Transcript Class #34: Monday, April 7

Class #34: Wednesday,
November 18
Climate Types (E, and H)
Past Climates: Proxy Data
and Mechanisms of Change
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The 6 major climate groups
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A: Tropical moist
B: Dry (can be subtropical or mid latitude)
C: Moist with mild winters (mid latitude)
D: Moist with severe winters (mid latitude)
E: Polar (high latitude)
H: Highland (rapid climate change with
elevation)
• 2nd letter: usually latitude (except B)
• 3rd letter: differences in temperature
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D Climate type
Severe (winter) Midlatitude
• Similar to C but severely cold winter
• Average temperature of coldest month
<27ºF
• Snow on ground for extended periods
• Average temperature of warmest month
>50ºF
• Overall, large change in temperature with
season
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D climate subtypes
• 2nd letter
– “f” no dry season
– “w” winter dry season
• 3rd letter
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“a” hot summer
“b” warm summer
“c” cool summer
“d” extremely severe winter
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D Climate Subtypes (continued)
• Humid continental
– Dfa, Dfb, Dwa, Dwb
– Dfa, for example, Chicago
• Subarctic
– Dfc, Dfd, Dwc, Dwd
– Long winter
– Brief cool summer
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E climate type
• Polar climate, very dry and cold
• Poleward of Arctic/Antarctic Circle, latitude
66.5º
• E climate subtypes
– ET Tundra: mosses, lichens, flowering plants,
woody shrubs, small trees, permafrost
– EF Ice caps: no vegetation; Greenland,
Antarctic Plateau
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H climate type: Highland
• Large variation of temperature and
precipitation over small horizontal distances
• Large diurnal temperature variation
• Can be dry or moist, depending on
orientation, humidity, and whether
prevailing winds are upslope or downslope
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Have today’s climates always been
the same?
• This question leads to the study of past
climates.
• So do the questions: Can we predict future
climates? What is the impact of humans on
climate?
• Two kinds of past climate:
– Historical, past few thousand years
– Paleoclimate, ancient, back billions of years
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Historical Climate
• Humans have kept records
• Instrumental record
– Since about 1600
• Historical data: proxy data
– Humans have kept some sort of record of
climate conditions
– Examples: dates of freezes of lakes and rivers,
farmers’ logs, animals in cave paintings, other
documents
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Another source of data for the
historical period is trees
• Tree rings are rings of growth in tree trunks in
regions with distinct growing seasons.
• A wider tree ring means more growth.
• Growth varies with temperature and precipitation,
depending on the species.
• Information from various species is most helpful.
• The study of tree rings is dendrochronology, and
is done by dendrochronologists.
• Figure shows dry periods in Iowa in 1700, 1740,
1820, 1820, 1890, and 1930 from tree rings.
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Proxy measurements of precipitation
from tree rings
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Dry periods:
1930s, the “Dust Bowl”
around 1700
1740
1820
1890
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Pollen for proxy information
• Pollen degrade slowly,
distinctive shapes for each
species
• Oldest sediments are
deepest
• Spruce need a cool climate
• Decline of spruce during
warming
• Pine need a warm and moist
climate
• Oak need it drier than pine
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Dating ancient climates
• Living things all contain carbon
• C14 begins to change to C12 in a radioactive
decay process at an exponential rate with a
half-life of 5760 years as soon as living
matter dies
• Carbon dating good to 50,000 yrs with an
uncertainty of about 15%.
• For older samples and rocks need another
method
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Dating really ancient climates
• Uranium-238 decays into Lead-206 with a
half-life of 4.5 billion years
• No other (except possibly human) sources
of Lead-206
• How we know how old the Earth and moon
are
• If equal amounts of Uranium-238 and Lead206, then 4.5 billion years old.
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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
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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
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