, are found at very high elevations, where the air is significantly cooler than it is at sea level. Image Credits: ©Chris Butler/Science Source It is not surprising that the geologic record holds evidence for the existence of glaciers and ice sheets in the past. Twenty thousand years ago, for example, a vast ice sheet extended as far south as New York and Pennsylvania. The evidence for these ice sheets, along with vast quantities of other evidence, indicates that at the time Earth was about 5 °C cooler than it is today. Evidence from sedimentary rocks and fossils also shows that Earth has gone through major changes in climate over the past 600 million years. What is surprising, however, is the evidence that earlier in Earth history—around 700 million years ago—the entire Earth could have been covered in ice. Geologists refer to this possible period as the Snowball Earth. What could cause Earth to become covered in ice? What caused other major changes in climate shown in the rock and fossil record? Geologists think that changes in climate are a result of a combination of factors, including changes in energy radiated by the sun, changes in Earth’s orbit and tilt, changes in the composition of the atmosphere, episodes of volcanism and mountain building, and changes in the locations of continents and oceans. Explain What conditions affect how warm Earth’s surface is? What could have been different 700 million years ago that resulted in more of Earth’s surface being covered in ice? Lesson 3 Earth and the Sun 219 EVALUATE CHECKPOINTS Check Your Understanding 1. A system can be described by its components and the processes that occur within it. 5. Organize the following statements into four cause-andeffect pairs. Earth’s tilt increases. Sunspot activity increases. The timing of the seasons changes. Identify the components of matter, energy, and force in the Earth-sun system. More radiation is emitted by the sun. a. matter:_______ Earth receives slightly less solar radiation. b. energy:_______ Seasonal differences in weather are greater. c. force:_______ Earth’s orbit becomes more eccentric (more elliptical). 2. Give examples of processes that occur within the Earthsun system: a. process that involves energy only: ________________ b. process that involves interaction between force and matter: ________________ c. process that involves interaction between energy and matter: ________________ 3. Which of the following statements accurately describe interactions in the Earth-sun system? Choose all that apply. a. Without gravity, Earth would not move at all through space. Earth’s axis and orbit precess (wobble). 6. For each change, identify the timescale over which the change occurs: hours, months, years, thousands of years, or billions of years. a. Earth rotates on its axis. b. The tilt of Earth’s axis changes. c. The sun gets hotter as it evolves. d. The shape of Earth’s orbit changes. e. The orientation of Earth’s axis changes as it orbits the sun. f. The number of sunspots observed increases and decreases. b. Different regions of Earth receive different amounts of solar energy. 7. Scientists think that between 1645 and 1715, the sun went through a period of emitting less solar energy. The image below shows Earth’s energy budget today. c. As Earth warms up, the amount of energy emitted by the sun increases, decreasing volcanic activity on Earth’s surface. FIGURE 20: Earth’s surface energy budget. d. If Earth were not tilted, it would be heated evenly over the entire surface. e. Energy absorbed by Earth’s surface flows into Earth’s interior to keep it warm. f. The total amount of energy that reaches Earth depends on its distance from the sun. 4. A student wants to explain why polar climates are cooler than tropical climates. Describe a 3D model that the student could make out of simple materials in order to explain this. Explain what each part of the model represents and how the student would use the model to explain differences in climate. 16 units absorbed by water, vapor, dust, ozone, and carbon dioxide 100 units incoming solar radiation 6 units scattered by air 30 total units reflected and scattered 20 units reflected by clouds 19 total units absorbed by the atmosphere 4 units reflected by water and land 3 units absorbed by clouds 51 units absorbed by water and land 70 units radiated as longwave radiation Describe three ways that the energy budget diagram for 1645–1715 would be different. 220 Unit 4 Solar System Formation 8. How and why could a change in solar output affect Earth’s water cycle? 15. What aspect of Earth’s orbit changes in cycles of 23 000 years? Of 41 000 years? of 100 000 years? 9. Which of the following are most likely causes of glacial periods over the past 2 million years? Choose all that apply. 16. What is the variation in the periodicity of long-term changes in eccentricity? a. 25 700 years a. decrease in tilt of Earth’s axis b. 100 000 and 413 000 years b. increasing eccentricity of Earth’s orbit c. 41 000 years c. decreasing rate of fusion in the sun over time d. 23 000 years d. decreasing volcanic activity on Earth’s surface e. increasing difference between high and low tides f. decreasing force of gravity between Earth and the sun g. increasing frequency of comets entering the inner solar system 10. Which of the following are Milankovitch cycles? Choose all that apply. 17. What would be consequence for incoming solar radiation and the seasons if Earth had no tilt? a 90° tilt? 18. In approximately 13 000 years, Earth’s axis will point toward the star Vega. Twenty-six thousand years from now, where will Earth’s axis be pointing? a. eccentricity b. sunspot cycle c. precession d. axial tilt MAKE YOUR OWN STUDY GUIDE 11. Explain how changes in the tilt of Earth’s axis can affect the differences between seasons. 12. At what time of year is the North Pole in complete darkness? a. during the June solstice b. during the spring equinox c. during the fall equinox d. during the December solstice 13. When it is summer is Australia, it is a. fall in the United States b. winter in the United States c. spring in the United States d. summer in the United States 14. Which of the following methods would scientists use to determine global changes in Earth’s climate over time? a. by examining ice cores b. by examining tree rings c. by examining sedimentary strata d. by examining glacial eccentrics In your Evidence Notebook, design a study guide that supports the main ideas in this lesson: The sun and Earth are part of a system of interacting components of matter and energy. Almost all of Earth’s surface energy originate as sunlight. Sunlight affects different parts of Earth in different ways depending on latitude, time of year, and surface characteristics. Gravity holds Earth in orbit around the sun, ensuring that a steady supply of energy reaches Earth. Changes in the amount of energy emitted by the sun and the amount reaching Earth in total and at different times of year can affect Earth’s global climate. Remember to include the following information in your study guide: Support main ideas about Earth-sun interactions with specific examples. Record explanations for patterns in the interactions between Earth and the sun. Evaluate evidence for the effects of changes in the Earth-sun system over time. Lesson 3 Earth and the Sun 221