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

How are seafloor sediment used to reconstruct past climates?

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.

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.

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.

What are diatoms?

Single-celled lake algae

What are varves?

• Tiny layers of lake sediment.

• Light and dark bands of sediment

How are tree rings used to reconstruct past climates?

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

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.

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

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.

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

• Disrupted deep ocean currents around Antarctica.

• Isolated from warm water

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.

When did the Devensian period reach its peak?

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

How many years does the Quaternary period span?

2.6 million years

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

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.

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.

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

Ice ages correspond to maximum orbital eccentricity.

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.

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.

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

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

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

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.

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.

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.

What do ocean currents do?

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

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.