Long-Term Climate Regulation

Transcription

1 C H A P T E R 1 Learning Objectives Long-Term Climate Regulation After reading this chapter, students should be able to: Have a grasp of long term climate cycles and the processes that affect climate fluctuations on timescales of millions of years or greater. Know how the sun has evolved over its history, and discuss the impacts this change has had on the Earth s climate and atmospheric composition. List the possible solutions to the faint young Sun problem and understand the strong and weak point of each possible solution. Understand the impact methanogens should have had on the Archean climate. Understand the reasons for the use of glacial evidence as a record of long term climate change. List the types of evidence used to indicate glacial episodes. Know what could have caused the Huronian (or Paleoproterozoic) Glaciation. Know what a Snowball Earth episode is and list the evidence for a Neoproterozoic Snowball Earth. Understand how the Earth could have entered into and escaped from a Snowball Earth episode. Offer hypotheses for how life could have survived a Snowball Earth episode. Know the long term climate fluctuations that occurred during the Phanerozoic. Explain how organic carbon burial could have led to cooling during the Carboniferous, and describe the evidence supporting this relationship. Know that the climate of the Mesozoic was warm, both in terms of mean surface temperature and equator-pole temperature gradient. Realize that the climate cooled during the Cenozoic, and know how the formation of the Himalayan Mountains could have led to this cooling. Review Questions 1.) Why does the Sun get brighter with time? As the Sun gets older, it converts more of its hydrogen to helium. As the He content increases, the sun s core becomes denser. This density change leads to a higher temperature within the sun, causing nuclear fusion within the core to proceed faster. The faster rate of fusion leads to a greater luminosity. 1

2 .) How might the carbonate-silicate cycle have helped to solve the faint young Sun problem? The carbonate-silicate cycle contains a negative feedback loop that would have mitigated the impacts of the decreased solar luminosity. If the Earth was relatively cool 4 billion years ago, then silicate weathering would have slowed down and the rate of drawdown of CO from the atmosphere into the oceans would have also decreased. Slowing down the removal of CO from the atmosphere allows CO concentrations to increase, thus creating a greater greenhouse effect that compensates for the lower solar luminosity. 3.) Why is methane thought to have been an important greenhouse gas during the Archean Era? Methane is thought to be important for a number of reasons. irst, methanogens are thought to have been among the first forms of life on Earth, so there should have been a significant biogenic flux of CH 4 into the atmosphere. Additionally, paleosol data place an upper limit on atmospheric CO concentrations that is too low to allow for liquid surface water given a strict CO -H O greenhouse. Thus, another greenhouse gas is needed to permit the presence of liquid water, and CH 4 is one of the more likely candidates for this. The other likely candidate, NH 3, is rapidly photolyzed and should not have had a long enough lifetime to be an effective greenhouse gas. (Note: The paleosol data are contentious, and so there is not widespread agreement on an upper limit for CO. This remains a topic for further research.) 4.) What triggered the Huronian glaciation at.3 b.y. ago? The most likely trigger for the Huronian glaciation was the decrease in CH 4 that resulted from the rise of O. The increased O concentrations would have eliminated much of the CH 4 present in the atmosphere, thereby reducing the greenhouse effect. 5.) What types of geologic evidence are used to infer past glaciations? Tillites, glacial striations, dropstones, and δ 18 O excursions are evidence of glaciations mentioned in the book. 6.) How many separate episodes of glaciation have occurred during Earth s history? There have been at least six episodes of extensive glaciation: the mid-archean at.9 b.y. ago, the Huronian at.3 b.y. ago, the Snowball Earth episodes of the Neoproterozoic at 750 m.y. ago and at 600 m.y. ago, the Late Ordovician glaciation about 440 m.y. ago, the Permo-Carboniferous glaciations about 80 m.y. ago, and the Pleistocene glaciation, which has lasted for the past m.y. 7.) What types of geologic evidence support the Snowball Earth model for the Late Precambrian glaciations?

3 Paleomagnetic evidence indicates that glaciers existed at or near the equator during this time. Additionally, banded iron formations indicate an anoxic deep ocean, which could have resulted from a separation of the ocean from the atmosphere resulting from the ice, and the placement of cap carbonates above glacial layers agrees with the CO build-up that could have been the means of escape from this Snowball Earth episode. 8.) How are carbon isotopes used to infer past atmospheric CO concentrations? Organisms fractionate carbon isotopes more strongly when atmospheric CO levels are high. Hence, a large difference in δ 13 C values between organic carbon and carbonates is considered as evidence for high atmospheric CO ratios. 9.) How are atmospheric CO levels affected by the presence of land plants? Vascular plants increase the partial pressure of CO in soils, allowing for faster weathering rates. This should lead to a speed-up of the drawdown of CO through carbonate deposition, and CO levels should subsequently decrease. 10.) What mechanisms might explain the warm climate of the Mesozoic Era? How might the equator-to-pole temperature gradient have been reduced? The warm Mesozoic climate was likely due to increased atmospheric CO concentrations and greater equator-to-pole heat transport. The increased CO concentrations likely resulted from higher outgassing rates corresponding to increased sea-floor spreading rates, while the cause for the greater equator-to-pole heat transport has been elusive. 11.) Why did climate cool during the past 40 million years? One theory is that climate cooled as a result of the collision of the Indian and Eurasian plates. This collision caused the uplift that led to the formation of the Himalayan Mountains and the Tibetan Plateau. This created both fresh weathering surfaces and high seasonal rainfall, allowing for high weathering rates. The high weathering rates led to the drawdown of CO as silicate minerals were weathered away. The decrease in CO levels lessened the greenhouse effect, cooling the Earth. Critical-Thinking Problems 1.) Evidence for low-latitude glaciation is found at both 0.6 Ga and.3 Ga. ( Ga means giga-anna, or billions of years ago.) These are two of the three possible snowball Earth events mentioned in the text. (We will neglect the event at 0.75 Ga because it is similar to the first one.) Your job is to estimate how thick the ice was at those times. 3

4 a. The variation in solar luminosity with time can be approximated by the following formula (derived by fitting the results of a computer model of the Sun s evolution) S0 S ( t / 4.6) where S solar flux at time t S W/m present solar flux t time in Ga (billions of years before present) Calculate the solar flux at 0.6 Ga and.3 Ga both in W/m and as a percentage of its current value. To find the percentage of its current value, plug in the values for t into the equation for 100% (S/S 0 ). At t 0.6 Ga, At t.3 Ga, 1 100% (0.6 / 4.6) ( S / S ) 100% 95% S W/m 130 W/m % (.3/ 4.6) ( S / S ) 100% 83% S W/m 114 W/m., and, and b. As we have learned previously, the effective radiating temperature of the Earth can be found from the formula σ 4 T e S (1 A) 4 where A is the planetary albedo and σ (5.67 x 10-8 W/m /K 4 ) is the Stefan- Boltzman constant. Calculate T e at 0.6 and.3 Ga, assuming an albedo of 0.65 (the value for clean ice and snow). Then, calculate the global average surface temperature, Ts, assuming that the atmospheric greenhouse effect, ΔT g, was the same as today (33K). Recall that T s T e +ΔT g. irst, we must solve for T e : T e S (1 A) 4 σ 1/ 4. At t 0.6 Ga, S 1300 W/m, so 4

5 1/ 4 S 1300W T e (1 A) (1 0.65) 1K 4, and 8 4 σ x10 W / K T s T e + ΔT g 1K + 33K 45K. At t.3 Ga, S 1140 W/m, so 1/ 4 S 1140W T e (1 A) (1 0.65) 05K 4, and 8 4 σ x10 W / K T s T e + ΔT g 05K + 33K 38K. c. The conductive heat flow through ice is given by T λδ Δz where λ ( W/m/K) is the thermal conductivity of ice, ΔT is the temperature difference between the top and bottom of the ice layer, and Δz is the thickness of the layer. We know that the current geothermal heat flux,, is about 0.06 W/m. Assume that had this same value at 0.6 Ga, but was 3 times higher at.3 Ga. Assume also that the top of the ice is at temperature T s, and that the water below the ice has a temperature of - C. What is the thickness of the ice at 0.6 Ga and at.3 Ga? irst, solve for Δz: λδt Δz. Now, plug in the given values for 0.6 Ga and.3 Ga. or t 0.6 Ga, T W / K (71K 45K) z λδ 867m km 0.06W or t.3 Ga, we will use three times the present heat flux and a surface temperature of 38K, so T W / K (71K 38K) z λδ 367m km 0.18W 5

6 .) Suppose now that the ice is transparent enough so that some sunlight makes it through. Let s see how that would change the ice thickness. a. Calculate the globally-averaged solar flux incident on Earth s surface during the Neoproterozoic glaciation (0.6 Ga). The globally averaged solar flux is equal to S/4. Thus, we take the answer to question 1a and divide it by 4. At t 0.6 Ga, 1 100% (0.6 / 4.6) ( S / S ) 100% 95% S W/m 130 W/m, so S/4 130 W/m / W/m, and (Note: This problem is slightly ambiguous, because one should probably reduce this flux by some amount to account for the presence of clouds. One could do this by multiplying S/4 by the quantity (1-A), as in the planetary energy balance equation. Use A 0.3, though, not A 0.65, as the relevant quantity is the amount of sunlight that hits the surface. Or, to think of it another way, the albedo of thin ice is considerably lower than that of a hard Snowball Earth. We will work on the wording of this problem for the next edition.) b. The solar flux at the equator is about 0% higher than the global average value. Suppose that 10% of this incident sunlight makes it through the ice. How thin must the ice layer be in order to conduct this heat back out? (Use the formula from Question 1-c.) As suggested, we will use the equation derived at the beginning of 1c: λδt Δz. We will again use W/m/K for λ, and for we will use S/4 39 W/m, and ΔT 73K 45K 8K. This gives us: ( W / K ) T 8K z λδ 1. 4m 39W (Note: If you use take A to be 0.3 in part a, then you should get m here, instead of 1.4 m.) 6

7 c. Is the ice thickness calculated in part (b) consistent with the assumption that it would transmit 10% of the sunlight through it? Determine this by consulting Box igure 1-. Box igure 1- from main textbook Yes, because according to Box igure 1-, for ice less than two meters thick, transmissivity should be approximately 10% or greater. So, the ice should stabilize at a thickness of somewhat less than m. Resource Guide Video/ilm: Planet Earth, Episode 3. The Climate Puzzle Annenberg/CPB Scientists piece together an unfolding mystery what caused the ice ages, how Venus' greenhouse effect may have parallels on earth, and what Antarctica's eerie ice rivers demonstrate. 7

8 Planet Earth, Episode 7. ate of the Earth Annenberg/CPB New theories about the global consequences of a nuclear winter and an ultra-violet spring are revealed in this final episode that explores the role of life in shaping Earth and its future. 8