Transcript Chapter16

Climate Change
Determining past climate
• The
methods used to study the past climate typically involve
physical, chemical, biological processes sensitive to: temperature
changes, the presence of water, solar activity, etc.
• The records must have survived over a long period of time.
• We must be able to date the records relatively accurately.
• Geological evidence – examples:
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♦ Advancing and retreating glaciers (surface temperature)
♦ Ice cores from Antarctica and Greenland (the amount of snow
accumulation, isotopic ratios, the crystal structure of the ice,
trapped air bubbles, solar activity, volcanic eruptions, fine dust
and plant pollen, microorganisms)
♦ Plant fossils in sedimentary rocks (plants are sensitive to longterm temperature changes)
♦ Core samples of the ocean floor sediments (contain shells and
microorganisms that live within some temperature range)
Dendrochronology (the rate of growth of tree rings-for trees which
are sensitive to temperature or drought stress)
Glaciers
Ice Cores from the Antarctic and Greenland
Sedimentary Rocks
Fossils in Sedimentary Rocks
Ocean Floor Cores
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Climate through the Ages: overview
Throughout the geological
history of the Earth, the
temperature has been warmer
than today (8 to 15 deg C)
There have been several periods
of glaciations (ice ages)
In the interglacial (warm )
periods the glaciers retreat and
the polar regions are ice free,
the sea level is elevated.
Some of the changes occur over
thousands of years, some as
quickly as a few years.
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Earth Temperature through the Ages
During the last 2 billion years the Earth's climate has
alternated between
♦ A frigid "Ice House", like today's world
♦ A steaming "Hot House", like the world of the
dinosaurs
Climate During the last 1000 years
Little Ice Age
Medieval Climatic Optimum
Eighteen hundred
and froze-to-death
Temperature trend
during the past 100-plus years
Mid-chapter summary
• The
Earth’s climate is constantly undergoing change. Evidence
suggests that throughout much of the Earth’s history the Earth’s
climate was much warmer than it is today.
• The
most recent glacial period (or Ice Age) began about 2 million
years ago. During this time, glacial advances were interrupted by
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warmer periods (interglacial periods). In North America, glaciers
reached their maximum thickness and extent about 18,000 to 22,000
years ago and disappeared completely from North America by about
6,000 years ago.
Over the last 100 years, the the Earth’s surface temperature has
increased by about 0.6 oC (about 1 oF).
Possible Causes of Climatic Change
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External: changes in the
1. incoming solar radiation
2. composition of the atmosphere
3. Earth’s surface
Natural phenomena may contribute to all three (1-3)
Human activity may contribute to 2 and 3
Internal: changes in the circulation patterns of the
ocean and the atmosphere, etc.
Climate Change and Feedback Mechanisms
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The climatic elements are intertwined: feedback
Negative feedback: counteracts changes, stabilizes climate
♦ Thermal (IR) radiation of the planet: increased
temperature leads to increased IR emission from the
planet -> the planet cools and the temperature drops.
♦ Cloud negative feedback: higher temperature leads to
more water vapor and therefore more clouds. The clouds
increase the planet’s albedo and less solar light reaches
the surface -> planet cools and the temperature drops.
Positive feedback: enhances the changes, destabilizes the
climate (runaway warming or cooling)
♦ Water vapor-greenhouse feedback: T increases, water
evaporates -> greenhouse effect -> T increases more
♦ Snow-albedo feedback: T increases, snow melts ->
albedo decreases -> less solar energy reflected at the
surface -> T increases even more
Climate and Plate Tectonics
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• they broke into seven smaller land masses
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• more likely to form -> snow-albedo feedback effect
The continents have changed their position during the
geological history of the Earth.
From one large concentrated land mass (super continent)
Continental drift affects ocean circulation and with it
the heat flow throughout the ocean.
As land masses move to higher latitudes, ice sheets are
180 million years ago
now
Climate change and plate tectonics
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• the atmosphere
Tectonic and volcanic activity adds CO2 and water vapor to
the atmosphere
Mountain building affects global circulation patterns in
degassing
degassing
volcano
underwater volcano
spreading
subduction
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Climate Change and Variations in the
Earth’s Orbit
Milankovitch Theory (1930s):
♦ Variations in the Earth orbit produce
changes in the amount of energy the
Earth receives from the Sun during
the seasons
Milutin
Milanković
1879-1958
♦ Three elements of the Earth orbit vary periodically
The eccentricity – 100,000 years cycle
Precession of the axis of rotation - 23,000 years
Obliquity – 41,000 year cycle
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♦ These variations result in three Milankovitch cycles
which have been observed in the Earth climate and
geological records.
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Variation in the Eccentricity
The Earth orbit changes between being less elliptical and more elliptical
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• Currently the eccentricity is small. The Earth is closest to the Sun in January
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Large eccentricity translates in a bigger difference in the amount of solar
energy that the Earth receives at the extreme points of the orbit
and farthest from the Sun in July by 3%. This results in about 7% difference
in the solar energy received on Earth
If the difference in the distance to the sun increases to 9%, the Earth will
experience about 20% variation in the received solar energy
This cycle has a period of about 100,000 years
Precession of the Earth axis
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and the SH winter will be milder
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Currently the Earth gets closest to the sun in January
Due to the precession in ~11,000 years it will be closest to
the sun in July: The NH winter will be colder than today
This cycle has a period of 23,000 years
Variations in the Obliquity
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• This variation has periodicity of 41,000 years
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The obliquity (the tilt of the Earth with respect to the
ecliptic) varies between 22 and 24.5 degrees
The obliquity determines the seasonal variations
♦ Large obliquity -> larger differences between the
winter and the summer
♦ Smaller obliquity -> less pronounced seasonal changes
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Milankovitch Cycles and Glaciations
100,000 year period is observed in the advancement
and the receding of the glaciers at high latitudes.
Milankovitch Cycles in the Atmosphere
Aerosols in the Troposphere
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• Impact on the energy balance:
Sources: factory and auto emissions, agricultural
burning, wildland fires, dust storms …
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♦ Some particles reflect sunlight very efficiently,
which results in cooling of the atmosphere.
♦ Others are selective absorbers and absorb some
of the incoming solar light or outgoing IR emission
of the Erath. These aerosols result in heating of
the Earth and the atmosphere.
Aerosols serve as condensation nuclei and change the
characteristics of clouds.
Aerosols stay in the atmosphere typically for a few
days before they settle down. They tend to
accumulate over the sources (industrial cities, large
fires …)
Volcanic Eruptions and Aerosols in the
Stratosphere
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particles of ash and dust are ejected high in the atmosphere
(stratosphere).
• Sulfur
and other volcanic gasses combine with water vapor and form
sulfuric acid droplets that are very reflective.
• This
reflective haze can stay in the stratosphere for several years
covering a big portion of the globe.
• The volcanic haze in the stratosphere results in cooling the Earth.
El Chichon
Mt. Pinatubo
The effect of Mount Pinatubo Eruption
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Sulfur Dioxide plume
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months after the
eruption, at an
altitude near 25 km
The eruption of Mt.
Pinatubo is followed
by a decrease in the
average global
temperatures
Variations in the Solar Output
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• The variation in the emitted energy is related to the
The total energy emitted by the sun varies with a
period of about 11 years.
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amount and the size of the dark spots on the surface
of the sun.
The sun spots are cold
regions with strong
magnetic storms.
The variation of the solar
energy is about 0.1%.
It does not have a big
impact on the climate.
Global warming and the effect of
greenhouse gases.
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• The greenhouse effect
Most common greenhouse
gases: H2O, CO2, CH4.
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results in heating the
atmosphere.
The H2O greenhouse
effect provides a strong
positive feedback
Other factors at play
♦ Clouds
♦ Jetstream
♦ Ocean currents
♦ …
Projections and uncertainties