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Water Vapor Feedback - Positive |
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Snow/Ice Albedo Feedback - Positive |
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Longwave Radiation Feedback - Negative |
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Vegetation-Climate Feedback - Positive |
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Cloud Feedback - Uncertain |
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Mixing Ratio = the dimensionless ratio of the
mass of water vapor to the mass of dry air. |
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Saturated Mixing Ratio tells you the maximum
amount of water vapor an air parcel can carry. |
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The saturated mixing ratio is a function of air
temperature: the warmer the temperature the larger the saturated mixing
ration. |
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è a warmer
atmosphere can carry more water vapor |
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è stronger
greenhouse effect |
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è amplify
the initial warming |
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è one of
the most powerful positive feedback |
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The snow/ice albedo feedback is associated with
the higher albedo of ice and snow than all other surface covering. |
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This positive feedback has often been offered as
one possible explanation for how the very different conditions of the ice
ages could have been maintained. |
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The outgoing longwave radiation emitted by the
Earth depends on surface temperature, due to the Stefan-Boltzmann Law: F = s(Ts)4. |
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è warmer
the global temperature |
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è larger
outgoing longwave radiation been emitted by the Earth |
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è reduces
net energy heating to the Earth system |
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è cools
down the global temperature |
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è a
negative feedback |
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Clouds affect both solar radiation and
terrestrial (longwave) radiation. |
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Typically, clouds increase albedo è a
cooling effect (negative feedback) |
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clouds reduce outgoing longwave radiation è a
heating effect (positive feedback) |
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The net effect of clouds on climate depends
cloud types and their optical properties, the insolation, and the
characteristics of the underlying surface. |
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In general, high clouds tend to produce a
heating (positive) feedback. Low clouds tend to produce a cooling
(negative) feedback. |
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ENSO is
the largest interannual (year-to-year) climate variation signal in the
coupled atmosphere-ocean system that has profound impacts on global
climate. |
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The delayed oscillator suggested that oceanic
Rossby and Kevin waves forced by atmospheric wind stress in the central
Pacific provide the phase-transition mechanism (I.e. memory) for the ENSO
cycle. |
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The propagation and reflection of waves,
together with local air-sea coupling,
determine the period of the cycle. |
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Based on the delayed oscillator theory of ENSO,
the ocean basin has to be big enough to produce the “delayed” from ocean
wave propagation and reflection. |
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It can be shown that only the Pacific Ocean is
“big” (wide) enough to produce such delayed for the ENSO cycle. |
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It is generally believed that the Atlantic Ocean
may produce ENSO-like oscillation if external forcing are applied to the
Atlantic Ocean. |
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The Indian Ocean is considered too small to
produce ENSO. |
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“Pacific Decadal Oscillation" (PDO) is a
decadal-scale climate variability that describe an oscillation in northern
Pacific sea surface temperatures (SSTs). |
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PDO is found to link to the decadal variations
of ENSO intensity. |
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The NAO is the dominant mode of winter climate
variability in the North Atlantic region ranging from central North America
to Europe and much into Northern Asia. |
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The NAO is a large scale seesaw in atmospheric
mass between the subtropical high and the polar low. |
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The corresponding index varies from year to
year, but also exhibits a tendency to remain in one phase for intervals
lasting several years. |
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The North Atlantic Oscillation is considered as
a natural variability of the atmosphere. |
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However, processes in the ocean and stratosphere
and even the anthropogenic activity can affect its amplitude and phase. |
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Surface winds of the NAO can force sea surface
temperature variability in the Atlantic Ocean. |
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Feedbacks from the ocean further affect NAO
variability. |
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The Arctic Oscillation switches phase
irregularly, roughly on a time scale of decades. |
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There has been an unusually warm phase in the
last 20 years or so, exceeding anything observed in the last century. |
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Tectonic-Scale Climate Changes |
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Orbital-Scale Climate Changes |
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Deglacial and Millennial Climate Changes |
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Historical Climate Change |
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Anthropogenic Climate Changes |
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Tectonic Scale: the longest time scale of
climate change on Earth, which encompasses most of Earth’s 4.55-billion
years of history. |
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Tectonic processes driven by Earth’s internal
heat alter Earth’s geography and affect climate over intervals of millions
of years. |
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On this time scale, Earth’s climate has
oscillated between times when ice sheets were presented somewhere on Earth
(such as today) and times when no ice sheets were presented. |
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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 are the causes and climate effects of
changes in sea level through time? |
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What caused Earth’s climate to cool over the
last 55 Myr (the Cenozoic Era)? |
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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. |
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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? |
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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. |
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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. |
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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. |
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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. |
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What can happen to the cold boundary? |
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The lithosphere has broken into a number of
rocky pieces, called plates. |
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There are a few large plates plus a number of
smaller one comprise the Earth’s surface (a total of 20 plates). |
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The plates range from several hundred to several
thousand kilometers in width. |
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Polar
position hypothesis |
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Chemical
Weathering Hypothesis |
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Seafloor
Spreading 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. |
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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. |
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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. |
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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. |
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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. |
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Changes in solar heating driven by changes in
Earth’s orbit are the major cause of cyclic climate changes over time
scales of tens to hundreds of thousands of years (23k years, 41k years, and
100k years) . |
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Earth’s orbit and its cyclic variations: tilt
variations, eccentricity variations, and precession of the orbit. |
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How do orbital variations drive the strength of
tropical monsoons? |
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How do orbital variations control the size of
northern hemisphere ice sheets? |
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What controls orbital-scale fluctuations of
atmospheric greenhouse gases? |
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What is the origin of the 100,000-year climate
cycle of the last 0.9 Myr (ice sheets melt rapidly every 100,000 years)? |
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Orbital-scale climate changes are caused by
subtle shifts in Earth’s orbit. |
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Three features of Earth’s orbit around the Sun
have changed over time: |
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(1)
the tilt of Earth’s axis, |
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(2)
the shape of its yearly path of revolution around the Sun |
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(3)
the changing positions of the seasons along the path. |
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Orbital-scale climate changes have typical
cycles from 20,000 to 400,000 years. |
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The 23,000-year cycle of precissional change
dominants the insolation changes at low and middle latitudes. |
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The 41,000-year cycle of tilt change dominants
the insolation changes at higher latitudes. |
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Eccentricity changes (the 1000,000 or
413,000-year cycles) is not a significant influence on seasonal insolation
chanes. |
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Monsoon circulations exit on Earth because the
land responds to seasonal changes in solar radiation more quickly than does
the ocean. |
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Changes in insolation over orbital time scales
have driven major changes in the strength of the summer monsoons. |
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Changes of 12% in the amount of insolation
received at low latitudes have caused large changes in heating of tropical
landmass and in the strength of summer monsoons at a cycle near 23,000 years in length. |
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The Vostok ice record shows a series of cyclic
variations in methane concentration, ranging between 350 to 700 ppb (part
per billion). |
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Each Ch4 cycle takes about 23,000 years. |
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This cycle length points to a likely connection
with changes in orbital procession. |
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The orbital procession dominates insolation
changes at lower latitudes. |
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Ice sheets reacted strongly to insolation
changes. |
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Summer insolation control the size of ice sheet
by fixing the rate of ice melting. |
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This figures shows a North Atlantic Ocean
sediment core holds a 3 Myr d18O record of ice volume and
deep-water temperature changes. |
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There were no major ice sheets before 2.75 Myr
ago. |
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After that, small ice sheets grew and melted at
cycles of 41,000 and 23,000 years until 0.9 Myr ago. |
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After 0.9 Myr ago, large ice sheet grew and
melted at a cycle of 100,000 years. |
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