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Chapter 4. CO 2 and Long-Term Climate. 彭于珈. Greenhouse Worlds. Compare with Venus Venus is a hot planet Distance0.72 AU Surface temperature 460 0 C Consider albedo. albedo 80 % receive20 %. albedo 26 % receive74 %. The Venus higher albedo reduces the amount reaching
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Chapter 4 CO2 and Long-Term Climate 彭于珈
Greenhouse Worlds • Compare with Venus • Venus is a hot planet • Distance0.72 AU • Surface temperature4600C • Consider albedo
albedo80 % receive20 % albedo26 % receive74 %
The Venus higher albedo reduces the amount reaching its surface to just over half that of Earth • Consider albedo • The CO2 in the atmosphere • Venus = 96 % • Earth = 0.02 % • Greenhouse effect • Venus2850C4600C • Earth310C150C
The Faint Young Sun Paradox • Nuclear reaction • fuses nuclei of H together to form He • caused Sun to expand and brighter • The models indicate that the earliest Sun shone 25% to 30%
Completely frozen Earth? • In astronomy Yes • In climate No • Some evidences indicate that Earth was not frozen solid anytime Why ?
Something must have kept the early Earth warm enough to offset the Sun’s weakness Thermostat (temperature regulator) • Recall the temperature on Venus • Where is the carbon reservoir? • Venus atmosphere • Earth rocks
Carbon Exchanges • Between Rocks and Atmosphere • Volcanicinput of carbon from rocks to the atmosphere • Removal of CO2 from the atmosphere by Chemical Weathering
Volcanic Input • When volcanic eruptions and the activity of hot springs • rate 0.15 x 1015 grams/year • But how could balance at the long intervals of geologic time?
Chemical Weathering • Hydrolysis • Main mechanism • Dissolution
Hydrolysis • Three key ingredients • Minerals silicate minerals • Water derived from rain • CO2 derived from the atmosphere Removal from the Atmosphere Silicate rock (continents) Carbonic acid (soil) Shells of organisms
Dissolution • The rate is faster than hydrolysis Removal from the Atmosphere Limestone (rock) In soils Shells of organisms Returned to atmosphere
Control Factors • Temperature • Precipitation • Vegetation • They are all mutually reinforcing to affect chemical weathering
Climate Factors that control Chemical Weathering Scientists estimate that the presence of vegetation on land can increase the rate of chemical weathering by a factor of 2~10 over the rate on land that lacks vegetation.
Chemical Weathering :Earth’s Thermostat ? • The average global rate of chemical weathering depends on the state of Earth’s climate. • But weathering also has the capacity to alter that state by regulating the rate at which CO2 is removed from the atmosphere.
The weathering thermostat works as a negative feedback • Negative feedback simply moderate the degree of climate change
Faint young Sun paradoxEarth was not frozen solid • The volcanism was much higher early in Earth’s history • Slower rates of weathering would have left more CO2 in the atmosphere • As Earth began to receive more solar radiation from the brightening Sun
Another Greenhouse Gas • CH4 & NH3 • Also warmed the early Earth • But such contribution is smaller than CO2 • water vapor • The most important greenhouse gas today • It acts as a positive feedback that amplifies changes in climate
Is life the Ultimate Control on Earth’s Thermostat? The Gaia Hypothesis • The biologists James Lovelock and Lynn Margulis proposed in the 1980s that life itself has been responsible for regulating Earth’s climate. • Chemical weathering thermostat • Carbon is at the center of the CO2 cycle • The action of land plantsCO2H2CO3 • The shell-bearing ocean plankton CO2CaCO3
430 Myr 2.1 Byr 2.3 Byr FIGURE 4-9 The Gaia hypothesis Over time, life-forms gradually developed in complexity and played a progressively greater role in chemical Weathering and its control of Earth’s climate. The Gaia hypothesis holds that life evolved in order to regulate Earth’s Climate. 2.5 Byr 3.5 Byr
Primitive single-celled marine algae 3.5 Byr ago
First primitive land plants 430 Myr ago
The first treelike plants 400 Myr ago