Causes of Climate Change (Edexcel A Level Geography)

Revision Note

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Jacque Cartwright

Written by: Jacque Cartwright

Reviewed by: Bridgette Barrett

Climate Change Over Time

  • Geological time is measured in aeons, eras, periods, epochs, and ages

  • We are now in the Quaternary period, which is divided into 2 epoch

    • Pleistocene epoch which began 2.6 million BP (before the present)

    • Holocene epoch, beginning at the end of the last ice age some 11,700 years BP 

  • Earth's climate has always swung between icehouse and greenhouse conditions, due to various reasons and over different lengths of time

  • During the Pleistocene epoch, there were 17 periods of glaciation (ice ages) and 17 interglacial (warming) periods  

    • Glacial periods saw glacial advance/expansion and sea levels dropped

    • Interglacial saw glacial retreat/contraction and sea level rise

    • Each cycle of warm-cold lasts about 100,000 years

Mean Antarctic temperatures and atmospheric CO2 concentration over the past 200,000 years
Mean Antarctic temperatures and atmospheric CO2 concentration over the past 200,000 years
  • 21,000 years ago,  32% of the Earth's surface was covered in ice

  • Currently, the Earth is in an interglacial period with glaciers retreating, 

Examiner Tips and Tricks

Remember that climate refers to a 30 year period of time, where temperature and precipitation has remained fairly constant over that period. Weather is the day-to-day conditions, which is dynamic. 

Weather is what you get and climate is what you expect.

You expect to need an umbrella in the UK; but not in the Mediterranean. 

Longer Term Climate Change

Milankovitch cycles of climate change

  • Milankovitch cycles describe the effects of changes in the Earth's movements on its climate over thousands of years

  • In the 1920s, Milankovitch suggested that variations in eccentricity, tilt, and wobble of the Earth's orbit resulted in cyclic changes in the amount of solar radiation reaching the Earth

  • He noted that this orbital forcing strongly influenced climatic patterns on Earth

  • These changes occur over thousands of years

Cycle

Time in Years (approx.)

Effect

Eccentricity (shape) 

100,000

The Earth is currently closer to the Sun in January than in July, meaning the seasons are more extreme in the Southern Hemisphere than in the Northern Hemisphere. Resulting in the northern hemisphere receiving roughly 7% less radiation in the summer, and 7% more in the winter, than the southern hemisphere in its equivalent seasons because the Earth is closer to the Sun in January than in July

Obliquity (tilt)

41,000

If the Earth’s axis were vertical, there would be no seasons – the same part of the Earth’s surface would be facing the Sun throughout the year. The more angled the axis, the more extreme the seasons. Currently, the Earth is tilted at 23.44° from its orbital plane, which is halfway between its maximum (24.5°) and minimum (22.1°) tilt and this angle is on the decreasing cycle

Precession (wobble)

26,000

The axis also traces a circle in space and has a 26,000-year time period. This is a gyroscopic motion due to the tidal forces exerted by the Sun and the moon on the solid Earth, it doesn’t help that the Earth is not a perfect sphere but has an equatorial bulge(expanded waistband). It changes which star we see as the North Star – currently it is Polaris, but 13,000 years ago, it would have been Vega

milankovitch-cycles
The shape, tilt and wobble of Earth's movement over thousands of years, affects long-term climate
  • Although the impacts of orbital change on insolation and its distribution across Earth's surface is small (±0.5°C), its overall effect is considered enough to 'tip' the climate into a major change

  • Yet evidence from ice cores, show that Earth's climate was 5-7°C colder, during the glacial icehouse periods

  • Positive feedback mechanisms fed and sustained the overall cooling

    • Small increase in snow and ice, raises surface albedo rates and lowers temperatures, which encourages further snowfall and further cooling cycles

    • Eventually, surface temperatures could drop from ±0.5°C to -7°C

Sunspots and flares

  • The energy emitted by the Sun varies because of sunspots, although the total variation in solar radiation is only about 0.1%

  • Sunspots are regions of intense and complicated magnetic fields that can produce solar flares – bursts of high-energy radiation and have been recorded for around 2000 years and really well over the last 400 years

  • Sunspots range from Earth-size “pimples” to swollen scars halfway across the surface

  • A solar flare is a violent eruption of plasma from the Sun, that is whipped up by intense magnetic activity

  • During the eruption, flares rise thousands of kilometres above the Sun, and the plasma temperatures quickly rise to 20 million °C

  • Large flares release 10^25 Joules, or about the energy of a few million volcanic eruptions on the Earth

  • Sunspots and solar flares are related:

    • Flares disturb the Earth’s atmosphere electrically and interfere with radio transmissions

    • The Aurora Borealis and Aurora Australis are results of flare activity that injects energetic particles into Earth’s magnetic field

    • Sunspot activity runs on an 11yr cycle of highs and lows

Examiner Tips and Tricks

Don't underestimated the effect that sunspots and orbital changes have on the overall climate. They may initially be small, but they are amplified through feedback mechanisms.

Volcanic eruptions

  • Eruptions eject large volumes of sulphur dioxide (SO2), water vapour, dust, and ash into the atmosphere

  • These gases and dust particles, once in the stratosphere, reflect some of the insolation, leading to cooling

  • Large volumes of gases and ash, influence climatic patterns for years

    • For example, in 1991 Mount Pinatubo, Philippines, ejected 17 million tonnes of SO2 into the atmosphere

    • Sunlight reduced by 10% bringing average global temperatures down by 0.6°C for a year

    • The effects of sulphate aerosols in the atmosphere are short-lived at around 2-3 years

Shorter Term Climate Change

Loch Lomond stadial

  • The term stadial refers to a brief cold period during an interglacial warm period

  • As ice sheets were melting, towards the end of the Pleistocene epoch's last glacial period (known as the Devensian) some 12,700 years BP, a short but severe glaciation returned to the North Atlantic region

  • This event called the Loch Lomond stadial, caused glaciers to grow in the Scottish Highlands

  • Temperatures across the British Isles ranged from -20°C in winter to 10°C during the summer

  • These conditions lasted for approximately 1300 years when temperatures rose suddenly and have continued to do so, ever since

  • One possible cause of this stadial was the sudden influx of cold, freshwater into the North Atlantic, from the melting polar ice sheets 

  • This freshwater would have disrupted the salt content driving the thermohaline circulation 

    • Ocean currents redistribute heat around the globe - cold currents move towards the equator and warm currents to the poles

    • The polar cold waters are denser, saltier sea water, which sinks to the ocean floor

    • Water at the surface then flows in behind it forming a current

    • The deep ocean current begins flowing to Antarctica, where it splits into the Indian and Pacific Oceans, and the water begins to warm 

    • Warm water is less dense and it surfaces in the South and North Atlantic Oceans and continues to flow around the globe and eventually returns to the North Atlantic, where the cycle begins again

    • Any disruption to this cycle changes the climate of the receiving continents

      • E.g., instead of the UK receiving warm, equatorial waters via the North Atlantic Drift, the circulation effectively stalled or reversed and the UK received the cold polar waters and climate

    • This stadial ended when the glacial meltwater supplies ran out, indicating how dynamic climate changes are

 

thermohaline-circulation
Thermohaline circulation: note how the UK receives warm waters from the equator and then returns cold water via the Canadian and USA eastern seaboards. This keep the UK within a temperate climatic zone with no annual temperature extremes 

Little Ice Age

  • Solar activity goes through an 11-year cycle of peaks and troughs

  • A solar maximum is the point in the cycle when the Sun is emitting the most energy

  • This coincides with maximum sunspot activity, solar flares and associated solar storms

  • Between 1645 and 1715 there was virtually no sunspot activity recorded and known as the Maunder Minimum

  • The Little Ice Age lasted from 1450 - 1850, with especially cold periods in 1660, 1770 and 1850 

  • Average temperatures were 0.5-1°C lower than now, which caused rivers and lakes to freeze regularly and Arctic sea ice was more extensive

  • During this time, the growing season became shorter and less reliable, and livestock survival rates decreased

  • These shortages increased the cost of food and people suffered illness and famine during that glacial period

Antarctic ice core data: changes in temperature over the last 11,000 years
Antarctic ice core data: changes in temperature over the last 11,000 years

Examiner Tips and Tricks

If you are asked to describe a pattern in the exam, make sure you start with a general overview of the main pattern, rather than starting with the finer details.

Learn approximate dates of geological time periods of the Pleistocene, Holocene and Devensian and relate them to events such as Maunder Minimum, Loch Lomond stadial and the Little Ice Age.

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Jacque Cartwright

Author: Jacque Cartwright

Expertise: Geography Content Creator

Jacque graduated from the Open University with a BSc in Environmental Science and Geography before doing her PGCE with the University of St David’s, Swansea. Teaching is her passion and has taught across a wide range of specifications – GCSE/IGCSE and IB but particularly loves teaching the A-level Geography. For the past 5 years Jacque has been teaching online for international schools, and she knows what is needed to get the top scores on those pesky geography exams.

Bridgette Barrett

Author: Bridgette Barrett

Expertise: Geography Lead

After graduating with a degree in Geography, Bridgette completed a PGCE over 25 years ago. She later gained an MA Learning, Technology and Education from the University of Nottingham focussing on online learning. At a time when the study of geography has never been more important, Bridgette is passionate about creating content which supports students in achieving their potential in geography and builds their confidence.