Global Change -- Past, Present, and Future
SECOND MIDTERM STUDY GUIDE
Second Midterm Thursday Nov.
(worth 15% of your grade)
Back to First Midterm Study Guide
Your second Midterm will consist of ten multiple choice questions (1 pt each), five short answer questions (out of seven choices, 3 pts each), focused more on specific concepts and topics, and one short essay (10 pts), which will focus on synthesis of concepts.
BRING A BLUE BOOK BUT no SCANTRON -- You'll mark your answers on your exam.
This Midterm will cover material beginning with Chapter 5, p. 98, From Greenhouse to Icehouse, the Past 50 Million Years, through Chapter 12, The Last Glacial Maximum, through page 227. Skip Chapter 11. Topics that weren't specifically discussed in class will be covered only peripherally, unless included below.
For each time period, evidence of change, and possible forcing mechanisms: know the (1) hypotheses regarding causes of climate change, (2) the empirical evidence supporting or refuting (matching or Not matching) the hypotheses, and (3) the model simulations and results, whether they support or refute the empirical evidence and the hypotheses.
Topics and Concepts that may be included:
History of climate fluctuations through time -- review
Range of time scales (see Figure1.2 and 1.3 pages 6-7, Ruddiman)
Do not memorize dates, but focus on patterns of climate variations through time
e.g. major glaciations lasting several million years every several hundred million year periods (not cyclic)
Know names, terms, geologic time periods:
Quaternary (last ~2.5 my) (my = million years),
Pleistocene (ice age, ~2.5 my-10,000 C-14 yr BP) (BP = Before Present),
Wisconsinan glaciation (~80,000-10,000 yrs BP),
Eemian, last interglacial, (~125,000 yrs BP)
Holocene (last ~10,000 yr)
Global Climate Change over the past 50 million years (Chapter 6)
About how warm was it 50 my ago? (how much permanent ice, if any?)
Patterns of ice sheet growth, mountain glaciers
Evidence of gradual cooling -- types of evidence? Where?
Vegetation proxies, leaf outlines, tropical versus high latitude plants
Oxygen isotope records -- delta O-18 (see also Appendix 1, p. 359)
Isotope Fractionation, ratio of O-16 to O-18
During Cold Periods:
Positive number = enriched in heavier O-18, as in deep tropical oceans
Negative number = enriched in lighter O-16, as in Polar ice caps
Measured in parts per thousand (%o)
Measured in ice cores in Greenland, Antarctica; measured in ocean sediment cores (see fig. 6-7 p. 101)
How is delta O-18 interpreted in terms of temperature and ice volume?
Deep Ocean Temperature dropped approximately 14degC over past 50 million years
Hypotheses for causes of global cooling over past 50 million years
Polar position of continent over one or more poles? Not supported, see Chapter 5
Gateway hypothesis --Antarctica over South Pole, isolated and surrounded by high latitude ocean -- did this separation cause warm waters to stop flowing poleward? Could this loss of warm water from the tropics caused cooling and ice development?
Timing is not right -- Antarctica separated from Australia ~35 my ago; Drake's Passage opened between Antarctica and S. America ~25 my ago, but first large increase in glacial ice on Antarctica ~13 my ago
Central American Seaway hypothesis -- closing of Panama Isthmus > 4 my, yet first large glaciation began at 2.75 my --
Before Isthmus closed, strong warm ocean flow westward from Atlantic into the Pacific; closing this passage could have started Gulf Stream, warm, saline water to flow to North Atlantic. Could this have discouraged sea ice formation, allowing more moisture to evaporate, which would have supplied water vapor for more snow -- causing ice development? Can you have it both ways?
Role of climate models (Ocean-GCMs) to help assess these questions
Changes in CO2 --
Ocean spreading rate hypothesis
May explain gradual cooling from about 55 million to 15 million years ago, but not past 15 my -- Why? (See fig. 6-12, p. 105)
Weathering hypothesis -- collision of Indian subcontinent with Asia created unusual mass of high elevation terrain, also evidence of increased sediment deposition in Indian Ocean. both consistent with increased weathering hypothesis
See also Negative Feedback Loop, Fig 6-18 -- Could increased chemical weathering over Himalayas actually slow down weathering globally, reducing cooling? Positive Feedback Loop, Fig. 6-19, p. 114 -- Increased physical weathering from Ice Fracturing could cause positive feedback by exposing more rock surface to weathering.
Orbital Controls on Climate Change (Chapter 7)
Tilt of Earth's Axis of rotation: varies on range of 22.2 to 24.5 degrees; present tilt is at 23.5 degrees, similar to 21,000 years ago at LGM
Tilt causes seasonal variations at mid to high latitudes
Steeper the tilt, greater the seasonality both summer and winter -- warmer summers, colder winters -- encourages ice sheet retreat, melting
Smaller the tilt, less the seasonality -- more moderate year round -- cooler summers, warmer winters -- encourages ice buildup at high latitudes
Long term cycle: 41,000 years
Orbital Eccentricity, from perfectly circular to more elliptical
Long term cycle: 100,000 years (average of four cycles); longer cycle at about 413,000 years
Orbital eccentricity affects the strength of the variation caused by the earth's wobble (precession of the equinoxes)
More circular orbit -- little variation in insolation between perihelion and aphelion
More elliptical orbit -- greater difference between amount of insolation received at earth during perihelion compared with aphelion
Perihelion: day in the earth's orbit when the earth is closest to the sun; earth receives more intense solar radiation at this time. (now Jan 3rd)
Aphelion: day when the earth is farthest from the sun, thus lower amount of insolation received (now July 4th)
Earth's Wobble: Precession of the Solstices and Equinoxes around Earth's Orbit, 23,000 year cycle
Two major types of precession:
(1) Rotation of the elliptically shaped orbit around the sun, so that the place in the orbit where the earth is nearest/farthest from the sun changes throughout a complete rotation (year) (fig. 7-10, p. 125)
(2) Direction that the earth's axis is pointing in the sky moves in a slow circle, resulting in a gradual change in where in the Earth's orbit the Southern or Northern Hemisphere is facing the sun (summer) (fig. 7-9, p. 125)
Result: The day of the year when Perihelion occurs (Earth closest to the Sun) moves back about one day each 63 years: 23,000 year cycle
Note: 11,000 years ago, Perihelion occurred near the Northern Hemisphere summer solstice, June 12st, enhancing summer heat
Precession primarily affects low and mid-latitudes, e.g. the 23,000 year cycle of South Asian and African monsoons
Tilt primarily affects high latitude climate variations
Insolation Control of Monsoons (Chapter 8)
Review Monsoon circulation -- switching of winds -- offshore in winter, onshore in summer
Driven by differences in pressure -- continentality (summer low over land; winter high over land)
Winter: dry, high pressure; Summer: wet, low pressure
Orbital Monsoon Hypothesis:
Suggests that variations in insolation over time (23,000 year wobble cycle) cause strengthening and weakening of monsoon circulation
Stronger summer radiation: more summer rain (winter always dry anyway, may have stronger offshore winds, but not as important to climate change)
See figure 8-5, page 141; threshold between monsoon lake expansion during enhanced high summer insolation, no record when summer insolation weak, no lake sediments deposited when lake is dry.
Evidence: lake expansion in North Africa during stronger monsoon cycles: dry river channels under the sand dunes, hippo fossils, diatoms in lake sediments, interbedded layers of lake sediments with dry land, soils, dune deposits; windblown freshwater diatoms in Atlantic ocean sediments
Upwelling in Atlantic: Stronger trade winds during weak monsoon create cold water upwelling offshore West Africa; reverse during strong insolation, strong monsoon
Monsoons on Pangaea 200 million years ago:
Evidence in ancient lake sediments deposited in basins in mid-Pangaea, now preserved in New Jersey and along the Atlantic coast
Lake depth (sediment thicknesses) and some varves (annual layers) reveal wet and dry cycles on a precessional cycle .
Insolation Control of Ice Sheets (Chapter 9)
Milankovich orbital parameters -- see Chapter 7
Terms: Accumulation (ice builds up); Ablation (ice melts or evaporates); Mass Balance: balance between accumulation and ablation; equilibrium line: point where accumulation and ablation are balanced -- this point moves up and down the ice sheet depending on whether it's growing or retreating
Milankovich theory: weak summer insolation at high latitudes and mild winters combine to cause ice sheets to grow
Conversely, strong insolation at high latitudes (hot summers) and corresponding cold winters, lower insolation, cause ice sheets to melt (Fig 9-2, p. 157)
Reasons why ice won't build up over Arctic Ocean even if it's cold enough year round (climate point)
What causes shift equatorward of climate point, point where ice begins to form (equilibrium line)
Orbital-scale Climate Interactions
Ice driven responses, high latitudes, No. hemisphere
Evidence, sea surface temperatures from marine cores, forams
Correlation between ice sheets and ocean temps
Response of atmospheric circulation to full glacial ice sheets
How do we know? Interplay between GCMs and climate proxies
Climate records in loess, Pacific marine cores, Arabian sea, lake cores
Global response to ice sheets? Northern vs Southern hemisphere forcings
Carbon dioxide and ice volume -- which comes first?
Feedbacks between CO2 and ice sheets
Why shift from 41 k cycle to 100 k cycle in ice sheet growth?
Causes of deglaciation, cycles, tipping points and feedbacks
Last Glacial Maximum
What was world like then? Evidence, modeling?
Ice sheets: Laurentide, Cordilleran, Fennoscandian, Barents Sea
Changes in atmospheric circulation patterns, Polar jet stream
Dunes and Loess distributions, Winds, sources: glacial dust, desert sand
Dust layers in ice cores -- how interpreted?
Data versus models, pollen as input
GCM simulations versus evidence, atmospheric circulation, lake levels
European environment -- effects of low sea level,
Tropical temperatures, evidence of drying in the Amazon basin
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Page last updated 11/2/09