Chapter 11
Climate Change and Variability
2. How has the meaning of climatic change evolved since the 1960s?
Answer: In the 1960s, “climatic change” was primarily used to refer to what we would now call “climatic variability.” The meaning of climate change in the 1970s generally referred to long-term changes in climate, perhaps on the order of thousands of years. The meaning of climate change shifted during the 1980s, this time referring to differences in climatic variables from one 30-year climate period to another. By the 1990s, climatic change came to imply that humans were altering the climate through the combined processes of the combustion of fossil fuels, deforestation, land use change, and deliberate attempts to modify the climate directly. Climatic change currently continues to imply that human influence on the climate system can occur on much shorter time scales than previously thought.
4. What theory prevails regarding the cause of the K-T extinction?
Answer: The prevailing theory of the cause of the K-T extinction suggests that a comet or asteroid impact near the tip of the Yucatan Peninsula of Mexico produced massive heat, dust chemical changes, and volcanic activity. This impact altered the atmosphere and caused the extinction of perhaps 85 percent of the species on earth, including all large mammals and dinosaurs.
6. What is the significance of the Younger Dryas?
Answer: The Younger Dryas is a brief but abrupt return to cold conditions that lasted from about 13,000 to 11,500 years ago. The Younger Dryas is named after the dryas, a herbaceous tundra plant whose pollen grains in sediment and ice layers identified this unique climatic period. There is a great deal of research focused on whether the Younger Dryas was a true global event. The Younger Dryas is important because it provides concrete evidence that very rapid changes in climate have occurred in the past, and may occur again in the future.
8. How does radiometric dating work?
Answer: Radiometric dating is based on the principle of radioactive decay and is used to determine the approximate time at which the evidence was preserved. Radioactive decay is the spontaneous breakdown of a radioactive isotope (the parent) into a new element (the daughter) through the addition or release of protons, neutrons, and/or electrons. Protons and/or neutrons spontaneously leave the nuclei of radioactive elements at very precise and known rates. The half life of a radioactive element is the amount of time required for one-half of the parent atoms to decay into the daughter product. There are dozens of radioactive elements that can be used for radiometric dating. Radiometric dating is a very useful tool that provides relatively precise dates of paleoclimatological evidence.
10. What are the advantages and disadvantages of using glacial evidence as a sign of climatic changes?
Answer: Glacial evidence can provide clear clues about the paleoclimates of recently glaciated areas dating back several thousand years. The paleochemistry of the atmosphere is preserved within air bubbles trapped in ice when the ice formed. Ice cores provide the only reliable evidence for the concentration of CO2 in the preindustrial record. The scientific soundness of using ice core data is considered a major advantage. Another advantage of ice core data is that it can provide a reliable record of climate change dating back several hundred thousand years. A disadvantage to using glacial evidence is that major parts of the earth have not been glaciated in the Pleistocene Epoch. This makes it difficult to reconstruct the paleoclimates of tropical, oceanic and many midlatitude locations. Glacial evidence can also raise questions about what conditions caused the glacier to exist. Glaciers can form due to low temperatures, wet conditions or a combination of the two. The remnants of glaciers prior to the Pleistocene are likely to have been eroded away by the more recent glacial episodes.
12. What are varves and how can they be used to identify climatic changes? What pitfalls exist in the use of varve data?
Answer: Varves are annual sets of silt and clay layers that are deposited on the bottoms of lakes and ponds that freeze in winter and thaw in summer. The thickness of varves can be related to climatic fluctuations where thicker varves may indicate a longer freeze-free period. The most extensive record of varves is a German data set that extends for 23,000 years. The problem with using varve data is that varves do not exit in lakes beneath glaciers due to the requirement of annual freeze and thaw cycles. This results in a series of “missing” years in the varve data when the glacier existed over the water body.
14. What is the purpose of content analysis?
Answer: Historical data are abundant and provide detailed evidence for conditions in recent times. A disadvantage to historical data that the results are not usually quantified, making it difficult to establish consistent findings among readers. Historians often quantify written observations through content analysis to alleviate the problem. Content analysis is an objective method of converting text into discrete categories to infer climatic conditions. Recurring patterns of text can be used to gain more meaningful information that can be used to estimate past climatic conditions.
16. How is eccentricity affected by the inverse square law?
Answer: Eccentricity refers to earth’s circular orbit around the sun. The earth’s orbit is not centered on the sun and the amount of eccentricity gradually changes from a more circular to more elliptical orbit over long time periods. The inverse square law affects eccentricity because it states that the intensity of radiation is inversely proportional to the square of the distance between the emitting and receiving body. The differing distance between the sun and earth throughout the year results in the earth receiving about seven percent less insolation in July than it does in January at present. This fact implies that eccentricity is not the major cause of the seasons.
18. Why do the dates of the equinoxes and solstices change throughout geologic time?
Answer: The dates of the equinoxes and solstices change throughout geologic time due to the precession of the earth on its axis. The earth’s axis wobbles like a spinning top because the earth is not a perfect sphere. Precession refers to the axis of the earth pointing in slightly different directions over time, with a periodicity of approximately 23,000 years. The combined effect of precession and the earth’s rotating elliptical orbit slowly changes the dates of the equinoxes and solstices. In 11,500 years, the earth will experience perihelion in July and aphelion in January. In 23,000 years, the northern hemisphere’s summer solstice will again occur in late June.
20. How might increased solar output result in a decrease in global temperatures?
Answer: There is some evidence to suggest that an inverse relationship exists between solar output and temperature. Latitudinal temperature differences would increase due to increased solar output as heating in the tropics would be relatively greater than at the poles. This increased energy gradient would enhance atmospheric circulation and increase the contrast in air masses between the tropics and polar regions. The increased air mass contrast, along with warmer tropical oceans, would create more precipitation. Some of this additional precipitation would be in the form of snow or ice in high-latitude regions which would increase the surface albedo. The increase in the amount of energy reflected from the surface would cause colder polar conditions, even with a greater input of solar energy.
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