How and why has climate changed in the geological past

• Methods used to reconstruct past climates 

  -Sea floor and sediment- the fossil shells of tiny sea creatures which accumulate in sea floor sediments, can be used to reconstruct past climates, the chemical composition of the shells indicates the ocean temperature in which they formed 
• Methods used to reconstruct past climates

  -Ice cores- from the polar regions they contain tiny bubbles of air records of the gaseous composition of the atmosphere in the past, the colder the climate the lower the frequency of oxygen
• methods used to reconstruct past climates

  -lake sediments- pollen analysis identifies past vegetation types and from this infers climate conditions 
• methods used to reconstruct past climates

  -tree rings- annukes vary in width each year depending on temperature conditions and moisture availability, the warmer and more moist the conditions the wider the annules 
• methods used to reconstruct past climates

  -fossils- plants and animals require specific environmental conditions to thrive, some such as coral reefs are highly sensitive to temperature, water depth and sunlight. Where they exist in the fossil record they can be used as proxies for climate 
• Long term changes 

  -during the mid cretaceous period around 100 million years ago average global temperatures were 6-8*c higher than today

  -subtropical conditions extended from antarctica to alaska and there were no polar ice caps

  -atmospheric co2 levels were 5 time higher than today

  -continents had a different configuration which affected ocean circulation and the earth's energy budget 
• Glaciation of Antarctica 

  -the entire continent of Antarctica is covered by vast ice caps that extends to sea level 

  -largest glacial system on the planet 

  -the ice cap is so thick only the tops of the highest mountains appear above the ice 

  -however 40 millions years ago the fossil record shows that the continent experienced subtropical conditions

  -the descent of antarctica into a permanent ice house state occurred rapidly around 30 million years ago
• glaciation of Antartica

  -continental drift, the movement of antarctica towards the south pole and away from south america and australia, isolating the continent, this allowed the antarctic circumpolar currents around antarctica from the warmer water further north 

  -the growth of south sandwich Islands submerged volcanic are disrupted deep water ocean currents around antarctica isolating the continent from warmer water from the south pacific atlantic and indian oceans 
• Quaternary glaciation

  -span of the last 2.6 million years 

  -main feature has been cyclic changes of climate, with long, cold periods or glacials interrupted by shorter, warmer interglacials

  -the most recent glacial is the Devenisan, it reached its maximum around 20,000 years years ago when approximately one third of the continental surface was covered by snow and ice
• The holocene

  -the current geological period is known as the holocene

  -it began at the end of the last glacial 11,700 years ago 

  -during ice sheets and glaciers have shrunk and sea level has risen by over 100m and with the exception of antarctica and greenland ice sheets have disappeared from the continental surface

  -warming has not been continuous 

  -a ‘little ice age’ occurred when global temperatures fell by 1*c and sent winters into a deep freeze 

  -the vast majority of climate scientists believe that climate change in the past 200 years has been driven by human activity 
• Milankovitch cycles

  -milutin mikankoitch argued that long term climate shifts such as glacial cycles are caused by astronomical events 

  -these external forcing mechanisms affect the amount of solar radiation reaching the planets surface and its spatial and temporal distribution 

  -he identified which climate cycles followed with shorter interglacials

  -the most significant changes in climate conditions often occur when these cycles coincide 
• Forcing mechanisms that have an effect 

  -obliquity- over around 40,000 years the earth's axial tilt varies from 22 degrees to 24.5 degrees. When the tilt is close to 22 degrees seasonal temperatures are reduced. As a result snow and ice accumulated during winter doesn't melt during the summer this allows glaciers and ice sheets to expand. This has a positive feedback effect increasing the reflection of incoming solar radiation and lowering temperatures even further 
• forcing mechanisms that have an effect

  -eccentricity- the earth's orbit around the sun follows an elliptical path. The eccentricity of the orbit varies from near circular to markedly elliptical. Ice ages correspond to periods of maximum orbital eccentricity.
• forcing mechanisms that have an effect

  -precession of the equinoxes

  • the earth rotates on its axis, so that the point in the earth's orbit when the planet is closeted to the sun changes over time. This shift or precession which occurs with a periodicity of around 22,000 years is due to the gravitational influence of the moon and Jupiter and affects the intensity of the seasons. Snow and ice accumulating in winter will not melt completely in the following summer, so that ice and snow cover expands eventually triggering a glacial period 
• Volcanic eruptions

  -explosive eruptions pump huge amounts of volcanic ash and sulphur dioxide into the stratosphere

  -eruptions like that have the potential to change the global climate 

  -although volcanic ash is quickly removed, sulphur dioxide is more persistent and has a cooling effect, it in converted to sulphur aerosols in the atmosphere
• Plate tectonics and continental drift

  -driven by plate tectonics and sea-floor spreading, the global distribution and configuration of the continents have changed dramatically over geological time

  -tectonic change on this scale have a direct impact on the global climate, they can explain the periodic extreme shifts of the earth's climate between greenhouse and icehouse conditions
• Ocean currents

  -are a vital component of the global energy budget, transferring surplus energy from the tropics to the poles

  -continental drift can modify ocean circulation and energy transfer

  -around 5 million years ago there was the formation of the isthmus of panama which joined the north and south american continents and closed the gateway between the pacific and atlantic oceans 

  -it is believe that this event intensified the gulf stream, conveying
• Natural greenhouse gases 

  -in the tertiary era, tectonic plate movements created extensive fold mountain ranges such as the Himalayas, uplist in these mountains increased rainfall, erosion and chemical weathering by rainwater charged with co2, therefore large volumes of co2 were removed from the atmosphere and transferred to storage in carbonate sediments in the oceans

  -at the same time the increase in nutrients in the oceans stimulated vast blooms of phytoplankton which also extracted co2 from the atmosphere. When the organisms died the co2 was then trapped in deep ocean sediments
• Solar output 

  -the sun's output is not constant but varies markedly over time. 

  -there is a positive correlation between the number of sunspots and solar energy output

  -only in the past 30 years have satellite been able to measure solar irradiance more accurately