Class #39: Friday, April 171 Mechanisms of Climate Change Natural and Anthropogenic

Review for test #4 on Monday, April 20 Chapter 13, Weather Forecasting –Skip Boxes 13.2 and 13.4 Chapter 14, Past and Present Climates Chapter 15, Human Influences on Climate –No questions on ozone holes, pages Class #39: Friday, April 172

3 What Mechanisms Have Caused Climate Change in the Past Overview: most sudden to the slowest Volcanic eruptions: acidity, overall cooling Asteroid impacts: overall cooling Solar variability: cooling or warming Variations in Earth’s orbit: Milankovitch cycles; cooling or warming Plate tectonics Changes in ocean circulation: can be rapid and long-lasting Natural variability: variations without forcing

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7 There may be a 26-million year periodicity in asteroid impacts

Class #39: Friday, April 178 The “Little Ice Age” occurred between about 1400 and 1850

Class #39: Friday, April 179 Milankovitch Cycles Precession, which is north star, 27,000 years Obliquity, tilt º, 41,000 years Eccentricity, more/less elliptical, 100,000 years Cold periods 20, 60, 160 K years ago

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Class #39: Friday, April 1711 Plate Tectonics and Continental Drift Pangaea, one large tropical supercontinent, 300 million years ago million years ago, a split occurred Laurasia: Asia, Europe, North America Gondwanaland: South America, Africa, India, Australia, Antarctica Collisions caused Himalayas, Rocky Mtns. Maybe ice sheets when continents became less tropical

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Class #39: Friday, April 1715 Ocean circulation The thermohaline circulation is a world- wide 3-dimensional ocean circulation Sinking motion occurs in the North Atlantic when cold salty water sinks This circulation can be cut off when melt causes water to be less dense and not sink Maybe responsible for cooling in a period of glacial melt

Class #39: Friday, April 1716 Global Warming is a fact!!! Over the past 2 decades the global average surface temperature has increased noticeably. A trend involves a steady change in one direction—upward for global average temperature. Not every location and/or every region shows the identical pattern.

More observations of global warming Widespread retreat of nonpolar glaciers Thinning of arctic sea ice Decreased N Hemisphere snow cover Increase of global mean sea level Longer growing season in NH Shortened duration of ice cover on NH lakes Class #39: Friday, April 1717

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Feedback: change leads to change leads to more change Positive feedback mechanism: reinforces (enhances) the original trend (change) Negative feedback mechanism: damps out an existing trend (change) Example of a positive feedback mechanism: warming, evaporation, water vapor, warming Class #39: Friday, April 1725

More climate feedback mechanisms Example of a negative feedback mechanism: warming, evaporation, water vapor, cloud, cooling Another positive feedback mechanism: –Called the ice/albedo feedback mechanism –Cooling, more ice, higher albedo, more cooling –Warming, less ice, lower albedo, more warming Class #39: Friday, April 1726

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