Chapter 14
Past and Present
Climates
Figure CO: Chapter 14, Past and Present Climates--Mendenhall Glacier in Alaska.
Courtesy of Anatoly Myaskovsky
Figure UN01: Susan Solomon
Courtesy of Susan Solomon, NOAA
Climate
• Is to weather what a friend’s personality is to his/her
mood
• Sums up the weather’s long-term behavior
• Is the collective state of the atmosphere for a given
place over a specified interval of time
Climate is defined by
• Location
– Globe, continent, region, city
– Chapter 14: regional and global scale
• Time: a specified interval
– 30 year average is normal
– 100 years or longer for history of climate
• Averages and extremes of variables
– Chapter 14: temperature and precipitation
Controls on Climate
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Similar to controls on temperature in Chapter 3
Latitude: solar energy input
Elevation: air temperature, snow vs. rain
Topography: moist vs. dry, temperature, distribution
of cloud patterns, solar energy reaching the surface
Controls on Climate (continued)
• Proximity to large bodies of water
– Thermal properties of water (absorption, heat capacity,
transparency, mixing) moderate temperature downwind
• Prevailing atmospheric circulation
– Intertropical Convergence Zone (ITCZ)
– Subtropical Highs
Classifying Climate
• Hard to do
– No clear boundaries
– Complex natural systems
• How it is done
– Important to life: hot or cold; moist or dry
– Most common system: based on vegetation
• Köppen climate classification scheme based on
vegetation and temperature
Köppen (KEPP-in) Scheme
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Modified by Trewartha and Horn
Related to geography and global circulation
In Chapter 14, uses 1, 2 or 3 letters
Has many other subdivisions within the categories
shown in our textbook
• Has 6 major groups: A, B, C, D, E, H
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: precipitation and dry periods
3rd letter: differences in temperature
Figure 01: Overview of the main climatic groups
Figure 02: Koppen Scheme
Reproduced from: Peel MC, Finlayson BL & McMahon TA (2007), Updated world map of the
Köppen-Geiger climate classification, Hydrology and Earth System Sciences, 11, 1633-1644.
Figure 03: Classification and global circulations
Tropical Humid Climates:
Af, Aw, Am
• All tropical (A) climates are humid
• Letter “f” means no dry season, rain year round,
usually closest to the equator
• Letter “m” means “monsoonal”, with a short dry
season and a very rainy season
• Letter “w” means “winter dry season” except no real
winter in tropics, just cool
Tropical Humid Climates (continued)
• Af
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Closest to the equator
Smallest annual range of temperature
6.9-10 inches of rain per month
Most thunderstorms in afternoon
Linked to ITCZ
Tropical rain forests
• Am
– Seasonal onshore winds during summer monsoon
– Climates with most yearly precipitation
– Jungle vegetation
Tropical Moist Climates (continued)
• Aw
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Farthest A climate from the equator
Often border Af
Tropical wet and dry
Wet summers, dry, cooler winters
Linked to the seasonal migration of the ITCZ
Vegetation is savannah or tropical grasslands with
scattered deciduous trees, as in the grasslands of Africa.
Figure 04: Monthly temperature and precipitation: Af, Aw, Am
Dry (B) Climates
• Potential evaporation minus precipitation greater
than zero
• More land of this climate type than any other
• Lubbock has a B climate
• Descending branch of the Hadley circulation near the
subtropical highs or
• Rain shadow of a mountain range
Dry Climate Subtypes
• 2nd letter
– “S” for steppe or semi-arid (like Denver)
– “W” for true desert (extremely dry)
• 3rd letter
– “h” for low-latitude, hot (yearly average temperature >=
64ºF)
– “k” for mid latitude, cool (yearly average temperature
<64ºF)
Dry Climate Subtypes (continued)
• BWh Extremely dry and hot; can have large sand dunes;
Sahara, Arabian peninsula, central Australia, most extreme B
climate
• BSk Least extreme B climate; mid latitude steppe, often high
plateau, Lubbock, Denver, San Diego; often rain shadow
• BSh Much of Mexico, lower latitude, subtropical steppe
• BWk Central Asia, very dry, mid latitude rain shadow,
continental interior
Figure 05: Monthly temperature and precipitation: BSh,BWh
Data from CIRA/Colorado State University and NOAA
Figure 06: Monthly temperature and precipitation: BSk,BWk
C Climate Type
• All C are moist, plentiful precipitation
• All C are midlatitudes
• Average temperature of coolest month between
27ºF and 65ºF
• Have many subtypes; Chapter 14 concentrates on a
few
• 2nd letter like A subtypes
– “f” no dry season
– “w” brief dry period in winter
C Climate Type (continued)
• 3rd letter
– “a” hot summer
– “b” warm summer
– “c” cool summer
C Climate Subtypes
• Cfb, Cfc Marine west coast
– Northwest coast of US, Canada
– Often cool ocean currents
• Cfa, Cwa Humid subtropical
– Southeastern US
– 30-100 inches of rain per year
• Csa, Csb Mediterranean
– Along a coast, mild winter
– Greece
– Dry summer, semi-permanent subtropical high
Figure 07: Monthly temperature and precipitation: CfbCfc
Figure 08: Monthly temperature and precipitation: Cfa,Cwa
Figure 09: Monthly temperature and precipitation: Csa,Csb
D Climate Type
Severe (Winter) Midlatitude
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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
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
Figure 10: Monthly temperature and precipitation: Dwb,Dfb
Figure 11: Monthly temperature and precipitation: Dfc,Dfd
D Climate Subtypes (continued)
• Humid continental
– Dfa, Dfb, Dwa, Dwb
– Dfa, for example, Chicago
• Subarctic
– Dfc, Dfd, Dwc, Dwd
– Long winter
– Brief cool summer
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
Figure 12: Monthly temperature and precipitation: ET,EF
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
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
Figure 13: Climate clues
Figure 14: Cave drawing of giraffe
Courtesy of Roberto D’Angelo
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.
Figure 15: Tree rings
Courtesy of Peter Brown, Rocky Mountain Tree-Ring Research
Tree rings as a proxy for precipitation
• Dry periods
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Around 1700
1740
1820
1890
1930s, the “Dust Bowl”
Figure 16: Tree ring history
Duvick, D. N. and T. J. Blasing, 1981. A dendroclimatic reconstruction of annual
precipitation amounts in Iowa since 1680. Water resources research, 17:11831189.
Pollen as a proxy for temperature
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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
Figure 17: Tree pollen
Adapted from T. Graedel and P. Crutzen. Atmosphere, Climate, and Change.
Scientific American Library, 1997. Spruce: © prism68/ShutterStock, Inc., Alder:
© psamtik/ShutterStock, Inc., Birch: © kosam/ShutterStock, Inc., Pine: ©
Arkady/ShutterStock, Inc., Oak: © Nina Morozova/Dreamstime.com
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
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 Lead206
• How we know how old the Earth and moon are
• If equal amounts of Uranium-238 and Lead-206, then
4.5 billion years old.
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
Figure B02: Iceberg satellite image
Courtesy of SSEC/AMRC, University of Wisconsin-Madison
Figure 18: 650,000 years in the past
Adapted from Brook, Edward J, Science 310 (2005): 1285-1286
Figure 19: Kilimanjaro ice fields
Thompson, L.G., H.H. Brecher, E. Mosley-Thompson, D.R. Hardy, and
B.G. Mark. 2009. Glacier loss on Kilimanjaro continues unabated.
Proceedings of the National Academy of Sciences
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 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
Figure 20: Oxygen ratio and temperature
Adapted from Imbrie, J. and J. Z. Imbrie, Science 202 (1980): 943-953
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
Figure T01_1: Earth’s
Geological Periods
Adapted from Bradley, R., Quaternary Paleoclimatology, Allen and Unwin, 1985.
What Mechanisms Have Caused Climate
Change in the Past
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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 longlasting
• Natural variability: variations without forcing
Asteroid Impacts
• There may be a 26-million year periodicity in asteroid
impacts
• The demise of the dinosaurs coincided with an
asteroid impact
Figure 22_1: Chicxulub impact
Courtesy of Virgil L. Sharpton, Lunar Planetary Institute
Figure 22_2: Chicxulub impact
Figure 23: Extinctions
Adapted from T. Graedel and P. Crutzen. Atmosphere, Climate, and
Change. Scientific American Library, 1997.
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
Sunspots and Climate
• The Maunder minimum in sunspot count occurred
between 1650 and 1700
• The Little Ice Age in Europe occurred between about
1400 and 1850
Figure 24: Sunspots
Source: NASA
Milankovitch Cycles
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Precession, which star is the north star, 27,000 years
Obliquity, tilt varies from 22 to 24.5°, 41,000 years
Eccentricity, orbit more/less elliptical, 100,000 years
Cold periods 20,000, 60,000, and 100,000 years ago
Figure 25A: Earth’s Orbital
variations
Figure 25B: Earth’s Orbital variations
Adapted from T. Graedel and P. Crutzen. Atmosphere, Climate, and
Change. Scientific American Library, 1997.
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
Figure 26A: Plate tectonics
Figure 26B: Plate tectonics
Figure 26C: Plate tectonics
Ocean Circulation
• The thermohaline circulation is a world-wide 3dimensional 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
• The cutoff may be responsible for cooling in a period
of glacial melt
Figure 27: The extent of the ice sheets over North America during the
Pleistocene
Figure 28: The temperature (orange) and ice accumulation (blue) in central
Greenland
Source: NOAA