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Detailed revision notes for Topic 3.1 OCR A-level Geography - CLIMATE CHANGE
Detailed revision notes on climate change Including many case study examples and facts, ending with synoptic link to L6 topics.
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Climate Change1. How and why has climate changed in the geological past
a) The Earth’s climate is dynamic.
Climate proxy - preserved physical characteristics of the past which act as records climate variability.
Methods used to reconstruct past climate: paleoclimatology.
➢ Marine sediments: Shells of foraminifera (tiny sea creatures) accumulate as sediments.
Chemical composition of shells indicate the ocean temperature in which they were formed.
➢ Lake sediments: Pollen grains, diatoms (microscopic plants), varves (layers of lake sediment
in light and dark bands. Light shows coarser sediment, run off summer) can show past
vegetation type.
➢ Ice cores from polar regions contain bubbles of air, records of the gaseous composition of
the atmosphere in the past. Scientists can measure the relative frequently of H and O atoms
with stable isotopes. The colder the climate, the lower the isotope.
➢ Tree rings (dendrochronology): Annule vary in width every year depending on temperature
conditions and moisture availability. Rings closer together due to drought/infestation.
➢ Fossils: Plants and animals require specific environmental conditions. Where they exist in
the fossil record they can be used as proxies for climate. Coral reefs sensitive to
temp./water/sunlight.
Past climate to reveal periods of greenhouse and icehouse Earth
Greenhouse/ Icehouse -
hothouse glacials and
interglacials
Global CO2 conc. High Low
Global temperature High Low
Global sea level High Low
Volume of ice Low High
• Long term, 100 million year transition to colder global climate
conditions
• Glaciation of Antarctica around 35 million years ago, shift to
permanent icehouse conditions.
Caused by a abrupt drop in CO2 levels from 1000/12000 ppm to
600/700 ppm.
Continental drift, movement of Antarctica towards the South pole isolated it from warmer currents.
Emergence of the South Sandwich Islands submerged volcanic arc disrupted deep water currents
around Antarctica, further isolating it from warmer water.
,• Quaternary Glaciation - current period of geologic time. Last 2.6 million years.
Glacials (100,000 years) & interglacials (10,000-15,000).
Most recent glaciation was the Devensian, max. Ice extent 20,000 years ago. ⅓ of the continent was
covered.
Within glacials, ice advance or retreat in much shorter pulses of colder “stadials” and “interstadials.”
• Holocene Epoch, our present interglacial. Started 11,700 years ago.
Ice sheets and glaciers have retreated. Only remain in high-altitude e.g. Himalayas, Andes and
high-latitude locations. Sea level rise.
‘Medieval Warm Period’ (1110-1300) - average global temperatures increased 1-2℃
‘Little Ice Age’ (1550-1850) average global temperatures dropped 1℃. Freezing winters in Europe.
Explosive volcanism e.g. Tambora 1815.
Low sunspot activity and reduced solar output.
Many scientists believe we have now entered a new glacial period - the Anthropocene.
How natural forcing has driven climate change in the geological past, including:
• Plate tectonics
Volcanic activity: eruptions release sulphur dioxide into the stratosphere which lowers global
temperatures. E.g. Mount Pinatubo, the Philippines 1991, 1.3℃ temp. decrease over 3 years. 20
million tonnes of S.
Continental drift: larger continents occupy high latitudes, +ive feedback from albedo.
The Milankovitch cycles: long-term climatic shifts are caused by astronomical events.
, (Orbital) Eccentricity of the Earth’s orbit
Changes in the shape of Earth’s orbit from nearly circular to
elliptical in a 100,000 period.
➔ Currently 6%
Maximum orbital eccentricity - 30% difference in solar
radiation from where the Earth is closest to the sun
(perihelion) and when it is furthest from the Sun (aphelion).
Corresponds to periods of Ice Ages.
Obliquity (tilt) of the Earth’s axis
Influences seasons. Periodicity: 40,000 years. Varies between
22 and 24.5 degrees.
➔ Currently 23.5 degrees
When tilt is closer to 22, seasonal temperature differences are
reduced. Ice accumulated in winter does not melt in the
summer, glacier and ice sheets advance.
+ive feedback. Albedo effect reflects insolation and lowers
temperatures further.
Precession of equinoxes
Periodicity: approx. 22,000
The earth wobbles on its axis, so the point of perihelion (earth
closest to the sun) changes over time.
Due to the gravitational influence of the moon and jupiter
affecting seasons.
If perihelion occurs in the Northern Hemisphere’s winter =
warmer winters and cooler summers.
Eventually leads to a glacial period, as winter ice accumulates
and does not completely melt in the summer.
• Solar output varies over time.
Follows 11 year cycle.
Number of sunspots can act as a proxy for solar output. Positive correlation between number of
sunspots and solar energy output.
• The role of natural atmospheric greenhouse gases.
Correlation between atmospheric CO2 levels and average global temperature. Icehouse -low,
reduced natural greenhouse effect. Hothouse - high.
Uplift of fold mountains e.g. Andes, Rockies increased rainfall and chemical weathering. CO2 was
removed from the atmosphere and transferred to storage in carbonate sediments in the oceans.
Stimulated phytoplankton blooms, also extracted CO2 from the atmosphere.
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