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Effective Radiating T of Earth

lRadiation emitted by a black body is proportional to thefourth power of its absolute T (F a T4)

lEffective radiating T for Earth = 255 K, or -18 C, or 0 FlThis is the average T of Earth if it had no atmosphere.lDifference between Earth’s present average surface T

(15 C) and its effective radiating T is due to thegreenhouse effect of the atmosphere; this temperature difference is 33 C. That is, the atmosphere increases Earth’s average T by 33 C.

lThe natural greenhouse effect is as important in determining Earth’s climate as is its distance fromthe sun.

lWhat was the surface T of the early Earth with afainter sun?

lSolar flux is predicted to be 30% less than todaylCalculated effective radiating T is 233 K or -40 ClIf greenhouse effect was the same, then Earth’s

surface T = 233 K + 33 K = 266 K, or -7 ClEarly Earth was not frozen; therefore greenhouse

effect must have been greater then than it isnow.

Faint Young Sun

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The early Earth had much higher atmospheric CO2 than today.

Decrease in atmospheric CO2 from higher amounts early inEarth history to the lower amount at the present time

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Photosynthesison land

Photosynthesisalso occurs inthe ocean, butonly in theupper portionpenetrated bysunlight; onlyrespirationoccurs in thedark part belowthe depth oflightpenetration.

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Simplified Carbon Cycle

ANIMALS

Photosynthesis

Respiration

Atmosphere

Burial

CO2 O2

CO2 + H2O + nutrients <==> CH2O + O2

PLANTS

Forms of Carbon in the Earth System

• Atmosphere - carbon dioxide (CO2)

• Biosphere - living and dead organic matter

• Hydrosphere - dissolved and particulate organic C; ! dissolved inorganic C (HCO

3-)

• Lithosphere - organic matter and CaCO3

! (limestones)

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Carbon is cycled through the terrestrial and marine ecosystemsprimarily by photosynthesis and respiration, although otherprocesses also play a role in the Carbon Cycle.

The concept of steady state: if the rate of input equals the rateof output, then the size of the reservoir (such as the water inthis bathtub) will be constant.

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The major reservoirs in which carbon exists on Earth in oneform or another: as a gas (CO2), in organisms, dissolved in theocean, or locked up in sedimentary rocks.

Distribution of Oceanic Carbon

0.2330

Deep DOC Surf DOC POC Plankton Bacteria0

200

400

600

800

1000

90

860

1015 g C

Types of CarbonMost organic carbon in the ocean is in the form of non-livingdissolved organic matter (DOC).

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Oceanic and Land Carbon Pools

Soil OC Ocean OC Land Bio Ocean Bio0

100

200

300

4001016 g C

300

100 80

0.3

Types of Carbon

Most organic carbon (OC) on land and in the ocean is in the formof non-living organic matter.

The rate at which carbon moves from one reservoir to another in ayear: if the Carbon Cycle is balanced, inputs and outputs of eachreservoir will be equal, and the reservoir sizes will neither increasenor decrease.

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Reservoirs and exchange pathways in the short-termCarbon Cycle

Quick Summaryl C switches back and forth between the

inorganic form and the organic form as itpasses through the biological part of the cycle.

l C is transferred from one reservoir to anothereither as part of the short-term biological cycleor the long-term geological cycle.

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Another view of thetransfer of carbonfrom one reservoirto another withhuman activities(e.g., burning offossil fuels,deforestation) nowincluded. Humanactivities represent anet input of CO2 tothe atmospherebecause naturalprocesses are inbalance.

The increase in atmospheric CO2 since measurements began inthe late 1950s.

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Various human activities that add CO2 to the atmosphere

X

Geological processes such as volcanism and weatheringalso add and remove CO2 from the atmosphere. Theseprocesses affect atmospheric CO2 levels over very longperiods of time.

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12

13

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Removal of C from the atmosphere by chemical weathering ofsilicate rocks. The C ends up in the ocean.

CO2 is one of the major products of volcanic activity. Most of thisC is derived from sedimentary rocks that have been buried deeplywithin the Earth.

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The “geological” part of the Carbon Cycle involving burial of Cin sedimentary rocks and then return of this C to the Earth’ssurface during weathering of rocks and through volcanic activity.

Plate tectonic processes carry old ocean crust and sea floorsediments into the Earth. The high temperatures cause meltingof these rocks leading to volcanoes and release of buried carbon.

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The relationshipbetween temperatureand precipitation onEarth. Hightemperatures andhigh rainfall lead toenhanced weatheringand erosion of rocks.

A negative feedback diagram showing how an increase in temperaturewill cause an increase in chemical weathering and therefore adecrease in CO2.

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The same negative feedback diagram showing how a decrease intemperature causes a decrease in chemical weathering andtherefore leads to less removal of CO2.

Feedbacksl Feedbacks - responses of a system (e.g., climate) to a

perturbation or change in the existing statel Positive feedback - system responds to enhance or re-

enforce the initial change (i.e., the change ismagnified or fed back)

l Negative feedback - system responds to diminish orcancel the initial change (i.e., the system attempts toreturn to its normal or undisturbed state)

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Negative feedback relationship between the rate of seafloorspreading (which affects input of CO2 to the atmosphere fromvolcanism) and the rate of chemical weathering (which affectsremoval of CO2) resulting in a relatively constant level of CO2and a stable climate (neither too hot or too cold).

The long-term or “geological” part of the Carbon Cycle illustratingthe importance of plate tectonics in replenishing CO2 used up in rockweathering.

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A model of the geological Carbon Cycle illustrating the inputand removal processes that control the long-term concentrationof atmospheric CO2.

Making a Habitable Planet

• Planet orbits within the Habitable Zone - toallow liquid water to exist on its surface

• Stays within the HZ throughout the planet’shistory - as its star heats up

• Large enough to hold an atmosphere withits gravity - to maintain a “greenhouse”

• Plate tectonics operates throughout theplanet’s history - to replenish CO2

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Significance of PlateTectonics to Life

• Chemical rock weathering reactions willdeplete the atmosphere of CO2 unless amechanism exits to replenish it.

• Plate tectonic processes, especially volcanicactivity, replenish the CO2 used up in rockweathering.

• Life cannot survive for very long on aplanet without plate tectonics.

The SnowballEarth Hypothesis- did the Earthever look likethis, completelycovered in ice?

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The geologists are showing a layer of rock (extending from their feetto their heads) that was deposited by melting glaciers. This layer isimmediately overlain by one (above their heads) that was depositedin a tropical marine environment. Questions - did glaciers flow intothe ocean in the tropics? If so, was the entire Earth frozen?

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The overlapping time frames for the Snowball Earth episodes andthe appearance of animals. Did the frozen Earth create theopportunity for animals to evolve?