Download Lecture4_Paleoclimate_Solar_Climate

Earth’s Climate System
• Climate – long term atmospheric conditions in a
region
• Earth’s climate includes interactions of:
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Atmosphere
Hydrosphere
Geosphere
Biosphere
Cryosphere
• Climate system – exchanges of energy and
moisture between these spheres
Earth’s Climate System
Earth’s Climate System
• Feedback loops – modify atmospheric
processes
– Positive feedback loops – enhance initial change
– Negative feedback loops – counteract initial
change
Determining Causes of Earth’s Climate
Change
• Paleoclimatology
• Proxy data – indirect
evidence using natural
recorders of climate
variability
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Sea floor sediments
Coral deposits
Glacial ice rings
Tree rings
Pollen
Historical documents
Temperature: the last 400,000 years
From the Vostok ice core (Antarctica)
Natural Climate Change
• External Forcing:
– The agent of change is outside of the
Earth-atmosphere system
• Internal Forcing:
– The agent of change is within the Earthatmosphere system itself
External Forcing
• Variations in solar output
• Orbital variations
• Meteors
Natural Causes of Climate Change
• Solar energy changes
– Variable energy from the
Sun over time
– Luminosity
– Sunspots
Solar Variations
 Sunspots correlate with solar activity
 More sunspots, more solar energy
• Sunspots are the most
familiar type of solar
activity.
SOLAR ACTIVITY
• Sunspots are the most
familiar type of solar
activity.
THE SOLAR CYCLE
• Sunspot numbers increase
and decrease
– over an 11-year cycle
• Observed for centuries.
• Individual spots last from a
few hours to months.
• Studies show the Sun is in
fact about
– 0.1% brighter when
solar activity is high.
The Seasons
• The Earth has seasons
because the axis is tilted.
• Earth rotates on its axis as
it orbits the Sun.
• Earth’s axis ALWAYS points
in the same direction…
• but Earth’s location in its
orbit around the Sun
determines whether the
NH or SH is pointed toward
the Sun.
The Seasons
• NH Summer Solstice
– June 20-21
• NH Autumnal Equinox
– September 22-23
• NH Winter Solstice
– December 21-22
• NH Vernal Equinox
– March 20-21
Earth’s orbit: an ellipse
• Perihelion: place in
the orbit closest to
the Sun
• Aphelion: place in
the orbit farthest
from the Sun
Earth’s orbit: an ellipse
• Perihelion: place
in the orbit
closest to the
Sun
• Aphelion: place
in the orbit
farthest from the
Sun
THE MAUNDER MINIMUM
• An absence of sunspots was well observed
– from 1645 to 1715.
• The so-called “Maunder minimum” coincided with a cool
climatic period in Europe and North America:
– “Little Ice Age”
• The Maunder Minimum was not unique.
• Increased medieval activity
– correlated with climate change.
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Natural Causes of Climate Change
• Variations in Earth’s Orbit
• Milankovitch Theories
– Eccentricity of Earth’s orbit
– Obliquity of Earth’s axis
– Precession of Earth’s axis
Orbital changes
• Milankovitch theory:
• Serbian astrophysicist in 1920’s who studied effects of solar
radiation on the irregularity of ice ages
• Variations in the Earth’s orbit
– Changes in shape of the earth’s orbit around sun:
• Eccentricity (100,000 years)
– Wobbling of the earth’s axis of rotation:
• Precession (22,000 years)
– Changes in the tilt of earth’s axis:
• Obliquity (41,000 years)
Eccentricity: period ~ 100,000 years
Natural Causes of Climate Change
• Volcanic eruptions
• Volcanic ejecta may
block sunlight
• Need many eruptions in
short time period
• Not observed in recent
history
Natural Causes of Climate Change
• Movement of Earth’s Plates
– Change ocean circulation
– Extremely slow process
– Climate change would be very gradual over
millions of years
• Linked to Pleistocene Ice Age, Little Ice Age,
Medieval Warm Period
Documenting Human-Caused Climate
Change
• Intergovernmental Panel on Climate Change
(IPCC)
– Global group of scientists
– Published assessments since 1990
– Predict global temperature changes of
1.4–5.8°C (2.5–10.4°F)
• Climate change models can mimic modern
conditions only if human emissions are taken
into account.
Atmosphere’s Greenhouse Effect
• Global warming –
increase in Earth’s
global temperatures
• Greenhouse effect –
keeps Earth’s surface
habitable
– Incoming heat energy is
shorter wavelengths
– Longer wavelengths –
some trapped, some
escape, net warming
effect
Earth’s Heat Budget
• Addition to or subtraction
from heat on Earth
• Incoming radiation from Sun
shorter wavelengths
• Outgoing radiation from
Earth longer wavelengths
• Rates of energy absorption
and reradiation must be
equal
Earth’s Heat Budget
Greenhouse Gases
• Water vapor
– Most important
– 66–85% of greenhouse effect
• Carbon dioxide
– Natural part of atmosphere
– Greatest relative contribution from human
activities
– Burning of fossil fuels
Atmospheric Carbon Dioxide
Greenhouse Gases
• Methane
– Second most abundant human-caused
greenhouse gas
– Great warming power per molecule
– Landfill decomposition
– Cattle
• Other trace gases
– Nitrous oxide, CFCs, ozone
Human-Caused Greenhouse Gases