Monday, December 4, 2017 The Pleistocene Glaciations (Chapter 14) Week 14 Assessment, closes Wednesday Dec 6

Transcription

1 Monday, December 4, 2017 The Pleistocene Glaciations (Chapter 14) Week 14 Assessment, closes Wednesday Dec 6 Week 15 Assessment will be last one, closes Wednesday Dec 13 Homework 5 due Wednesday, Dec 6

2 Monday, December 4, 2017 The Pleistocene Glaciations (Chapter 14) Evidence for glaciations (p ) Milankovitch cycles (p ) Introduction to Feedbacks important for glacials/interglacials (p )

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4 Sample question (we didn t use clickers for this one) Which of the following is probably not a likely explanation for producing conditions that would result in an ice age on Earth? (a) Reduced methane from reaction with oxygen (b) Increased weathering of rocks (c) Massive biomass burning (d) Reduced solar illumination in northern hemisphere summer (e) Increased organic carbon burial

5 Which of the following is probably not a likely explanation for producing conditions that would result in an ice age on Earth? (a) (b) (c) Reduced methane from reaction with oxygen (would cool Earth, leading to ice age) Increased weathering of rocks (would reduce CO2 in atmosphere, leading to cooling) Massive biomass burning (would put CO2 in atmosphere, warming planet - correct answer!) (d) Reduced solar illumination in northern hemisphere summer (tricky this one allows ice from winter to remain at the end of summer, allowing an ice sheet to grow!) (e) Increased organic carbon burial (removes CO2 from atmosphere)

6 Let s examine answer (d). For the past 500,000 years and longer, ice ages have occurred with regularity that is explained by changes in Earth s orbit. Abundances of greenhouse gases like CO 2 and methane (CH 4 ) have tracked the temperature changes CO 2 T CH 4

7 The most recent ice ages occurred in the Pleistocene 1.8 mya- 10,000 ya (Chapter 14). This represented a new era of glaciations that are timed closely to changes in Earth s orbit glacials (cold periods) and interglacials (warm periods) are an example of a two-state climate system Figure 14-9

8 There are many lines of evidence (called proxies ) for a progression of cold periods and warm periods where a large ice sheet grew in the Arctic, covering large land masses, and reducing sea levels by 100 s of feet. When this ice sheet melted, it inundated coastlines, forcing humans to migrate to dry land. A good example of this occurred in Vietnam, and is part of their history, as villages were located on coastlines, which disappeared 14-16,000 years ago as the large ice sheet in the Arctic melted and sea levels rose.

9 Key to Pleistocene ice ages northern hemisphere summer i.e. can you melt the snow from the previous winter? If no, then ice sheet grows. If yes, ice sheet shrinks Figure 14-1

10 What is the evidence for these glacial periods? Morraines (piles of rocks and boulders) Where remnants of ice sheets are currently melting Where great ice sheets once dominated the landscape

11 U-Shaped Valleys

12 Glacial Deposits Long Island, New York

13 Factors that influence summer insolation Tilt (obliquity) of earth s orbit with respect to plane of rotation about the sun (larger tilt, less sun in winter, but more sun in summer) varies from 22 to 24.5 degrees (i.e. the arctic circle oscillates north and south) period is 41,000 years

14 Factors that influence summer insolation Precession where the orbital axis is pointed presently, the north star influenced by gravity from Sun and Moon changes timing of summer compared to distance from sun Is also affected by Venus and Jupiter (like torque pulling on a top, making it wobble) period is 19,000-23,000 years

15 Factors that influence summer insolation Eccentricity the shape (i.e., how circular it is) of the earth s orbit about the sun varies from 1.00 (circular) to 1.06 Currently (nearly circular, but closest to sun in Dec., farthest from sun in June) period is 100,000 years Small effect on total insolation

16 Changes in climate due to changes in Earth s orbit

17 Solar insolation in June, Northern Hemisphere Calculated variation of June sunlight Variations broken down into orbital components Actual variations from ice cores Figure 14-8

18 Timing of the growth and melting of ice sheets for the last million years or has reflected variations in Earth s orbit that have favored the build up of ice in the northern hemisphere (i.e., less light in summer resulted in greater buildup of ice) Three factors contributed to this precession, obliquity, and eccentricity. However, we noted that the climate signal at 100,000 years (the eccentricity mode) was far larger than expected based on changes in sunlight. We already know that CO 2 abundances and temperatures have correlated strongly during the current ice age epoch (the Pleistocene). How does CO 2 vary with temperature?

19 Amazingly, the prediction that the timing of ice ages would reflect these three orbital parameters occurred BEFORE any ice cores were ever drilled. It is pretty remarkable that the isotopes of water, which reflect the amount of ice that is piled up in the northern hemisphere ice sheet, oscillate at intervals that are a near-perfect match for the orbital predictions. These predictions were made by a Serbian geophysicist Milutin Milanković, who, when World War I war broke out in 1914, was interned by the Austro-Hungarian army. His interest was in solar climates and temperatures prevailing on the planets. During his internment in Budapest he was allowed to work in the library of the Hungarian Academy of Sciences, where, by the end of the war, he had finished a monograph which was published in 1920, titled Théorie mathématique des phénomènes thermiques produits par la radiation solaire (Mathematical theory of thermal phenomena caused by solar radiation). (adapted from Wikipedia)

20 But the measurements from ice cores show that the 100,000 year cycle is bigger than expected. Calculated variation of June sunlight Variations broken down into orbital components Actual variations from ice cores Figure 14-8

21 What is needed to amplify the signal from the 100,000 year cycle? Some kind of positive feedback loop! CO 2 /biological pump Shelf-nutrient Iron fertilization Coral reef Cloud/albedo

22 Which one of the following represents a negative feedback between Earth's temperature and atmospheric CO 2? (a) Increased marine algal productivity and formation of aerosolproducing methane sulfonic acid during glacial periods. (b) growth of coral reefs during interglacial periods (c) Exposure of continental shelves and release of phosphorus during glacial periods. (d) Growth of terrestrial biomass during deglaciations and destruction of terrestrial biomass during glacial periods. (e) Increased deposition of iron-containing dust from deserts into the oceans during glacial periods

23 Which one of the following represents a negative feedback between Earth's temperature and atmospheric CO 2? (a) Increased marine algal productivity and formation of aerosolproducing methane sulfonic acid during glacial periods. (b) growth of coral reefs during interglacial periods (c) Exposure of continental shelves and release of phosphorus during glacial periods. (d) Growth of terrestrial biomass during deglaciations and destruction of terrestrial biomass during glacial periods. (e) Increased deposition of iron-containing dust from deserts into the oceans during glacial periods

24 We will look at each of these in detail next time!