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Learn about how climate change affected life on Earth. See the video “Noah’s Snowball.” 446 Unit 4: Life Over Time Climate and Life Throughout its history, Earth’s climate has changed many times. Often the changes are gradual. They may seem small. However, an average global temperature change of just a few degrees can have a large impact on climate. Small changes in climate then cause big changes for plants and animals. Before there were humans to record events, Earth recorded its changes in its rocks and fossils. For example, scientists get a sense for Earth’s climate at different times in the distant past by looking at fossils, the remains and traces of living things. If scientists find fossils of tropical plants in places near the arctic circle, then they may conclude that the climate in those places was different in the past. Scientists have found that warmer climates lead to a greater diversity of organisms. One researcher examined fossils of tiny organisms called phytoplankton (FY-toh-PLANK-tuhn). During cooler climate periods, there were fewer types of phytoplankton than during warmer periods. The same is true for other organisms. Peter Wilf and Conrad Labandeira studied fossil plants. They were especially interested in the marks they found on the plants. The marks were left by plant-eating animals who bit the leaves. The warmer the climate was, the more types of plants there were—and the more kinds of animals were eating the plants. SOURCE: Images © 2000 AAAS The chew marks of the hispine beetle larva on living ginger in Panama (left) look similar to fossilized chew marks found in Wyoming (three photos right). Frontiers in Science 447 Before and after photographs of the sky show that distinct bands appeared due to dust and ash from the 1991 volcanic eruption of Mt. Pinatubo. Mass Extinction Turn of Events Several times in Earth’s past, many kinds of animals and plants have disappeared in a relatively short time. These events are called mass extinctions. While we don’t know for sure what causes them, most scientists think climate change plays a role in mass extinctions. What caused this extinction? Using fossils, scientists have concluded that Earth’s climate became cooler. Material from erupting volcanoes may have blocked sunlight long enough to cool the Earth. The cool temperatures and lack of sunlight may have killed plants and animals. The largest mass extinction in Earth’s history happened at the end of the Permian (PER-meeuhn) Period about 248 million years ago. Scientists estimate that 90–95 percent of animal species that lived in the water died out. About three quarters of the vertebrates, or animals with backbones, living on land died out too. Scientist Peter Ward has been studying the Permian extinction. He looked at ancient African rivers and found evidence that rivers had became clogged with soil. Plants normally holding soil in place may have been wiped out, causing the soil on the riverbanks to loosen. The plant extinction would also have led to animal extinction. View the “Noah’s Snowball” segment of your Scientific American Frontiers video to learn about another theory of how climate change affected life on Earth. IN THIS SCENE FROM THE VIDEO Fossil hunters examine evidence of early life in China. DEEP FREEZE Can you imagine what Earth would be like if it were completely covered in ice? Geologists Paul Hoffman and Dan Schrag suggest Earth was frozen solid until about 600 million 448 Unit 4: Life Over Time years ago. They think Earth’s climate changed by just a few degrees, but it was enough to make the ice caps cover the planet. The only life that survived was bacteria that were kept warm by volcanoes. And it was the volcanoes that changed the climate again, say Hoffman and Schrag. Suddenly eruptions melted the ice. Ocean levels rose. The scientists think this change might have taken only a hundred years. Not everyone agrees with the snowball hypothesis, but it could explain why new forms of life began to appear. What Hit Them? Not all scientists agree about what caused the Permian extinction. If an asteroid hit Earth, it would push massive amounts of dirt and dust into the air. This could block sunlight and create a cooler climate. An increase in volcanic eruptions is another possible cause. The most famous extinction of all took place at the end of the Cretaceous Period. The extended winter that may have followed a meteor impact caused many large land animals—including dinosaurs—to become extinct. In a new climate some species thrive and survive. They spread out and, over time, evolve to fill empty niches or unique roles in the environment. For example, before the Cretaceous extinction, the only mammals were small. After the dinosaurs became extinct, large mammals could fill the roles of large plant-eaters and meat-eaters. Even today, climate change continues. Earth’s average temperature rose about half a degree Celsius in the twentieth century. Studying how past climate changes shaped life helps scientists predict how it may affect us in the future. A large plant-eating mammal, Chalicotherium grande, roamed Asia millions of years ago. Scientists have learned a lot about climate change and mass extinctions by studying fossils. There are many questions still to be answered. • What caused changes in Earth’s climate? As you study this unit, work alone or with a group on one of the projects listed below. Use the bulleted steps to guide your project. Museum Display What organisms survived the Permian extinction? What organisms went extinct? • Create a museum display using art and text. • Use visuals to show the organisms and the modern relatives that have close connections to them. Design a Robot Often, scientists design robots to study dangerous or distant locations. • Design an artificial robot that would be well-adapted to survive an event that causes a mass extinction. • Explain why the design would help the robot remain in operation. Species over Time Find out more about species that have gone extinct during recorded history. • Choose one species that is now extinct. • Present a timeline giving a history of that species. • Describe what some of its ancestors and surviving related organisms are. • Describe when it was last seen. Include some of the possible reasons for why it died out. CAREER CENTER CLASSZONE.COM • What else might have caused mass extinctions? • How might climate change affect life on Earth in the future? Learn more about careers in paleontology. Frontiers in Science 449