climate change test 2
28. What two things do changes in δ18O values in the last 50 Myr tell us about climate changes?
"A record of δ 18 O c over the last 70 million years has been compiled from benthic foraminifera liv- ing on the ocean floor (Figure 7-7). Although the trend is shown as a single line, it is actually derived from hundreds of individual analyses that scatter in a wide band because of local temperature condi- tions specific to each site and because of short-term changes. This signal begins to trend erratically toward more positive values near 50 million years ago, and intervals of fastest change occur near 35 million years ago, 13 million years ago, and in the last 3 million years. These changes toward more positive δ 18 O c values are caused by a combination of (1) cooling of the deep ocean, and (2) growth of ice sheets on land. Both of these factors are critical aspects of the transi- tion from a greenhouse to an icehouse climate."
55. Why are air bubbles in ice cores younger than the ice in which they are sealed?
"Air moves freely through snow and ice in the upper 15 m of an ice sheet, but flow is increasingly restricted below this level. Bubbles of old air are eventually sealed off completely in ice 50 m or more below the surface." At a depth of about 50 meters below the surface, air can no longer circulate at all and is trapped in place as small bubbles, a process called sintering. These air bubbles form a permanent record of the past atmosphere. At the time the air bubbles are trapped, their slow diffusion beneath the surface makes their average age greater than that of the overlying atmosphere and the snow falling on top of the ice sheet. But the age of the bubbles is younger than that of the ice in which they are trapped because the surrounding ice was deposited many years earlier. The difference in age between the younger air bubbles and the older surrounding ice depends on the rate at which the ice accumulates. If deposition of ice is fast (0.5 m/year), the age difference between the bubbles and the ice enclosing them will only be a few hundred years. If deposition is slow (0.05 m/per year), the age offset can be as large as 2,000 years or more."
42. Describe an evidence showing variations in the strength of the tropical North African monsoon over the last several hundred thousand years.
"Because the huge North African land surface is situated at tropical and lower subtropical lat- itudes, it is strongly influenced by the overhead Sun. As a result of strong solar heating during northern hemisphere summer, a low-pressure region devel- ops over west-central North Africa and draws in moisture-bearing winds from the tropical Atlantic Ocean (Figure 9-2A). During typical summers, this monsoonal rainfall penetrates northward to 178N lati- tude (the southern edge of the Sahara Desert) before retreating southward later in the year. During northern hemisphere winter, the overhead Sun moves to the Southern Hemisphere, and solar radiation over North Africa is weaker. Cooling of the North African land surface by back radiation causes sinking of air from above, and a high-pressure cell develops at the surface over the northwestern Sahara Desert (Figure 9-2B). Strong and persistent trade winds associated with this high-pressure cell and with similar circulation over the adjacent North Atlantic blow southwestward from North Africa across the tropical Atlantic. Because the trade winds of the winter monsoon carry little moisture, winter precipitation is rare in North Africa. Only two areas receive much rain during this season: (1) the northernmost Mediterranean margin, where storms occasionally form over the nearby ocean, and (2) the southwest equatorial coast (the Ivory Coast), where the moist intertropical convergence zone (ITCZ) remains over the land."
11. Be able to read (and explain in simple terms) a chemical reaction showing how chemical weathering remove CO2 from the atmosphere. (p. 87)
"Carbon dioxide (CO 2 ) is removed from the atmosphere, incorporated in groundwater to form carbonic acid in soils, used in the chemical weathering of CaSiO 3 , and deposited in the CaCO 3 shells of marine organisms."
33. When is Earth closest to the Sun in its present-day orbit? How does this position affect the amount of radiation received by Earth?
"Larger tilt angles turn the poles more directly toward the Sun in the summer and increase the amount of solar radiation received."
19. Why was Earth ice-free even at the poles 100 Myr ago?
"Most reconstructions have inferred that the global mean spreading rate was faster near 100 million years ago than it is at present. If this conclusion holds up to future scrutiny, the BLAG hypothesis would pre- dict that the rate of input of CO 2 to the atmosphere should have been higher 100 million years ago than it is today (Table 5-2). This prediction would agree with geologic evidence of a warmer climate 100 mil- lion years ago, and with the absence of large polar ice sheets."
37. Earth's tilt is slowly decreasing today. As the tilt decreases, are the polar regions receiving more or less solar radiation in summer? In winter?
"The increase in tilt that turns the North Pole more directly toward the Sun at its summer solstice on June 21 also turns the South Pole more directly toward the Sun at its summer solstice six months later (December 21). On the other hand, the increased angle of tilt that turns a particular polar region more directly toward the Sun in summer also turns the pole away from the Sun in winter in the other hemisphere."
39. How does eccentricity combine with precession to affect long-term insolation on Earth?
"The shape of Earth's orbit around the Sun has also varied in the past, at times becoming more circular and at other times more elliptical (or "eccentric") than today." "The positions of the solstices and equinoxes in rela- tion to the eccentric orbit have not always been fixed at their present locations" "The cause of these changes lies in a long-term wobbling motion similar to that of a spinning top." "Earth's wobbling motion, called axial precession, is caused by the gravitational pull of the Sun and Moon on the slight bulge in Earth's diameter at the equator." "A second kind of precessional motion is known as precession of the ellipse. In this case, the entire elliptically shaped orbit of the Earth rotates, with the long and short axes of the ellipse turning slowly in space" "The combined effects of these two precessional motions (wobbling of the axis and turning of the ellipse) cause the solstices and equinoxes to move around Earth's orbit, with one full orbit around the Sun completed approximately every 22,000 years (Figure 8-11). This combined movement, called the precession of the equinoxes, describes the absolute motion of the equinoxes and solstices in the larger reference frame of the universe."
29. What hypotheses are available to explain the last 50 Myr cooling?
"the size of the Antarctic ice sheet has increased irregularly toward the present, with a major growth phase near 13 million years ago. Greater amounts of ice-rafted debris in nearby ocean sediments suggest additional increases in Antarctic ice during the last 10 million years." "Fossil remains of vegetation indicate a progressive cooling over the last 50 million years.
17. How could chemical weathering be both the driver and the thermostat of Earth's climate?
A plausible explanation of the faint young Sun paradox is that the weakness of the early Sun was compensated for by a stronger CO 2 greenhouse effect in the atmosphere Later, when the Sun strengthened, increased chemical weathering deposited the excess atmospheric greenhouse carbon in rocks, and the weakened greenhouse effect kept Earth's temperatures moderate
47. What does the 2.75-Myr δ18O history tell us about ice sheets in the Northern Hemisphere?
By 2.75 million years ago, global cooling had altered the position of the equilibrium line threshold enough that it began to intercept and interact with the summer insolation curve. At intervals of 41,000 or 23,000 years, as summer insolation minima crossed the equilibrium line threshold (Figure 10-19C), the climate point moved southward over the continents (Figure 10-19D), and ice sheets began to grow. But""before these ice sheets could grow very large, sum- mer insolation had already reversed direction and begun to increase toward the next maximum, and the equilibrium line had moved back northward off the continents. As a result, the ice sheets melted away. To some degree, this small glaciation phase fits the Milankovitch theory, which predicted discrete intervals of glaciation, each lagging just behind indi- vidual summer insolation minima and ending during subsequent summer insolation maxima. On closer inspection, however, the δ 18 O oscillations during this period are not fully consistent with his theory. Insolation changes at high northern latitudes show large variations at the 23,000-year precession cycle, both in monthly changes and in the caloric summer index used by Milankovitch (see Chapter 8). Yet the δ 18 O signal during the small glaciation phase is domi- nated by strong 41,000-year variations, with much weaker changes at 23,000 years (see Figure 10-13). A plot of the relative amplitude of these orbital cycle variations using the power spectrum method (see Chapter 8) highlights the mismatch between the rhythms present in the insolation signal and those found in the δ 18 O (ice volume) response (Figure 10-20 top and middle)."
13. If Earth's surface froze solid (i.e. the snowball earth), what would happen to CO2 emissions from volcanoes and to CO2 removal by chemical weathering?
CO2 emmisions from volcanoes are not affected by surface temperature changes, but CO2 removal from chemical weathering would slow down, eventually resulting in an increase in atmospheric CO2 that would oppose cooling effects.
10. What climate factors affect the removal of CO2 from the atmosphere by chemical weathering?
Chemical weathering (chemical breakdown of minerals or rocks): higher temps in subtropical areas favor chemical weathering, temp and precip act together, vegetation enhances it, plants extract co2, deliver to soil, combines with groundwater to form carbonic acid, enhances the rate of chemical breakdown of materials
54. How are ice cores dated?
For cores without annual layering, one common technique for dating ice is to construct an ice flow model based on the physical properties of the ice sheet and the assumption of a smooth steady flow of ice below the surface. These models produce good estimates of the age of the ice, but are not as accurate as annual layer counts.
49. Which parts of the Milankovitch theory have proved to be accurate? Which parts are insufficient?
High summer insolation heats land, glacier ablates. Low summer insolation keeps land cool, glacier persists or grows Changes in climatic cycles of glacial-interglacial periods were initiated by variations in the Earth's orbital parameters (Earth-Sun geometry factors)
53. Which season determines whether or not ice sheets grow or melt? Why?
Ice sheets exist because ice accumulates more than melts.Both depend on temperature insolation season Summer insolation controls the size of ice sheets.
45. Why does the size of a growing or melting ice sheet lag well behind changes in insolation?
Ice sheets have a large inertia (see residence time in previous classes), so it takes a long time to respond to insolation changes (i.e. growing/melting ice sheet lags well behind insolation changes).
20. What evidence shows that the world was warmer 100 Myr ago than today?
Most reconstructions have inferred that the global mean spreading rate was faster near 100 million years ago than it is at present. If this conclusion holds up to future scrutiny, the BLAG hypothesis would predict that the rate of input of CO 2 to the atmosphere should have been higher 100 million years ago than it is today (Table 5-2). This prediction would agree with geologic evidence of a warmer climate 100 million years ago, and with the absence of large polar ice sheets."
48. Why are northern ice sheets more responsive to insolation changes than Antarctic ice?
This is mainly because Antarctic is the coldest continent, where temperature is so much below freezing all year round.
3. What are the commonly used proxy data for climate studies? Where are they distributed? What climate information (or variables) can they tell us? What are their time spans and resolutions? Why does the importance of different proxy data change for different time scales?
Tree rings, ice cores, deep lake sediments, and coral reefs.
51. What controls orbital-scale fluctuations of atmospheric greenhouse gases?
Ultimately, changes in Earth's orbital parameters (eccentricity, tilt, presession) control changes in greenhouse gases, but feedbacks involving ice sheets, climate, ecosystems, and oceans also play a role.
35. What aspect of Earth's orbit changes in cycles of 41,000 years? Of 100,000 years? Of 23,000 years?
angle of Earth's tilt has varied through time in a narrow range. The present tilt (23.58) is near the middle of this range, and the angle is currently decreasing at a slow rate. Cyclic changes in tilt angle occur mainly at a period of 41,000 years, the interval between successive peaks or successive valleys (see Box 8-1). The 41,000-year cycle is fairly regular, both in period (wavelength) and in amplitude."
36. Which latitudes are most affected by changes in the tilt of Earth's axis?
at high latitudes changes in tilt amplify or surpress strength of seasons
32. Why does Earth have seasons?
axis tilt= obliquity orbit around the sun position of tilted earth in respect to the sun
16. What is the central concept behind the BLAG (spreading rate) hypothesis?
changes in climate driven mainly by changes in the rate of co2 input into atmosphere and ocean by plate tectonic processes BLAG hypothesis proposes cycling of carbon provides long-term stability to the climate system by moving a roughly constant amount of total carbon back and forth between the rocks and the atmosphere over long intervals of time results in atmospheric CO 2 levels constrained to vary only within moderate limits. But the long delays between carbon weathering and burial permit small imbalances to occur between the rate of carbon burial and the return of CO 2 to the atmosphere. These imbalances could drive climate changes over intervals of tens of millions of years. p.110 "This hypothesis predicts that atmospheric CO 2 concentrations and global climate are driven by the global mean rate of seafloor spreading, which controls the rate of CO 2 input at ocean ridge crests and subduction zones. The spreading rate hypothesis also invokes chemical weathering as a negative feedback that partially counters changes in atmospheric CO 2 and global climate initiated by varying rates of seafloor spreading."
40. How do orbital variations drive the strength of tropical monsoons?
changes in insolation recieved on earth, land and sea cool down and heat up at different rates, low pressure and rising air on land (fast heating), high pressure and sinking air in ocean(slow heating)
21. Why does higher CO2 make sense as one explanation for this greater warmth?
co2 traps heat like blanket duh
23. What are the possible causes of mismatches between the models and geologic observations?
could lie in either geological data, in the climate simulation, or some combo of the two problems w models: sometimes scientists asking more from models than present science development could deliver (crude treatment of ocean circulation at the time, didn't include upwelling in global models, etc) data from vegetation suggests moderate climate, GCM data suggests hard freezing winters problems with data: proxy data used to reconstruct past climate might have given incorrect "target signal" geochem analysis of plankton reconstructed cretaceous temps; shells yield dif data based on how well preserved they were; new data shows it was considerably warmer than original target signal values
18. What explains the changes in Earth's climate over the last several hundred million years?
driven mainly by changes in the rate of co2 input into atmosphere and ocean by plate tectonic processes (BLAG) changes in the average rate of seafloor spreading over mils of yrs control rate of delivery of co2 from the large subsurface rock reservoir of carbon
44. What is the equilibrium line? Why is it important?
equilibrium line that separates temperatures cold enough to permit net ice accumulation (glaciation) from those warm enough to cause net melting of snow and ice. The position of this threshold line relative to the insolation curve changes very slowly through time as the gradual cool- ing of the last several million years proceeds. The ice sheet response results from the interaction between the equilibrium line threshold and the summer insolation signal. When insolation values fall below the equilibrium line threshold existing at that time, ice sheets grow. When they rise above it, ice sheets melt. Both the growth and melting of ice sheets lag several thousand years behind the insolation forcing."
7. What are greenhouse eras? What are icehouse eras? Give examples. How many icehouse eras have existed during the earth's history?
greenhouse era- co2 in atmosphere traps warmth like blanket- no ice sheets on land We live on an "icehouse" Earth (ice sheets on land), one that is quite cold compared to other periods in Earth history.
15. What are the major characteristics of the climate of Pangaea (~200 Myr ago)?
huge size and the reduction of influence of oceanic moisture-> interior of Pangaea had extremely dry continental climate. widespread aridity at lower latitudes, especially in the Pangaean interior Precipitation values comparable to those in semi-arid grassland areas such as the western plains of the US today pervasive aridity reflects two factors: (1) the large amount of land at subtropical latitudes beneath the dry, downward-moving limb of the Hadley cell, and (2) the large amount of land in the tropics, causing trade winds to lose most of their ocean-derived water vapor before reaching the continental interior ocean around Pangaea received far more rainfall than the land; more than it does today vulnerable to seasonal extremes bc moderating effects of ocean moisture failed to reach much of Pangaea's interior wide range of seasonal temperatures could explain the lack of ice sheets on Pangaea; winter temperatures were cold enough to pro-vide the snowfall needed for ice sheets to grow, but the hot summers caused rapid melting of the snow, preventing glaciation. strong reversal between summer and winter monsoon circulations; large seasonal swings in land temp+small swings in ocean temp= contrasting responses=monsoons.
12. Why is chemical weathering a plausible thermostat for Earth's climate?
it reacts to the average state of Earth's climate and then alters that state by regulating the rate at which CO2 is removed from the atmosphere.
8. What factors explain why Earth has remained habitable for most of the 4.55 Byr history?
most of earths carbon is in rocks, not atmosphere, so net greenhouse heating of earth is pretty small
26. What are oxygen isotope ratios? What is isotope fractionation?
most of the oxygen in nature occurs either as the very abundant O16 isotope or the less abundant O18.
2. What are instrumental data? Can a thermometer be exposed under sunlight when it is used to measure air temperature? Give your reasons.
no because it would just be measuring the temp of the glass of the thermo itself, not the atmosphere (glass heats quickly)
24. What regions of the continents were flooded by high seas 100 Myr ago?
over geo time scale, ocean levels have risen/fallen by 200m against continental margins. in times of high sea level, ocean flooded shallow low-lying interiors of continents and formed large seas, eroding coastal sediments When sea level is low, the coastline tends to be situated near the topographic break between the relatively flat continental shelf and the steeper continental slope (Figure 6-6A). At these times, erosion prevails on continental margins, and most of the eroded sediment is carried across the continental slope and dispersed down in the deeper ocean. When sea level is high, the ocean floods the low-gradient continental margin to depths of 100 meters or more (Figure 6-6B). At such times, sediment is deposited on the submerged continental shelf." In the Cretaceous world of 100 to 80 million years ago, the coastlines and interiors of most continents were flooded by an ocean that rose well above the modern level. Areas flooded included much of southern Europe and interior regions of North America penetrated by seaways linked to the Gulf of Mexico and the Arctic Ocean. Since that time, sea level has slowly fallen to its modern position, close to the lowest level on record"
22. How well do model simulations capture the distribution of temperatures 100 Myr ago?
p 124
14. Do glaciations always occur when continents are located in polar positions? (p. 102-105)
polar position hypothesis: ice sheets should appear on continents that were located at polar or near-polar latitudes; no ice should appear at times when continents were located outside of polar regions hypothesis seems plausible bc modern ice sheets occur on the polar continent of Antarctica and the near-polar landmass of Greenland Modern ice sheets exist at high latitudes bc cold temps caused by low angles of incident solar radiation, high albedos resulting from the prevalent cover of snow and sea ice that reflect most solar radiation, and sufficient moisture to replenish the ice despite melting along their lower margins Over the last 450 million years, seafloor spreading has slowly moved continents across Earth's surface between the warmer low latitudes and colder high latitudes; but if latitudinal changes alone predict climate, these movements should have produced predictable changes in glaciations over 10s/100s mil yrs. 3 icehouse eras since 450mil yrs ago cretaceous- greenhouse earth warm period of dinosaurs supercontinents form; gondwana moved across magnetic south pole position of south magnetic pole 445mil yrs ago points to evidence of glaciation in modern sahara desert area when this happened glacial era was brief (1 mil yrs or less), not easily explained by slow motion of gondwana across south pole lack of glaciations between 425 and 325 mil yrs ago even though gondwana was moving across pole; land existed at south pole for 100 mil yrs without major ice sheets forming--->polar position NOT only requirement for large scale glaciation eventually pangea forms, antarctica is directly over pole but largely free from ice TLDR; polar position hypothesis accounts for earth's glaciation history in last half-billion yrs; cannot explain absence of ice during some intervals of last 500mil yrs
25. What were the major factors explaining higher sea level 100 Myr ago?
presence of marine sediments deposited simultaneously on coastal margins and in shallow interiors of continents at levels well above present sea level= past sea levels HIGH Deposition of marine sediments on several continents at the same time indicates that the changes in sea level are global in scale higher sea levels attributed to two kinds of factors: (1) tectonically driven changes that altered the volume of the ocean basins and their capacity to hold water, and (2) changes in the volume of water in the ocean basins caused by variations in climate."
1. What are proxy climate data? How are they different from instrumental data?
proxy- gather from natural recorders of climate variability, e.g., tree rings, ice cores, fossil pollen, ocean sediments, coral and historical data INSTRUMENTAL- past 200 yrs or so, using tools like thermometers, satellites, etc
4. Why are ocean sediments and ice cores so important to study the past climate?
sediments delivered to the sea floor may stay for 10s&mils of yrs, some distincly show layers where erosion has occurred, deep ocean less disturbed and sediment deposition slower than land by erosion. after ice sheets reach max size and begin to retreat, they form long curving ridges called moraines, ice can protect debris, also 10s/1000s yrs old record. p56&57
9. Why does the faint young Sun pose a paradox?
suns ongoing nuclear reaction means it got hotter over time, but if sun was ever cooler than it was now then the whole earth would've been covered in ice, which theres no record or evidence for. sedimentary deposits show earth was not frozen for its first 3 bil years, life forms 3.5 bil yrs old wouldn't have survived
5. Which two major groups of organisms are most important to climate reconstructions over the past several million years?
trees- outer softwood layers deposit into millimeter thick layers that turn into hardwood, annual layers corals- form annual band of calcium or magnesium carbonate that hold geochemical info about climate; live 10s100s of yrs
6. Why do we care about the past climate change?
understanding past climate helps us understand how it has changed over time
27. What kinds of changes in vegetation and ice show that Earth has cooled in the last 50 Myr?
vegetation disappeared as climate became more frigid and ice spread across the continent. Today, the only vegetation on Antarctica is lichen and algae found in summer meltwater ponds in ice-free regions of a few coastal valleys. A similar long-term cooling trend is evident in north polar regions. Palm-like and other broad-leaf evergreen vegetation existed in the Canadian Arctic at 808N near 55 million years ago (Figure 7-3 left), as did the ancestors of modern alligators that would presumably have been ill-adapted to extreme cold. Sea ice was apparently absent, even along the coastal Arctic margins." "Gradually the warm conditions in the Arctic gave way to colder climates. The appearance of conifer forests of spruce and larch by 20 million years ago indicates cooling, and the ring of tundra that has encircled the Arctic Ocean in the last few million" "years indicates deepening cold. Tundra is scrubby grasslike or shrublike vegetation that lives on thawed layers lying above permafrost, ground that is frozen each winter by intense cold. The appearance of tundra and permafrost is thought to be linked to the onset of extremely frigid winters brought on by expanding sea ice. The shapes of tree leaves have also been used to reconstruct past climate. Leaves of trees living today in the warm tropics tend to have smoothly rounded margins, while leaves of trees in cooler climates generally have irregular edges, usually jagged or serrated in outline."
