CHAPTER 20 (Moran and Morgan, 1997) Why has the earth's climate undergone significant climatic change over its history? Based upon the past climatic records, several theories have been proposed that attempt to explain the possible causes of the climate variability. Essentially all theories involve changes in the energy budget of the earth-atmosphere system. Some theories are based upon changes in the receipt of incoming solar radiation (the solar constant, astronomical changes, aerosol loading of the atmosphere by volcanoes or anthropogenic air pollution). Other climatic change theories invoke changes in the terrestrial radiation associates with a change in the emissivity of the system. STUDY NOTES CHAPTER 20 Figure 20.1 -- Take several minutes to consider the various ongoing external and internal processes that can affect the climate of the earth-atmosphere system. Some of these processes are described in detail in the text. For example, the large blue arrow identifying changes in solar radiation arriving at the top of the atmosphere may be the result of changes in the solar constant due to sunspots, or due to astronomical changes. Volcanic activity is one way in which changes in the atmospheric composition could occur, resulting in changes in the atmospheric concentrations of such gases and aerosols that could change the transmission of solar and terrestrial radiation through the atmosphere. Figure 20.2 -- Inspect this diagram that provides estimates of the approximate period or characteristic time interval of a particular periodically varying forcing phenomenon. The horizontal axis is logarithmic, spanning 9 orders of magnitude, starting with a decade time span on the left and going to one billion years on the right. Note that in this diagram, the human interactions appear as a candidate for climatic change only during the last 100 to 150 years since the Industrial Revolution. Orbital variations, describing the variations in the earth's orbit around the sun and the tilt of the earth's spin axis have variations on the order of several thousand years to 100,000 years. Changes in the continent-ocean configuration and to mountain building, both associated with plate tectonics undergo longer-term changes, with periodicities ranging from nearly 100,000 years to more than 100 million years. Figure 20.5 -- A diagram that depicts the long term variation in the observed sunspot numbers. Figure 20.6 -- Changes in the earth's orbital configuration that could cause climatic changes as proposed by Milankovitch include three different effects. CHAPTER 20 (Moran and Morgan, 1997) CAUSES OF CLIMATIC VARIABILITY In this chapter, we explore the possible causes of climatic variability by examining those phenomena that could shift the radiative equilibrium of the Earth-atmosphere system. Considered are phenomena that might alter the solar constant (sunspots, astronomical changes), the planetary albedo (volcanism), or the gas and aerosol composition of the atmosphere (air pollution). Some observed and reconstructed climatic variations closely match oscillations of certain forcing phenomena. But, establishment of cause-effect relationships is made difficult by the complexity of processes operating within the Earth-atmosphere system. For this reason, it is not possible to predict the climatic future with a reasonable degree of certainty. CHAPTER OBJECTIVES After reading this chapter, the student should be able to: describe the short-term variations in solar energy output. explain how Milankovitch cycles might explain large-scale fluctuations in the Earth's glacial ice cover. identify the type of volcanic eruption that is most likely to impact climate. describe the historical trends in greenhouse gases. summarize the role of human activity in the upward trend of greenhouse gases. explain how an enhanced greenhouse effect and global warming might impact society. describe how aerosols might affect the global radiation budget. explain how properties of the Earth's surface influence climate. describe how researchers attempt to predict the climatic future. discuss the validity of various scenarios of the climatic future. 20 Causes of Climatic Variability 470 Explaining Climatic Variability 471 Climate and Solar Variability 473 Volcanoes and Climatic Variability 477 Greenhouse Gases and Climatic Variability 480 Aerosols and Climatic Variability 482 The Earth's Surface and Climatic Variability 483 The Climatic Future 483 Conclusions 495 Special Topic: Interaction of Climate Controls 484 Special Topic: Chaos and Predicting the Climatic Future 486 Mathematical Note: Radiative Equilibrium and Climatic Change 488 Key Terms 487 Summary Statements 487 Review Questions 487 Questions for Critical Thinking 489 Selected Readings 489