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Read the following scenario and answer the question below. …

Read the following scenario and answer the question below.  Many geologists have proposed naming the current time period of Earth’s history the Anthropocene epoch. The rationale for doing so includes the fact that erosion rates and greenhouse gas levels in the atmosphere have increased rapidly over the past 300 years. Almost all environmental scientists agree that increases in greenhouse gases contribute to global climate change. Carbon dioxide, methane, nitrous oxide, hydrochlorofluorocarbons, and water vapor are the main culprits. Human activities, chief among them the extraction and burning of fossil fuels for energy, have significantly increased these “greenhouse gases” in our atmosphere in the last 300 years. With rising standards of living in developing countries, emissions of carbon dioxide and other greenhouse gases are expected to continue to rise. If unchecked, carbon dioxide levels will reach twice preindustrial levels by midcentury and double again by the end of the century. Computer models have shown that this rise alone could raise Earth’s temperatures by 3 to 10 degrees Fahrenheit by 2100. Preindustrial levels of atmospheric carbon dioxide have been estimated to be approximately 275 ppm (parts per million). If unchecked, carbon dioxide levels are expected to reach ________ ppm by the year 2100.