Discovering Past Climates

Tree Rings & Climate

• Growth affected by:• Temperature• Rainfall

• Tree growth is indicated by the size & colour of rings

• Hold information for no more than a few hundred years (unless from an archaeological site)

Analyzing Tree Rings

• Width of the ring• Wide ring indicates cool, wet weather - Rapid tree

growth• Thin rings indicate dry, hot conditions - Growth is slower

• Colour of the ring• Dark ring - Growth during late summer• Light ring - Growth during spring

Ice Cores & Climate• Layers of ice & snow accumulate over the years• Ice cores (long cylinders of ice) are removed, using a drill• The extent of climate records depends on the depth of the ice

core• EPICA (Antarctica) obtained climate records 800 000 years ago

Evidence of Past Climates from Ice Cores • Ice is deposited annually when the summer melt is not

enough to get rid of the winter’s snow• Ice cores hold 4 types of clues, which remain trapped

until the ice melts• Dissolved particulate matter in the ice• Physical characteristics of the ice• Composition of trapped air bubbles• Composition of the ice

Dissolved Particulate Matter in the Ice• Dust, ashes, salts, plant pollen &

other matter are brought to Earth when it rains or snows

• These samples, when frozen, give clues about events like:• Volcanic eruptions• Meteorite impacts• Forest fires• Vegetation cover

Physical Characteristics of the Ice• Indicate the conditions

of the temperature & humidity at the time the ice crystals formed• Snowflakes• Hail• Glaciers• Pack ice

Composition of Trapped Air Bubbles• When water freezes, air

bubbles of are trapped in the ice

• Provides information on atmospheric composition when the ice was formed

• Analysis of air at different depths in an ice core can help in tracking concentration of greenhouse gases change over hundreds of thousands of years

Composition of the Ice• Isotopes: different atoms of a particular element that have

same # of protons but different # of neutrons• Oxygen has 3 naturally occurring isotopes• Oxygen-16• Oxygen-17• Oxygen-18

• The difference in molecular weight affects the physical properties of water (freezing point)• Oxygen-18 freezes at a higher temperature than oxygen-16• Oxygen-18 evaporates slower than oxygen-16

• Concentration of isotopes in different layers of ice tells us the temperature at the time the ice was formed• Ice that forms during high temperatures contains more oxygen-18• During lower temperatures, polar ice contains less oxygen-18

Oxygen Isotopes & Global Temperature

How Scientists Determine Patterns of Climate Change

Evidence of Past Climate Change from Sedimentary Rock• Sedimentary rock is formed

over long periods of time from sediments of rock

• The composition of the sediments provide evidence of past climate conditions• Pollen• Microscopic organisms• Shells

Sedimentary Layers in Glacial Lakes• Sediment records reflect regularly repeating annual

changes in sediment deposition• Sedimentary layers are called varves

• Thick deposits of light-coloured sediment • Summer months

• Fine, dark-coloured sediment• Winter months

• Estimate rainfall & temperature over a long period of time

Fossil and Preserved Organisms• Types and abundance of fossilized remains provide clues

about past climate and environment

How Scientists Infer the Rate of Climate Change

Dendrochronology Activity

Homework• Page 359 # 1, 2, 4 - 8