ES 1111

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ES 1111

• Chapter 4 Evidence of Climate Change

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Looking into the Past

• Must be careful with past information there
  could be explanations other than climate change!
• In 17th 18th centuries, the river Thames in
  London froze in winter
• Much colder winters?
• Other explanation River was able to freeze more
  readily back then?
• (1) Old London bridge acted to slow river flow
  down
• (2) Lack of embankments back then (river wider)
• (3) Lack of waste heat from industrial plants
• Weather records show that winters back then were
  1º C cooler

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Constructing Past Climates

• Proxy measurements must be used to construct a
  picture of past climate
• Plate tectonics (folding, faulting) and erosion
  complicates the picture
• Much of the record is confused or destroyed
  (90-99 eroded)
• Little remains to tell us about the climate over
  the first 90 of Earths lifetime
• Dont know arrangement of continents and oceans
• Dont know atmospheric composition precisely
• First sedimentary rocks currently observed
  3,700 million years old
• First signs of life 3,800 million years ago
  (provide little evidence of climate bacteria)

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Ice Age

• Times in Earths history when ice covered a large
  part of the Earths surface
• First proposed by Swiss naturalist Louis Agassiz
• Erratics rocks found in unusual areas
• Agassiz found blocks of granite transported 100
  kilometers from Alps

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From 2.7 to 1.8 Billion Years Ago

• Snowball Earth Widespread, global glacial
  conditions
• Evidence of glaciers in parts of Africa located
  near the Equator at the time
• 3 discrete glaciations found in Wyoming between
  2.5 and 2.2 billion years ago
• Glacial rocks from this period are then covered
  by carbonates and magma

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Sudden Shift in Climate

• Catastrophic event caused increase in CO2
• Earth remained free of ice caps for 1 billion
  years
• Snowball Earth appeared again several ice ages
  observed following warm-up
• At end of Precambrian dramatic evolutionary
  changes in species found
• Cambrian explosion after last ice age in this
  period, acceleration in evolution observed

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The Big Five

• Five clearly established events of mass
  extinction
• Over last 600 million years, 99.9 of all species
  have died
• Now 1 million species, 75 insects

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100 Million Years Ago

• Warmest climate period supported by proxy data
• 6º C to 12º C warmer than present day
• Configuration of continents played a role
  seaway at Equator
• Today circumpolar ocean current around
  Antarctica makes Antarctica colder than the Arctic

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Changes in Earths Surface

• Figure 4.7, Page 83

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End of the Mesozoic

• Sudden cooling observed
• One of the Big Five
• Dinosaurs extinct

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Pleistocene

• Repeated ice ages 32 of Earth covered by ice
• Evidence of 7 glacial periods
• Each glacial period occurs every 100,000 years
• Each glacial period had substantial fluctuations
  in climate from extreme cold to near
  interglacial warmth.
• Fluctuation in climate dominated by cycles of
  21,000, 41,000, and 100,000 years.
• From sediment/ice core data changes can be
  sudden!

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Heinrich Layers

• Heinrich layers sediment layers found in ocean
  cores thought to be produced by debris carried
  out to sea by icebergs (melting, then deposition)
• Vital question how influx of freshwater can
  alter deepwater circulation and therefore climate
• Similar to the scenario in The Day After
  Tomorrow

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End of Pleistocene

• Last ice age reached greatest extent about 18,000
  years ago
• Ice sheet 3 kilometers thick as far south as
  Great Lakes
• Total ice volume of 84-98,000,000 km3 (30 million
  today)
• Sea level lower by 90-120 meters
• Global average temperatures 5º C colder than now
• Dramatic warming started 15,000 years ago
• Rapid change in circulation pattern seen (dust)
• Conclusion Climate is capable of sudden, large
  shifts

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The Holocene

• Recent 10,000 years
• Warm, stable interglacial period
• Extraordinary quiet phase compared to earlier eras

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6,000 Years Ago

• Laurentide ice sheet disappeared
• Peak in post-glacial warming
• Temperatures 2-3º C warmer than now
• Ancient trees found farther north than trees
  exist today

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5,500 Years Ago

• Cooler and Drier conditions
• Historical records can be used now
• Desiccation of Sahara decline in rain 4,000
  years ago
• Tree line retreats southward
• Mountain glaciers growing

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9th and 10th Centuries

• Warming in Europe and North Atlantic
• Vikings settle Greenland
• Limited geographical coverage of records makes it
  difficult to say global climate was warmer

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16th to 19th Centuries

• Cooler period
• Firmer foundation of evidence
• Best known example of climate variability in
  recorded history
• Glaciers expanded
• Cool summers, severe winters
• Not a period of sustained cold
• Little Ice Age concentrated in winter half

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Since Late-19th Century

• Global warming
• Evidence comes primarily from instruments
• Annual global surface temperatures have risen
  0.62º C slightly greater in Southern Hemisphere
• Warmest years in the record have all fallen in
  1990s
• Warmest 20-year periods 1925-1944, 1978-1997
• Slight cooling in-between these years

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Global Warming

• Warming seems to be due to decrease in areas
  affected by exceptionally cold temperatures
• To a lesser extent, global warming is due to
  increases in exceptionally warm temperatures
• In recent decades night minimum temperatures
  have increased more rapidly than daytime maximum
  temperatures
• Diurnal temperature range has decreased 0.08º C
  per decade
• Most pronounced warming northern continents
• Marked cooling NW Atlantic Ocean, lesser amount
  in north central Pacific
• Changes appear most clearly in winter
• Warming trend of 40s in higher latitudes of
  northern hemisphere southern warmed less

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Consequences of Global Warming

• Rise in global temperatures are only part of the
  concern
• In terms of economic impact, increases in extreme
  weather events would be the most damaging
  consequence
• Climatologists have concluded for recent decades
  thus far
• No consistent trend in interannual temperature
  variability
• No consistent pattern for rainfall variability
• No pattern for intense rainfall or extratropical
  cyclones
• Decrease in tropical cyclones in north Atlantic
  (elsewhere not as reliable)

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Climate Change

• No shortage of evidence of climate change in
  every time scale
• Evidence shows climate can change suddenly and
  rapidly naturally
• Scale of these changes had major impacts the
  world over
• Complete picture of the size of these changes,
  and where/when they took place is not possible
  (uncertainty increases as we go back in time)
• More questions remain than answers
• Accept anthropogenic global warming as real until
  new evidence/data becomes available?