10.1 How and why has climate changed in the geological past?

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How are seafloor sediment used to reconstruct past climates?

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1

How are seafloor sediment used to reconstruct past climates?

  • Fossil shells of foraminifera accumulate in ocean floor sediments.

  • Foraminifera shells indicate the ocean temperature in which they were formed.

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2

How are ice cores used to reconstruct past climates?

  • Contain records of the gaseous composition of past atmospheres.

  • Contain frequency of hydrogen and oxygen atoms with stable isotopes.

  • Colder the climate = fewer isotopes.

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3

How is lake sediment used to reconstruct past climates?

  • Contain diatoms + varves

  • Diatoms record evidence of past climates in shells.

  • Lighter band of sediment indicates spring/summer climate.

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4

What are diatoms?

Single-celled lake algae

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5

What are varves?

  • Tiny layers of lake sediment.

  • Light and dark bands of sediment

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6

How are tree rings used to reconstruct past climates?

Annules vary in width each year depending on temperature and moisture.

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7

How are fossils used to reconstruct past climates?

  • Plants and animals require specific environmental conditions to thrive.

  • Where they exist in the fossil record can be used as proxies for climate.

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8

How do changes in atmospheric CO2 explain the glaciation of Antarctica?

  • CO2 levels dropped abruptly 35 million years ago.

  • From 1000-2000ppm to 600-700ppm

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9

How does continental drift explain the glaciation of Antarctica?

  • Movement of Antarctica away from South America and Australia.

  • Isolated Antarctica from warm water due to Antarctic Circumpolar Currents.

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10

How does the growth of the South Sandwich Islands' explain the glaciation of Antarctica?

  • Disrupted deep ocean currents around Antarctica.

  • Isolated from warm water

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11

How does the obliquity of the Earth caused climate change? (External forcings)

  • Earth's axial tilt varies from 22° to 24.5°.

  • When close to 22°, seasonal temperature differences are reduced.

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12

When did the Devensian period reach its peak?

When approximately 1/3 of the continental surface was covered by snow and ice.

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13

How many years does the Quaternary period span?

2.6 million years

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14

What are the features of the Holocene?

  • The current glacial period

  • Began 11,700 years ago

  • Ice sheets and glaciers have shrunk

  • Several relatively short warming and cooling periods

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15

What are the impacts of the obliquity of the Earth on glaciers and ice sheets? (external forcings)

  • They expand

  • Positive feedback effect increased.

  • Reflection of incoming solar radiation

  • Further lowering temperatures.

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16

How does the eccentricity of the Earth's orbit cause climate change? (External forcings)

  • Earth follows elliptical orbit around the sun

  • Eccentricity varies over periods of 90,000-400,000 years.

  • With maximum eccentricity, solar radiation reaching the earth's surface varies by 30% when closest to the sun and furthest from the sun.

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17

How are ice ages influenced by the eccentricity of the Earth’s orbit? (External forcings)

Ice ages correspond to maximum orbital eccentricity.

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18

How does the precession of the equinoxes cause climate change? (External forcings)

  • Point in the Earth's orbit where it is closest to the sun (perihelion) changes every 22,000 years.

  • Perihelion occurs during northern hemisphere's winter, winter = warmer.

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19

How do volcanic eruptions cause climate change? (Internal forcings)

  • Explosive eruptions release huge amounts of ash and sulphur dioxide

  • Change global climate in the short term.

  • Sulphur Dioxide converted to sulphate aerosols

  • Reflect solar radiation back into space.

  • Lowers temperature in the troposphere.

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20

What is an example of a volcanic eruption causing climate change? (Internal forcings)

  • Mount Pinatubo eruption (June 1991) injected 20 million tonnes of Sulphur Dioxide into the stratosphere.

  • Lowered Earth's climate by 1.3C

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21

How does continental drift cause climate change? (Internal forcings)

  • Changes landmass positions over time

  • Movement can alter ocean currents and atmospheric circulation.

  • Changes in currents and air patterns can lead to cooler temperatures

  • Contribute to glacier formation

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22

How do natural greenhouse gases cause climate change? (Internal forcings)

  • Close relationship between atmospheric CO2 and average global temperature.

  • Periods of glacial conditions corresponded with low levels of atmospheric CO2

  • Reduce the natural greenhouse effect

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23

How does continental drift explain how CO2 is removed from the atmosphere? (Internal forcings)

  • Tectonic plate movements created extensive fold mountains (Himalayas)

  • Led to increased rainfall and chemical weathering

  • Caused large volumes of CO2 to be removed and transferred to storage in carbonate sediments in oceans

  • Stimulated growth of phytoplankton to extract from the atmosphere.

  • Phytoplankton died and CO2 became trapped in ocean sediments.

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24

How does solar output vary over time? Does this cause climate change? (Internal forcings)

  • Solar output follows an 11-year cycle.

  • The difference in energy output between maximum and minimum solar activity is only 0.1%

  • This is not enough to impact the global climate significantly.

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25

What are examples of solar output causing climate change? (Internal forcings)

  • End of the 17th century saw zero sunspot activity for several decades.

  • Explains the severe winters in Europe at this time.

  • The Little Ice Age

  • Past 50 years has had relatively high sunspot activity.

  • Coincides with the recent warming of the Earth’s global climate.

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26

What do ocean currents do?

Transfer surplus energy (warm water) from the tropics to the poles.

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27

How does continental drift affect ocean circulation?

  • Gulf Stream intensified by South America and North America being connected.

  • Causes evaporation and precipitation to increase

  • More precipitation deposited in the north, diluting the salinity of the North Atlantic and the Arctic.

  • Reduced salinity weakened the downwelling of water in the North Atlantic

  • Led to the expansion of sea ice

  • Amplified by positive feedback.

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