CO2 and Long-Term Climate Chapter 4 CO2 and Long-Term Climate 彭于珈

Greenhouse Worlds Compare with Venus Venus is a hot planet Distance0.72 AU Surface temperature4600C Consider albedo 接收輻射量與距離成反比，金星是地球的1.93倍 雖然很高，可是也不全是這個關係，也要考慮其它因素。如反射率和co2含量。

albedo80 % receive20 % albedo26 % receive74 %

The CO2 in the atmosphere Consider albedo The CO2 in the atmosphere Venus = 96 % Earth = 0.02 % Greenhouse effect Venus2850C4600C Earth310C150C The Venus higher albedo reduces the amount reaching its surface to just over half that of Earth

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% 核融合的過程，使太陽越長越大。 以前太陽比現在小25~30 %

Completely frozen Earth? In astronomy  Yes In climate  No Some evidences indicate that Earth was not frozen solid anytime 天文上說地球曾經很冷過。 但氣象卻否 Why ?

Thermostat (temperature regulator) Recall the temperature on Venus 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 回想前面金星與地球，金星因為co2很多，所以很熱。 地球的carbon在rock中 金星的carbon在atmosphere中

Carbon Exchanges between Rocks and the Atmosphere

Carbon Exchanges Between Rocks and Atmosphere Volcanic input of carbon from rocks to the atmosphere Removal of CO2 from the atmosphere by Chemical Weathering 天文上說地球曾經很冷過。 但氣象卻否

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? 有機碳在rock中氧化，最後以co2形式釋出到大氣裡。

Oxidation of organic carbon in sedimentary rocks

Chemical Weathering Hydrolysis Dissolution Main mechanism 天文上說地球曾經很冷過。 但氣象卻否

Removal from the Atmosphere Hydrolysis Three key ingredients Minerals  silicate minerals Water  derived from rain CO2  derived from the atmosphere Removal from the Atmosphere 天文上說地球曾經很冷過。 但氣象卻否 Silicate rock (continents) Carbonic acid (soil) Shells of organisms

Removal from the Atmosphere Dissolution The rate is faster than hydrolysis Removal from the Atmosphere Limestone (rock) In soils Shells of organisms Returned to atmosphere 天文上說地球曾經很冷過。 但氣象卻否

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. 全球平均風化速率視地球氣候型態而定。 但風化也會影響大氣中co2的移除。

The weathering thermostat works as a negative feedback Negative feedback simply moderate the degree of climate change

Faint young Sun paradox Earth 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 現在可以解釋為何地球不會完全冰凍

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 講完之後就講：是否還有其它原因可以控制這個thermostat 如生活。生物圈，蓋亞假說。

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 plantsCO2H2CO3 The shell-bearing ocean plankton CO2CaCO3

430 Myr 2.1 Byr 2.3 Byr 2.5 Byr 3.5 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. 35億年前：單細胞生物出現 25億年前：細胞核 23億年前：大氣中出現O2 21億年前：多細胞生物 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