Key Idea 1: Climate Change through Geologic Time

• Eccentricity:

  • orbit changes from being more elliptical to more circular - eccentricity measures how much the shape of earth's orbit departs from a perfect circle (these variations affect the distance between the earth and sun)
  • cycle last 96,000-100,000yrs + earth's orbit at elliptical maximum equals a 20-30% difference in solar radiation recieved in Jan/July

  
• Eccentricity:

  • in a more elliptical orbit, earth is further away from the sun reducing the solar energy that reaches the earth and cooling global temperatures in some months + when earth's orbit is more circular, solar energy is more consistent
  • eccentricity affects the length of the seasons; as eccentricity decreases, the length of the seasons evens out
• Obliquity (Axial Tilt):

  • obliquity is the angle earth's axis of rotation is tilted as it travels around the sun - cycle lasts 41,000yrs
  • not all earth recieves same amount of energy - solar energy entering at shallower angle at higher latitudes travels futher through atmosphere/spreads across larger area
  • tilt of earth's axis varies from 21.8-24.4$^o$ relative to the plane of its orbit (plane of the elliptic); earth's tilt is currently 23.5$^o$

  
• Obliquity (Axial Tilt)

  Increased tilt acts to amplify seasonal difference, whilst decreased tilt diminishes it:

  • greater axial tilt = more extreme seasons (each hemisphere recieves more solar energy in summer as tilted towards sun and less during winter)
  • less axial tilt - sun's solar energy more evenly distributed over summer/winter + less tilt increases difference in energy recieved between equatorial/polar regions
• Precession:

  • earth wobbles on its axis causing long-term changes in where different seasons occur along earth's orbital path - cycle lasts 21,000-23,000yrs
  • precession of the equinoxes; northern hemisphere summers occur when earth near furthest point from sun of its orbit (aphelion) + 10,000yrs ago northern hemisphere summers occured when earth nearest to sun on its orbit (perihelion)

  
• Precession:

  • axial precession makes seasonal contrasts more extreme in one hemisphere/less extreme in the other; southern hemisphere summers currently hotter and moderate northern hemisphere seasonal variations
  • could expand/stretch winter or summer months + axial wobble amplifies tilt and eccentricty
• Ocean Currents:

  • driven by wind, tides and water density/depth changes + topography of ocean floor affects speed/direction of currents
  • surface currents control top 10% (surface currents near shore controlled by wind/tides) + deep ocean currents control other 90% (mostly affected by wind)
  • currents affected by coriolis effect (spin of the earth); affects wind patterns and therefore currents - gyres spin clockwise in northern hemisphere and counterclockwise in southern hemisphere
• Thermohaline Circulation - cool water near poles freezes leaving salt behind making the water saline (increases it's density, so it sinks); forms conveyor belt that moves southwards. Process helps regulate global temperatures because it transports large amounts of heat from the tropics to the polar regions.
• Paleogene (66-23Ma):

  • pangea continued to break up; India/Africa collided into Eurasia to uplift the Himilayas/Alps + Atlantic opened wider seperating further Europe from America
  • enormous subduction zones from Pacific 'Ring of Fire' consuming large amounts of oceanic crust results in volcanoes/earthquakes
  • climate went from greenhouse in early eocene to icehouse by early oligocene (glaciers first appeared in Antartica) - tropical species of eocene gradually replaced by cold-tolerant species in oligocene
• Antartic Circumpolar Current (ACC):

  • movement of Australia away from Antartica/opening up of Drake's Passage enabled the circumpolar event which cooled the climate
  • water from South Atlantic/India/South Pacific began to be trapped in this current; progressively cooled whilst cycling around Antartica
  • as Australia moved further away, Drake's Passage deepened and cold ocean current strengthed; resulted in further cooling leading to glacial expansion (amplified by earth's albedo effect) - cooled Antartica by 3$^oC$ resulting in Ice Age
• • Paleogene 56Ma - Drake's Passage; South America used to connect to Antartica (blocked cold currents from circulating around Antartica resulting in moderate climate) + opening of Drake's Passage resulted in colder Antartica - North and South America not connected (allowed movement of ocean currents between Atlantic/Pacific at the equator).

  
• Atmospheric Circulation affects climate at:

  • Equator (0$^o$) - warm air rises (less dense), as air cools it moves north/southwards but some remains and condenses to form clouds resulting in rain (tropical climate; warm air/high rainfall); warm air from high concentration of sun energy.
  • Sahara Desert (30$^o$N) - as warm air moves northwards it sinks and cools at 30$^o$N resulting in dry conditions from lack of cloud creation/rainfall + descending air warms by compression giving cloudless/stable conditions resulting in desert conditions.
• Atmospheric Circulation affects climate at:

  • UK (50-60$^o$N) - here warm air rises, cools and condenses to form clouds/rain + influenced by polar/ferrel cells (warm air from tropics meets cold polar air leading to instability, and areas of low pressure/depressions creating rain); unstable weather/variations between seasons.
  • Poles (90$^o$) - cold air sinks giving high pressure resulting in dry, stable conditions (lack of rainfall); high pressure from divergence of air north/south.
• • trade winds form when warm air rises at the equator, resulting in more air rushing in to replace the lost air; then combined with the coriolis effect (spin of the earth), the trade winds blow east to west and towards the equator
  • westerlies are created in high pressure areas; blow west at mid-latitudes (and influenced by coriolis effect)

  
• • earth's tilt affect global atmospheric circulation as it affects the amount of sunlight parts of the earth recieve; if earth is tilted a lot, the sun may be less concentrated at higher latitudes (could affect the rising of warm air disrupting global atmospheric circulation)
  • with tilt, the cells (hadley, ferrel and polar) shift futher northward changing the weather conditions for some seasons
• Atmospheric Circulation:

  • meeting of trade winds in equatorial region forms the Inter-Tropical Convergence Zone (ITCZ) + trade winds (pick up latent heat crossing warm/tropical oceans) forced to rise by convection currents
  • unstable, warm and moist air rapidly cooled adiabatically to produce cumulonimbus clouds/frequent afternoon thunderstorms and low pressure (influence equatorial climate) + at ground level, ITCZ experiences gentle/variable winds (doldrums)
• Atmospheric Circulation:

  • as rising air cools to temperature of environment, uplift ceases and it begins to move further from equator + further cooling, increase in density and diversion by coriolis force causes air to slow/subside (forms descending limb at Hadley Cell)
  • air subsides 30$^o$N/S of equator to create sub-tropical high pressure belt with clear skies and dry/stable conditions + on earth's surface, cell is completed as some air returned to equator as north-east trade winds
• Atmospheric Circulation:

  • remaining air diverted polewards, forms warm south-westerlies (collect moisture crossing seas); warm winds meet cold Artic air at polar front (60$^o$N) and are uplifted forming low pressure area (forms rising limb of Ferrel/Polar Cells)
  • resultant unstable conditions produce heavy cyclonic rainfall (associated with mid-latitudes depressions); some rising air returns to tropics/some towards poles (cools/descends to form area of stable high pressure) - air to polar front is cold easterlies
• Major Monsoon in Southeast Asia results from:

  • extreme heating/cooling of large land masses in relation to smaller heat changes over adjacent sea areas (affects winds and pressure)
  • northward movement of the ITCZ during northern hemisphere summer
  • uplift of the Himilayas (6Ma) became sufficiently high to interfere with the general circulation of the atmosphere
• South-West/Summer Monsoon:

  • overhead sun appears to move northwards to tropic of cancer in June drawing the convergence zone associated with ITCZ + increase in insolation over northern India/Pakistan/Central Asia means heated air rises, creating large area of low pressure
  • so, warm moist equatorial maritime air/tropical maritime air (from Indian Ocean) is drawn northwards first then diverted northeast due to coriolis force
• South-West/Summer Monsoon:

  • air is humid, unstable and conductive to rainfall; precipitation most substantial on India's west coast (air rises over western Ghats); monsoon storms allow for rice planting
  • rainfall totals heightened as air rises by orographic/convectional uplift + 'wet' monsoon maintained by release of substantial amounts of latent heat
  • average arrival date; 10 May, Sri Lanka + 5 July, Pakistan border (range of 7 weeks)
• North-East/Winter Monsoon:

  • overhead sun/ITCZ/sub-tropical jet stream all move southwards; central asia experiences intense cooling (allows high pressure system to develop)
  • airstreams moving outwards from this high pressure area are dry as their source area is semi-desert + become drier as they cross Himalayas/adiabatically warmer as they descend to Indo-Gangetic Plain
  • south-west monsoon begins retreat from northwest of India on 1st September, and takes till 15th November (11 weeks) to clear southern tip
• Monsoon - a seasonal prevailing wind blowing from south-west between May and September bringing 'wet' monsoon, or from north-east between October and April bringing 'dry' monsoon in South Asia.

  • monsoon affects 1/4 of world's population + monsoon rainfall unreliable; low rainfall leads to drought, and excessive rainfall leaves areas prone to flooding