Tectonic-Scale Climate
Change
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The faint young Sun paradox and its
possible explanation. |
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Why was Earth ice-free even at the
poles 100 Myr ago (the Mesozoic Era)? |
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What caused Earth’s climate to cool
over the last 55 Myr (the Cenozoic Era)? |
Faint Young Sun Paradox
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Solar luminosity was much weaker (~30%)
in the early part of Earth’s history
(a faint young Sun). |
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If Earth’s albedo and greenhouse effect
remained unchanged at that time, Earth’s mean surface temperature would be
well below the freezing point of water during a large portion of its 4.5 Byr
history. |
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That would result in a “snowball”
Earth, which was not evident in geologic record. |
Three Possible Solutions
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Solution 1: Additional heat sources
must have been presented |
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Unlikely: The geothermal heat from the early Earth is sometimes
suggested one such additional heat source to warm Earth. However, the
geothermal heat flux is not big enough to supply the required energy. |
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Solution 2: The planetary albedo must
have been lower in the past |
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Unlikely: It would require a zero albedo to keep the present-day
surface temperature with the 30% weaker solar luminosity in the early Earth. |
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Solution 3: Greenhouse effect must have
been larger |
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Most Likely: The most likely solution to the faint young Sun paradox is that Earth’s greenhouse
effect was larger in the past. |
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But (1) why and (2) why that stronger greenhouse effect reduced to the
present-day strength? |
Chemical Weathering
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The precipitation process in the
atmosphere dissolve and remove CO2 from the atmosphere. |
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Rocks exposed at Earth’s surface
undergo chemical attack from this rain of dilute acid. |
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This whole process is known as chemical
weathering. |
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The rate of chemical weathering tend to
increase as temperature increases. |
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Weathering requires water as a medium
both for the dissolution of minerals and for the transport of the dissolved
materials to the ocean |
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è The rate of chemical weathering increases as precipitation increases. |
Negative Feedback From
Chemical Weathering
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The chemical weathering works as a
negative feedback that moderates long-term climate change. |
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This negative feedback mechanism links
CO2 level in the atmosphere to the temperature and precipitation
of the atmosphere. |
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A warm and moist climate produces
stronger chemical weathering to remove CO2 out of the atmosphere è smaller
greenhouse effect and colder climate. |
Earth’s Thermostat –
Chemical Weathering
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Chemical weathering acts as Earth’s
thermostat and regulate its long-term
climate. |
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This thermostat mechanism lies in two
facts: |
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(1) the average global rate of chemical weathering depends on the
state of Earth’s climate, |
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(2) weathering also has the capacity to alter that state by regulating
the rate which CO2 is removed from the atmosphere. |
Climate Changes in the
Last 500 Myr
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Climate in the past 500 million years
have alternated between long periods of warm climate and short periods of
cold climate. |
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During the last 500 million years,
major continent-size ice sheets existed on Earth during three icehouse ear:
(1) a brief interval near 430 Myr ago, (2) a much longer interval from 325 to
240 Myr ago, and (3) the current icehouse era of the last 35 million year. |
Plate Tectonics and
Climate
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How can one account for the alternating
periods of climatic warmth and coolness observed in the geologic record? |
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è Part of the answer must lie in the tectonic
activity and the positions of the
continents. |
The Polar Position
Hypothesis
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The polar position hypothesis focused
on latitudinal position as a cause of glaciation of continents. |
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This hypothesis suggested that ice
sheets should appear on continents when they are located at polar or
near-polar latitudes. |
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To explain the occurrence of icehouse
intervals, this hypothesis calls not on worldwide climate changes but simply
on the movements of continents on tectonic plates. |
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This hypothesis can not explain the
climate of the Late Proterozoic Era, when both continents and glaciers appear to have been situated at
relatively low latitudes. |
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It can not explain the warm Mesozoic
Era when high-latitude continents were present but were almost completely
ice-free. |
Tectonic Control of CO2
Input – The Seafloor Spreading Rate Hypothesis
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During active plate tectonic processes,
carbon cycles constantly between Earth’s interior and its surface. |
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The carbon moves from deep rock
reservoirs to the surface mainly as CO2 gas associated with
volcanic activity along the margins of Earth’s tectonic plates. |
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The centerpiece of the seafloor
spreading hypothesis is the concept that changes in the rate of seafloor
spreading over millions of years control the rate of delivery of CO2
to the atmosphere from the large rock reservoir of carbon, with the resulting
changes in atmospheric CO2 concentrations controlling Earth’s
climate. |
Negative Feedback in
Seafloor Spreading Hypothesis
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The seafloor spreading hypothesis
invokes chemical weathering as a negative feedback that partially counters
the changes in atmospheric CO2 and global climate driven by
changes in rates of seafloor spreading. |
Tectonic Control of CO2
Removal – The Uplift Weathering Hypothesis
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The uplifting weathering hypothesis
asserts that the global mean rate of chemical weathering is heavily affected
by the availability of fresh rock and mineral surfaces that the weathering
process can attack. |
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This hypothesis suggests that tectonic
uplifting enhances the exposure of freshly fragmented rock which is an
important factor in the intensity of chemical weathering. |
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This hypothesis looks at chemical
weathering as the active driver of climate change, rather than as a negative
feedback that moderates climate changes. |
Can These Two Hypotheses
Explain Tectonic-Scale Climate Changes?
Greenhouse Earth – 100Myr
Ago
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Earth’s climate 100 Myr ago was warm
enough at poles to keep ice sheets from forming. |
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Why? |
One Reason: Higher CO2
Level
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CO2 level was higher 100 Myr
ago. |
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Although higher atmospheric CO2
level can account for the warm climate 100 Myr ago, this mechanism alone can
not explain the extremely small equator-to-pole temperature gradient at that time. |
Ocean Heat Transport
Factor
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The deep ocean 100 Myr ago was filled
with warm saline deep water formed in the tropics or subtropics, rather than
with cold water from high latitudes. |
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A strong flow of warm deep water from
the tropics to the poles could have contributed to the poleward heat flux
needed to warm the poles. |
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Also, the warm salty deep water could
have caused faster overturning of the deep ocean and greater poleward heat
transport. |
Why the Cooling over the
Last 50 Myr?
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The collision of Indian and Asia
happened around 40 Myr ago. |
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The collision produced the Himalayas
and a huge area of uplifted terrain called the Tibetan Plateau. |
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The Himalayas Mountains provided fresh,
readily erodable surfaces on which chemical weathering could proceed rapidly. |
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At the same time, the uplifting of the
Tibetan Plateau create seasonal monsoon rainfalls, which provided the water
needed for chemical weathering. |
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Therefore, the collision of India and
Asia enhanced the chemical weathering process and brought down the
atmospheric CO2 level to the relatively low values that prevail today. |
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This reduced the greenhouse effect and
cooled down the climate over the last 50 Myr. |
Summary: Tectonic Control
of Climate
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Plate Tectonics probably does influence
climate over long time scales. |
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The main influence of plate tectonics
on climate appears to be indirect: by modulating CO2 levels in the
atmosphere through the chemical weathering process. |
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This, in turn, affects climate by way
of the greenhouse effect. |
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Such change, in combination with the
long-term increase in solar luminosity, can account for the main features of
the long-term climate changes. |