Causes of Climate Change (Edexcel A Level Geography)

• 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

• 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, 

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

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

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

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: 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