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Climate Sensitivity and Feedback |
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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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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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Climate changes of these scales in the past
several tens of thousands of years occurred within the time span of
recorded human civilization. |
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These change can be resolved by 14C-dated
records. |
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The major boundary conditions that have driven
climate changes during the last 21,000 years have been the changes in: |
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(1)
size of ice sheet |
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(2)
seasonal insolation |
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(3)
level of greenhouse gases in the atmosphere. |
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Climate Sensitivity: the relationship between
the measure of forcing and the magnitude of the climate change response. |
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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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č a warmer
atmosphere can carry more water vapor |
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č stronger
greenhouse effect |
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č amplify
the initial warming |
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č 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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č warmer
the global temperature |
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č larger
outgoing longwave radiation been emitted by the Earth |
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č reduces
net energy heating to the Earth system |
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č cools
down the global temperature |
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č a
negative feedback |
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Clouds affect both solar radiation and
terrestrial (longwave) radiation. |
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Typically, clouds increase albedo č a
cooling effect (negative feedback) |
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clouds reduce outgoing longwave radiation č 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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