1. CO 2 and Long-Term Climate

2. Greenhouse Worlds: Venus and Earth Mean temperatures at surface –Venus: 460 o C –Earth: 15 o C At first glance, it would seem that distance from the Sun is a major factor Why is Venus so much warmer than Earth?

3. Venus is closer to the Sun Mean distance from the Sun –Venus: 108.2 million km –Earth:149.6 million km Venus is 72% (or.72 Astronomical Unit*) of Earth’s mean distance (1.0 A.U.) * An astronomical unit (A.U.) is the average distance between Earth and the Sun and is used for distance measurement in the Solar System.

4. Venus Receives More Insolation Amount of insolation received varies inversely with the square of its distance from the Sun. Venus receives nearly twice the solar radiation as Earth does. But, this isn’t the reason... Earth Venus (1) 2 (0.72) 2 0.581 1.93 1 ==

5. Venus Upper atmosphere –Thick cover of sulfuric acid clouds –High albedo (80%) –Only 20% of insolation reaches the surface.

6. Earth Clouds reflect 26% of insolation 74% of insolation reaches Earth’s surface.

7. Less Insolation Reaches the Surface of Venus Even though receives 1.93 times the insolation the Earth does –The amount reaching its surface is 52% of Earth’s –This is due to the high albedo of the cloud cover on Venus 1.93 x 0.20 0.74 = 0.52

8. The Cause... The Thick Atmosphere of Venus It’s atmosphere is 90 times as dense as that of Earth. 96% of the atmosphere is carbon dioxide Venus is said to have a “runaway greenhouse effect.”

9. Venus and Earth Are Greenhouse planets Contain nearly equal amounts of carbon The difference is where they store carbon –Venus: Primarily in its atmosphere –Earth: Most is stored in rocks Limestones Also in reservoirs of coal, oil, and natural gas –On Earth the major greenhouse gas is water vapor Greenhouse heating from atmospheric carbon is relatively small (31 o C) –On Venus the major greenhouse gas is CO 2 Enormous net greenhouse warming (285 o C) even though atmospheric water vapor is nearly absent

10. The Faint Young Sun Paradox

11. Earth’s Sun Formed from the solar nebula 4.55 Byr “Shines” as a result of an ongoing nuclear reaction in its core

12. Fusion in the Sun’s Core Four hydrogen (H) nuclei (each with a mass of about 4.030 mass units) join to form a helium (He) nucleus with a mass of only about 4.003 energy units. The mass that seems to be lost is converted to radiant energy –4 million metric tons of matter are converted into energy every second

13. An Expanding Sun The earliest Sun had 25% to 30% lower luminosity. As nuclear fusion caused the Sun to expand, –It became brighter

14. Hertzsprung – Russell (H-R) Diagaram Shows the relationship of a star’s mass to its luminosity The Sun will eventually expand to a red giant and then end its life as a white dwarf

15. All Water on the Early Earth Should Have Been Frozen A decrease in the Sun’s brightness of just a few percent would cause all water on Earth to freeze. The geologic record shows that Earth has never been completely frozen.

16. Evidence of Liquid Water on Earth Throughout Geologic Time Sedimentary rocks are a prominent part of the rock record. Most sedimentary rock indicate a liquid water depositional environment.

17. Evidence of Liquid Water on Earth Throughout Geologic Time Primitive life dates back to at least 3.5 Byr ago. Continued presence of life on Earth along with a succession of increased complexity isn’t congruent with extreme cold. 3.2 to 3.5 Bry old Procaryotes from Australia Cambrian marine life

18. Why then, with a weak Sun wasn’t Earth completely frozen for the first 3 billion years of its existence?

19. A Warming Process Must Have Been Present There must have been a process that warmed Earth. But, it must not be doing so today –Combined with the strengthening of the Sun Earth would be uninhabitable. Somehow, Earth has remained within a moderate temperature range during the period of the Sun’s increasing output.

20. A Thermostat Process A process that : –Warmed Earth when it otherwise would have frozen –Reduced heat upon detecting increasing warmth from the strengthening Sun Greenhouse Gases could have been part of the mechanism. –More abundant during early Earth history –Decreased as Earth warmed

21. Effect of Greenhouse Gases

22. Carbon Exchange between Rocks and the Atmosphere Over long periods, slow exchanges can produce large cumulative changes in atmospheric CO 2

23. Carbon Reservoirs Largest carbon reservoir is in rocks. Inverse relationship between size of reservoir and rate of exchange Over millions of years slow exchanges can result in large changes in atmosphere CO 2.

24. Volcanic Sources of CO 2 Heat in Earth’s interior causes rocks to melt.

25. Volcanic Eruptions

26. Volcanic Sources of CO 2 Heat in Earth’s interior causes rocks to melt.

27. Yellowstone National Park

28. A Balancing Act Rate of carbon input is roughly balanced by a similar rate of natural removal –Probably prior to industrial revolution Volcanic input of carbon is irregular because volcanoes don’t erupt on a “schedule.” If volcanic input of carbon stopped... –It would take 4,000 years for atmospheric CO 2 to fall to zero. A geologically short period of time

29. Other Reservoirs Would Compensate Near surface reservoirs would lose CO 2. –Vegetation –Soil –Surface Ocean They would take 24,700 years after end of volcanism to lose all their carbon. Deep-Ocean carbon reservoir would also lose. –With this reservoir it would take 278,000 years for a complete termination of volcanic carbon input to completely deplete all reservoirs. This is 0.01% of all Earth history

30. Is the Volcanic Source of CO 2 the Natural Thermostat? Volcanoes alone could not have delivered the amount of carbon needed to: –Prevent the atmosphere from running out of CO 2 –But not overheat the planet Volcanic processes are driven by Earth’s internal heat. –Volcanism doesn’t react to external changes and then act to moderate their effects like a thermostat.

31. Chemical Weathering of Continental Rocks The major long-term process of CO 2 removal Avoids long-term buildup of CO 2 levels over time –Of the types of chemical weathering previously discussed, two types are important in the carbon cycle. Hydrolysis Dissolution

32. Hydrolysis The main mechanism for removing CO 2 from the atmosphere Three key ingredients –Water derived from precipitation –Minerals in continental rocks –Carbon dioxide from the atmosphere

33. Continental Rocks On the average, composition of granite –Composed of silicate minerals –Typically cations (Na +, K +, Fe +2, Mg +2, Al +3, and Ca +2 are: Chemically bonded to the negatively charged silicon-oxygen tetrahedron (SiO 4 -4 )

34. The Silicon-Oxygen Tetrahedron

35. Olivine – A Silicate Mineral

36. Example Using Wollastonite CO 2 dissolves in rainwater (and in groundwater) –Forms carbonic acid Carbonic acid reacts with wollastonite Weathered products release Si +4, Ca +1, and HCO 3 -1 –Eventually end up in the ocean and is deposited in shells of marine organisms. –Eventually forms limestone wollastonite

37. Example Using Wollastonite Accounts for 80% of carbon buried per year in sediments and rocks wollastonite

38. Diatomite Silica deposited in the deep ocean

39. Limestone Limestone ridge in the Canadian Rockies Limestone in France

40. Coral Limestone

41. Barrier Reefs Great Barrier Reef Australia

42. Skeletal Limestone - Coquina Formed from wave-broken fragments of shells, corals, and algae.

43. Chalk Fine-grained, light colored, and porous from microscopic marine organisms (plankton).

44. White Cliffs of Dover Kent, England

45. Coccolithophorids (Coccoliths) Primary constituent of chalk in the White Cliffs of Dover Calcareous platelets Secreted by yellow-green algae Extremely small

46. Bioclastic Limestone Coarse-grained with shell and coral fragments Fine-grained carbonate mud from coralline algae

47. Dissolution of Limestone Rainwater and CO 2 combine in soils forming carbonic acid Calcite in limestone is chemically dissolved Dissolved ions flow to the ocean in rivers. CaCO 3 + H 2 CO 3 Ca + 2HCO 3 calcite carbonic acid calcium bicarbonate

48. Dissolution of Limestone Forming Caves Great Onyx Cave, KY

49. Howe Caverns, NY

50. Carlsbad Caverns, NM

51. Stalactites

52. Dissolution of Limestone Rates Faster than hydrolysis of silicates Returns all of the CO 2 to the atmosphere –Within the relatively short time it takes dissolved ions to reach the sea and become incorporated into the shells of marine organisms. No net removal of atmospheric CO 2 during the overall process.

53. Chemical Weathering may act as Earth’s thermostat Rates are sensitive to climate

54. Climate Factors That Control Chemical Weathering

55. Temperature Controlled laboratory experiments indicate –Weathering rates double for each increase of 10 o C. Lab studies are difficult to transfer to studies of the real Earth Only a few silicate minerals have been examined Natural rates are difficult to determine in the field Rapid carbonate dissolution complicate studies – May dominate total dissolved ions in rivers - Do not control CO 2 levels in atmosphere

56. Precipitation Increased precipitation results in a greater rate of chemical weathering Groundwater in soils increases –Formation of carbonic acid increases

57. Effects of Temperature and Precipitation are Linked Warm tropics –High humidity and rainfall –Rapid chemical weathering High Latitudes –Cold and dry –Little chemical weathering

58. Effects of Temperature and Precipitation are Linked Smaller-scale complications creating region variations –Hot regions may have high evaporation rates –Dry out soil –Evaporated water may fall in another region

59. Vegetation Plants –Extract CO 2 from the air –Delivers it to the soil Combines with groundwater to form carbonic acid –Can increase the amount of chemical weathering by a factor of 2 to 10 over the rate on land without vegetation. –More vegetation increases rate of CO 2 extraction from air and increases amount of carbon in the biomass.

60. Chemical Weathering: Earth’s Thermostat Mechanism involves two facts: –The state of Earth’s climate affects the rate of global chemical weathering –Weathering can affect the state of Earth’s climate It regulates the rate at which CO 2 is removed from the atmosphere –Chemical Weathering acts as negative feedback

61. Chemical Weathering for a Warming Climate - Negative Feedback Increase in temperature, precipitation, and vegetation Increase in weathering rate More CO 2 removed from the atmosphere Slows the warming

62. Chemical Weathering for a Cooling Climate - Negative Feedback Initial cooling is reduced Less CO 2 is removed from the atmosphere due to decreased chemical weathering

63. Negative Feedback Explains the Faint Sun Paradox Favored over volcanism, which did occur at high rates on the early Earth –High rates could have produced enough CO 2 to warm Earth –But, it’s highly unlikely that the slowing of volcanism over a 4 Byr period was paced exactly the rate needed to counter the strengthening Sun.

64. Early Earth Earth was cooler –Less precipitation –Less vegetation Chemical weathering was slower Slower CO 2 removal –100 to 1,000 times as much CO 2 in the atmosphere as today

65. Strengthening Sun Warmer temperatures More –Precipitation –Vegetation More chemical weathering More CO 2 removed from the atmosphere Offset warming from the stronger Sun.

66. A Snowball Earth? Evidence of several glaciations between 850 and 550 Myr ago –If these were at or near the poles, then climate was similar to today –If these were in the tropics then it’s possible Earth would have been close to a frozen state This is unresolved...

67. The Gaia Hypothesis Proposed by –James Lovelock Independent Scientist, Environmentalist, Researcher, Author –Lynn Margulis Department of Geosciences University of MA at Amherst

68. Gaia Hypothesis that life evolved in order to regulate Earth’s climate Named after the Greek Goddess known as Earth or Mother Earth (the Greek common noun for "land" is ge or ga). It is written that Gaia was born from Chaos, the great void of emptiness within the universe

69. Modern-Day Biologic Processes Cited by Gaia supporters Important parts of the processes of chemical weathering and carbon cycling –Carbon is at the center of the CO 2 cycle –Terrestrial plants contribute CO 2 to the soil and form carbonic acid –Shelled ocean plankton extract CO 2 from the ocean and store it in their calcium carbonate shells

70. Critics of Gaia Cite that... Most of the active roles played by organisms in the biosphere today –Are a relatively recent development in Earth’s history The role of life in the distance past –Probably smaller –Or nonexistent Through Earth’s long history life has differed considerably from those of today –Life forms that existed for over 90% of Earth history Too primitive to have an effect on chemical weathering to drive Earth’s thermostat

71. Development of Life on Earth Primitive organisms similar to the modern-day Bacteria Oscillatoria

72. Development of Life on Earth Stromatolites (2.9 Byr ago) Fossil Record is poor or absent

73. Stromatolites: Stromatolites: Builders of Limestone and Producers of O 2 Cyanobacteria –older incorrect term is blue-green algae –Photosynthetic bacteria Secret CaCO 3 in daily cycles Traps sand and forms layers in various mound-like formations

74. Shark Bay, Australia Present-Day Stromatolites

75. Development of Life on Earth Evidence is provided by Banded Iron Formations (BIFs)

76. BIFs and the Atmosphere How are these rocks related to the atmosphere? Their iron is in iron oxides, especially hematite (Fe 2 O 3 ) and magnetite (Fe 3 O 4 ) Iron combines with oxygen in an oxidizing atmosphere to from rust- like oxides that are not readily soluble in water As photosynthesizing organisms increased in abundance, free oxygen was released into the oceans, causing the precipitation of iron oxides. Cited as evidence of Gaia by supporters

77. Development of Life on Earth “ Cambrian Explosion” –450 Myr ago –Sudden appearance of shelled marine organisms –Cited by critics that life didn’t play a role in transferring products of weather on land to the seafloor in preceding 4 billion years Fossil Record is poor or absent

78. Development of Life on Earth Fossil Record is poor or absent 430 Myr ago simple land plants with roots and stems Similar to the modern-day Psilotum 400 Myr ago more complex treelike plants appeared Similar to modern cycads

79. Role of Marine Algae and Terrestrial Microbes on the Early Earth Gaia supporters argue that critics underestimate the role of these organisms in Earth’s early history. Claim that modern-day bacteria play a greater role in weathering than is recognized –Therefore, they must have been important in Earth’s early history when they were the only terrestrial life-forms

80. The Gaia Hypothesis is Unproved While fascinating, “the jury is still out.” Better quantitative measurements are needed of separate contributions of the following factors to the rate of chemical weathering: –Biological –Chemical –Physical