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Natural & Anthropogenic sources of climate variability

Natural & Anthropogenic sources of climate variability

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Natural & Anthropogenic sources of climate variability

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  1. Natural & Anthropogenic sources of climate variability 69EG6517 – Impacts & Models of Climate Change Dr Mark Cresswell

  2. Lecture Topics • Introduction – a brief history of time • Natural – Milankovitch cycles • Natural – Solar Cycles • Natural – Volcanic activity • Anthropogenic – Fossil fuels • Anthropogenic – Land use change


  4. Is this a new phenomenon?

  5. Introduction #1 It is known that our climate has experienced warmer and cooler phases throughout the past Sea levels regarded as rising at an “alarming” rate today have been considerably higher in the past

  6. Introduction #2 • Proxy reconstructions for the climate of the Quaternary Period are considerably more abundant and reliable than for earlier periods • The Quaternary spans the last 2Ma of Earth history and is separated into two Epochs, the Pleistocene (2Ma to 10Ka) and the Holocene (10Ka to present)

  7. Introduction #3 • Although deglaciation had been taking place for at least 4,000 years, a rapid deterioration (cooling) in climate occurred at about 10 to 11Ka • This event is known as the Younger Dryas Cooling. The North Atlantic polar front readvanced far southward to approximately 45°N (only 5 or 10° north of the glacial maximum position)

  8. Introduction #4 Holocene thermal maximum: 6 to 7 thousand years ago

  9. Introduction #5 • Quantitative estimates of mid-Holocene warmth suggest that the Earth was perhaps 1 or 2°C warmer than today • Most of this warmth may primarily represent seasonal (summer) warmth rather than year-round warmth

  10. Introduction #6 • Beginning about 1450 A.D. there was a marked return to colder conditions. This interval is often called the Little Ice Age, a term used to describe an epoch of renewed glacial advance

  11. Is this a new phenomenon?


  13. Milankovitch Cycles #1 • The distance between the Earth and Sun changes for a variety of reasons as does the quantity of solar energy reaching Earth • The Earth follows an elliptical orbit around the Sun. Orbital stretch/shrink ~100,000 yrs

  14. Milankovitch Cycles #2 • Milutin Milankovitch (a Serbian astrophysicist) worked out ways in which the Earth-Sun geometry changed as a function of orbital cycles

  15. Milankovitch Cycles #3 Milankovitch Cycles: • Variations in the Earth's orbital eccentricity—the shape of the orbit around the sun. • Changes in obliquity—changes in the angle that Earth's axis makes with the plane of Earth's orbit • Precession—the change in the direction of the Earth's axis of rotation, i.e., the axis of rotation behaves like the spin axis of a top that is winding down; hence it traces a circle on the celestial sphere over a period of time

  16. Milankovitch Cycles #4 Illustration of ECCENTRICITY

  17. Milankovitch Cycles #5 Illustration of OBLIQUITY

  18. Milankovitch Cycles #6 Illustration of PRECESSION OF THE EQUINOXES

  19. Milankovitch Cycles #7 Calculated Orbital Variation


  21. Solar Cycles #1 • There is really no such thing as a “solar constant” • We already know that orbital effects can change the quantity of solar radiation reaching the Earth • The Sun generates variable quantities of energy due to its own internal variability • Solar activity is know to have cycles – with a periodicity of about 11 years

  22. The 11 year solar cycle

  23. Solar Cycles #2 The 11 year solar cycle Historical overview of solar sunspot cycles

  24. Solar Cycles #3 The 11 year solar cycle Historical overview of solar sunspot area from 1870s to 2000

  25. Solar Cycles #4 • As well as sunspot activity, the Sun can interact with our atmosphere by generating solar flares leading to a powerful solar “storm” (enhanced solar wind) • Solar flares can damage satellites, and can also affect the Van Allen belts producing Aurora (Northern Lights)

  26. Solar Cycles #5 Coronal Mass Ejections (CMEs) are geomagnetic disturbances on the Sun surface that generates the Aurora Borealis.


  28. Volcanic Activity #1 • Active volcanoes generate large quantities of dust and smoke • Particulates in the atmosphere block out solar radiation, preventing it from penetrating through to the ground surface • The main effects of volcanic eruptions is to cool the affected regions (not dissimilar to a “nuclear winter”).

  29. Volcanic Activity #2 • When Mount Pinatubo erupted in the Philippines June 15, 1991, an estimated 20 million tons of sulphur dioxide and ash particles blasted more than 12 miles (20 km) high into the atmosphere

  30. Volcanic Activity #3 • A research team ran a general circulation model developed at the Max Planck Institute with and without Pinatubo aerosols for the two years following the Pinatubo eruption • The climate model calculated a general cooling of the global troposphere


  32. Fossil Fuels #1 • The burning of fossil fuels is believed to be the major source of anthropogenic climate forcing • Burning oil, gas and coal generates a wide variety of gases and particulates – the most important of which is carbon dioxide (C02) • The natural Greenhouse effect is enhanced by extra C02 to create the Enhanced Greenhouse Effect. Without the natural greenhouse effect global temperatures would be around 253 K (-20ºC) but is actually 288 K (15 ºC)

  33. Fossil Fuels #2 • Other greenhouse trace gases include Methane (CH4), Nitrous Oxide (N2O) and water vapour. Contribution of land, oceans and human activity to carbon flux


  35. Land Use Change #1 • Human activity can affect the way in which the Earth surface responds to solar radiation • Modifying land surfaces can profoundly affect heating and vulnerability to climate change • The gradual commercial deforestation of the tropical rainforest regions in South America have removed a valuable carbon sink and released carbon as this timber is burned or decays

  36. Land Use Change #2 • The “slash and burn” policy attributed to subsistence farmers in Africa and South America have removed tree species • Removal of trees can lead to landslips, soil erosion and development of dustbowls • Changes in Sahelian grasslands (removal) may have modified the albedo and soil moisture regime leading to droughts in the region

  37. Other Sources of Variability • Urban heat island • Ocean circulation • Geothermal activity • Tectonic plate movement Any questions ?