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 Younger Dryas and what is its significance?
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.