class #39: friday, april 17

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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 446-449

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

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The “Little Ice Age” occurred between about 1400 and 1850

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Milankovitch Cycles

• Precession, which is north star, 27,000 years• Obliquity, tilt 22-24.5º, 41,000 years• Eccentricity, more/less elliptical, 100,000 years• Cold periods 20, 60, 160 K years ago

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Plate Tectonics and Continental Drift

• Pangaea, one large tropical supercontinent, 300 million years ago

• 160-230 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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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

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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

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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

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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

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