New Readings
description
Transcript of New Readings
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The Greenhouse Climate
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We Know the Last 100 Myr Pretty Well…
• Why?
• Know– Continental Positon– Shape of the Ocean Basins– Temperature– Sea Levels
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Cretaceous: “abundance of chalk”
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Pangaea was breaking up….
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Sea Level was MUCH Higher….
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Climate Evidence
• Warm-adapted vegetation and critters north of the Arctic circle– Brown-leaf evergreens– Turtles and crocodiles
• No continental glaciation• Tropical conditions to 40– Coral growing at the
paleolatitude of New York– NO HARD Freezes in
temperate latitudes.
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180 Myr Ago
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160 Myr Ago
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120 Myr Ago
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80 Myr Ago
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60 Myr Ago
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Why so much warmer?
• Climate models– Geography– CO2 4-6 times current
levels
• Ocean Circulation– Today the atmosphere
transports about double the heat that the oceans transport
– In Cretaceous that ratio was probably reversed
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CO2 probably drove the warming
• Note that CO2 effect is not linear
• At low levels snow and ice contribute to positive feedback– They reflect a lot of
solar radiation back to space
• At high levels there is much weaker effects– Little snow to melt– CO2 Saturation
• But water vapor feedback works in the opposite direction
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Regression
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Transgression
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But what causes changes in sea level?
• Changes in Climate– Ice vs no ice….about 50
meters of change in Cretaceous
– Thermal contraction of water ~ 7 meters
• Change in ocean basin volume– Spreading rates (50 to 150
meters)– Continental Collisions (~10
meters)
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Fast Spreading
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Slow Spreading
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Continental Collision• The size of continents
effects the size of oceans (duh….)
• The collision of India with Asia decreased the continental area by 2 million Km2 (and increased the oceans by that amount)– Net effect was about 10
meters of sea level drop
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Effects of Sea Level on Climate• Water has a large
heat capacity and moderates regional climates– That’s why
maritime (western ends of continents) climates tend to be milder
• High stands of sea level extend the reach of maritime climates
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Climatic Factors
• Ice Sheets– Antarctic ~ 66 meters of sea level rise– Greenland ~ 6 meters
• Thermal Expansion/Contraction– The volume of water changes about 1 part in
7000 per degree C– Given ocean cooling since the cretaceous,
that works out to about 7 meters of sea level drop.
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The 55 Myr Spike• We found this spike in
the data…..– Increase in global
temperatures of 5-9 C–Major acidification of
the oceans– Extinction of about half
of the forams• Why?–Major release of
carbon–Methane on the
seafloor
• We found this spike in the data…..– Increase in global
temperatures of 5-9 C–Major acidification of
the oceans– Extinction of about half
of the forams• Why?–Major release of
carbon–Methane on the
seafloor