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Lecture 5: The Science of Climate Change (Global Warming (Greenhouse…

- - - - variations in shape of earth's orbit about sun, varies from elliptical to circular. circular favours glacial (cooling), elliptical favours interglacial (warming as earth passes closer to sun)
    - - inclination of earth's axis in relation to its plane of orbit around sun. today, earth tilted on rotational axis at 23.5 degrees relative to perpendicular orbital plane. over 41000 years, angle fluctuates btw 21.5-24.5, influencing latitudinal distribution of solar radiation, not amt. of radiation. smaller tilt (associated w/ cooling), small temp. diff. btw summer & winter
    - - wobbling of earth's axis which affects direction of tilt of axis. wobbles from pointing at Polaris (North Star) to star Vega. When points at Vega, it is the "new" N. Star. periodicity of 23000-26000 years. in 12000 years, NH will experience summer in Dec, winter in June.
- - - - pluvial: wetter periods in dry land environments, often lasting thousands of years. 2 possible explanations for this in N. Africa: movement southwards of precipitation normally over Mediterranean in winter OR movement northwards of ITCZ during warmer interglacial. during Pleistocene, inter-montane basins & tectonic troughs in Andes filled w/ meltwater from neighbouring glacial systems & higher precipitations. Some basins became smaller or non-existent.
    - - determine age of fossils, rocks & sediments. radiogenic isotopes formed from decay of radioactive parent element. dating technique: decay follows a particular pattern. every radioactive isotope has half-life, time taken for half isotope to decay. e.g. fossil H2O in Nubian sandstone in Egypt dated this way to 25000-35000 years ago, suggesting past pluvial existed then.
    - - studying layers of shells that settle on ocean floor. ocean drilling provides sediment cores (deeper sample, older material). sediments contain remains (bones, shells) that settle when organisms die. recent material will bury older material. useful recorders of climatic change as no.s & types of organisms change w/ climate. e.g. foraminifera, near surface plankton, v. sensitive to H2O temp. changes. presence & absence indicate fluctuations in climate.
    - - glacier advance, sea lvl falls, vice versa. falling rate slow (1-5m/1000 years), rising faster (5-10m/1000 years). in SE Asia, fall in sea lvl linked mainland Asia w/ islands of W. Indo across Sunda Shelf. dist. from sea could be responsible for reduced precipitation & increase in continentality. current deltas at river mouths flowing to sea formed when sea lvl rose during Holocene.
  - - - involves study of tree rings. each year, trees increase width of trunks by growth of concentric rings, each distinct. under climatic stress conditions (lack of moisture or excessive warmth), growth retarded. favourable conditions, thick rings. effective when v. old trees are cut down (issue: preserve or cut down?)
    - - examining diff. types, no. of pollen preserved in peat (top soil, tight & condensed, not easily eroded) & sediments. pollen & spores from vegetation are small & often wind-blown can be deposited, preserved indefinitely in lake beds. BUT not all species produce same quantity. e.g. research on terrestrial pollens show 4-7C drop in Java during glacial age.
  - - - streams erode downwards, creating V-shaped valleys & in winding manner. glaciers carve U-shaped valleys by widening the lower reaches of V-shaped valleys, in a straight manner.
      - erratic: flowing ice capable of moving large sized sediments tillites: sediments of varying sizes found after an ice sheet melts. found along past margin of ice sheet, showing extent of glaciation moraines: sediment ridges formed when ice recedes & stops @ various pt.s & build up. vary in height & composition depending on materials & length of time ice remains
      - glaciers advance (warm), retreat (cold). in tropics, happens only in higher parts of mountains. advancing glaciers require considerable supply of moisture. Andean snowline of glacial period is 400-1000m lower than present. some Andean plateaus, higher than 4000m, which were covered w/ small ice caps during the Pleistocene are ice free now.
    - - ice cores contain microscopic bubbles of ambient air permanently trapped by being compressed, when snow falls onto a glacier. conc. of CO2 & other gases yields long term record of their varying lvls in air. as snow accumulates, lays down record of environmental conditions @ time of formation, preserving it. amt. of gases varied significantly (low during glacial times, high during interglacials) but reached around same lvls in past 4 glacial periods, suggesting there have been processes that keep Earth from being too cold or warm