class #35: friday, november 201 past climates: proxy data and mechanisms of change

Class #35: Friday, November 20 1

Class #35: Friday, November 20

Past Climates: Proxy Data

and Mechanisms of Change


Glaciers, Icebergs, Bubbles, and Dust

• Climate clues buried in ice just as in lake sediments

• When snow and ice exceed melting, glaciers form. Ice crystals crush under pressure, trapped air expelled, and bubbles form

• Ice 30-m thick can flow downhill. At the coast, calving produces icebergs when the glacier breaks, with as much as 90% underwater

• Gas bubbles with CO2 and CH4


Dust

• Dust in ice cores can be volcanic activity, or dry and windy conditions

• Acidic dust with sulfuric acid indicates volcanic activity

• Dust storms in Africa can be detected in polar ice cores


Marine sediments

• Current warmth on last slide is unusual

• Ratio of Oxygen-18 to Oxygen-16 in shells of marine animals tells about amount of continental ice that was present when they lived

• This method works back to 2-3 million years

• Warm periods about every 100,000 years


Fossil records are oldest

• Use Uranium dating for the oldest

• Types of plants and animals give climate clues

• Some plants live under very narrow conditions of temperature and humidity


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


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


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


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


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


Ocean circulation

• The thermohaline circulation is a world-wide 3-dimensional ocean circulation

• Sinking motion occurs in the North Atlantic when ice melts

• 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 #35: friday, november 201 past climates: proxy data and mechanisms of change

Documents

continental ice

ice crystals

polar ice coresclass

little ice age

bubbles formice

years ago160

underwatergas bubbles

climate cluessome plants