Download “Freeze – Fry” Episodes of the Late Precambrian

“Snowball Earth”
MET 112 Global Climate Change - Lecture 11
ƒ A hypothetical state in which all surface water is
frozen.
ƒ Some geologists think it may actually have
happened.
ƒ Other scientists find the idea hard to support.
“Freeze – Fry” Episodes
of the Late Precambrian
Eugene Cordero
ƒ Evidence:
San Jose State University, Spring 2004
Outline
ƒ Reference: Hoffman and Schrag’s Scientific
American Article.
ƒ Positive and negative feedbacks
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“Freeze-Fry” Episodes
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The Cooling Phase – Outline of Theory
ƒ Something happened to cause an imbalance in the
long-term carbon cycle
ƒ
ƒ The Earth cooled
ƒ Ice began to expand from the poles
ƒ The
, accelerating the
cooling
ƒ Evaporation decreased (due to cooling of oceans)
ƒ Water vapor concentration decreased,
ƒ A hypothesis that:
– Earth experienced
– from very cold (“Snowball Earth”) to
– Period of very hot temperatures
ƒ This accelerated the cooling even more
ƒ Ice continued to expand until Earth was frozen
– Many times back and forth!!
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The Situation
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Before cooling
ƒ A snowball Earth would have a high albedo,
perhaps as high as 60%.
ƒ It would be difficult to “thaw out” such a planet
(see next slides)
Reflection
Albedo =
Incoming
Sunlight
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After cooling
With Greenhouse Gases Restored
Reflection
Reflection
Albedo =
Albedo =
Incoming
Sunlight
Incoming
Sunlight
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Thawing Out the Earth
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Resolutions
ƒ Restoring greenhouse gases to original levels
is not sufficient
ƒ Much higher levels of CO2 would be required
ƒ Question: how much higher CO2 would be
required to thaw out the earth?
–
ƒ One possibility – a large increase in volcanic
eruptions over a long period
–
ƒ Another possibility – a decrease in silicate-tocarbonate conversion
ƒ Why would this happen?
ƒ How could CO2 levels get so high?
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The Warming Phase -- Outline
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Warming (Continued)
ƒ The long-term carbon cycle became
unbalanced in the opposite sense (more CO2
going into atmosphere than going out)
ƒ
– Eventually, CO2 levels became very high
ƒ Earth began to warm up
ƒ
ƒ Albedo decreased, accelerating the warming
ƒ Evaporation increased, increasing water
vapor
ƒ Greenhouse effect became stronger,
accelerating the warming
ƒ (Continued)
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ƒ After about 10 million years of warming, ice
started to melt at the equator
ƒ
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Getting Rid of the Excess CO2
Evidence for such an event
ƒ Layers of carbonate
rocks found directly
above glacial rock debris
ƒ Carbonate rocks are
generally formed in warm
oceans
ƒ Now, atmosphere is incredibly hot (+50°C)
because of high CO2 levels
ƒ Hot oceans
⇒
evaporation rate (globally)
⇒
precipitation rate (globally)
⇒
rates of chemical weathering
⇒
rate of silicate-to-carbonate conversion
⇒ Rapid removal of CO2; rapid formation of
carbonate rocks
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Climate Feedbacks
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Positive Feedbacks
ƒ Earth/Atmosphere is delicate balance
–
ƒ Processes that accelerate a change
– Note: Feedbacks cannot initiate change; they
can only alter the pace of change
ƒ Important examples:
– Ice-albedo feedback
– Water-vapor feedback
ƒ Slight changes in balance can cause
–
ƒ These changes can be enhanced or diminished by positive
or negative feedbacks
ƒ Positive feedback:
–
ƒ Negative feedback:
–
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Ice-Albedo Feedback (Cooling)
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Ice-Albedo Feedback (Warming)
Initiating Mechanism
Initiating Mechanism
Earth Cools
Earth Warms
Ice Coverage Increases
Ice Coverage
Albedo Increases
Albedo
Absorption of Sunlight Decreases
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Absorption of Sunlight
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Water Vapor Feedback (Warming)
Water Vapor Feedback (Cooling)
Initiating Mechanism
Initiating Mechanism
Earth Warms
Earth Cools
Evaporation
Evaporation
Atmospheric Water Vapor Content
Atmospheric Water Vapor Content
Greenhouse Effect
Greenhouse Effect
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Positive Feedback Exerted by Water Vapour
in the Atmosphere
Positive Feedback Exerted by Snow and Ice
Enhanced greenhouse
warming
Increased warming
by trace gases
(including water vapour)
Increased water
vapour
Increased air
temperatures
Greater waterholding capacity
Figure 12.6 Positive feedback exerted by water vapour (as a greenhouse gas) in the atmosphere.
Possible Role of Cloud in Warming or
Cooling the Atmosphere
Increased warming by
trace gases
(including water vapour)
Increased air
temperatures
Increased water
vapour
Increased cloud
amount
Increased trapping
of terrestrial radiation
g
n
mi
ar
W
Increased reflection
of solar radiation
Co
ol
in
g
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Net warming
or cooling
Figure 12.7 Role of cloud in both warming and cooling the atmosphere.
Greater absorption
of solar radiation
Decreased albedo
Melting of snow
and ice
Figure 12.8 Positive feedback exerted by snow and ice albedo in the atmosphere.