Download Fig. 2-12, p. 43

Fig. 2-12, p. 43
Greenhouse effect
Greenhouse effect: analogy to real greenhouse
Energy radiatively trapped, not convectively trapped
Greenhouse effect
Balance without
GE
Incoming solar
Outgoing infrared
T increases until
IR balances solar
Average T is 255
K
• -18 C
•0F
Greenhouse effect
Balance with GE
Energy gets out
less efficiently
T has to be
higher for same
output
T ~288 K
• 15 C
• 59 F
Greenhouse effect
Atmosphere acts as “blanket”
Water vapor, carbon dioxide, methane enhance
blanket
Clouds enhance blanket, but also block sunlight
Greenhouse Enhancement
Global warming is occurring due to an
increase in greenhouse gases
Carbon dioxide
Methane
Nitrogen Oxide
Chlorofluorocarbons
Positive feedbacks continue the
warming trend.
Negative feedbacks decrease warming.
Incoming Solar Radiation
Conduction, convection, and infrared
radiation warm the atmosphere from
below, not sunlight or insolation from
above.
Scattering
Reflection, albedo
Incoming Solar Radiation
Observation: Blue skies and white
clouds
Sky is blue, clouds are white
¾ Clouds scatter all wavelengths of visible
light
Incoming Solar Radiation
Observation: Blue skies, red skies, and
white clouds
Selective scattering of incoming solar
radiation
Air scatters blue light best
At sunset, blue light has been scattered
away
Incoming solar radiation
Energy balance rules
Planet: Radiation in = radiation out
Solar in = reflected out + longwave out
Atmosphere: Total in = total out
Balance solar, longwave,
conduction+convection
Surface: Total in = total out
Balance solar, longwave,
conduction+convection
Annual Radiation Balance
Out of 100 units of incoming solar
energy:
51 units reach the Earth’s surface.
Earth absorbs 147 units, radiates 117
units, 30 unit surplus, warm.
Atmosphere absorbs 130 units, radiates
160 units, 30 unit deficit, cool.
Surplus/deficit balanced by nonradiative transport
Fig. 2-16, p. 48
Stepped Art
Fig. 2-17, p. 49
Fig. 2-17, p. 49
Tropical surplus Æ weather
Fig. 2-18, p. 49
Things to ponder
If it were much colder …
Less water vapor
Less greenhouse effect
Less warming
Snowball effect?
If it were much warmer …
More water vapor
More greenhouse effect
Eventually decompose
carbonates, clathrates?
Runaway greenhouse
Follow the water
Venus, Earth, and Mars started similar
Similar amount of water, carbon dioxide
Similar initial temperatures (within 20 deg
C)
Now they are very different
Venus is too hot
Mars is too cold
Earth is just right
¾ What
happened?
Venus: atmosphere
Surface temperature is 750 K (890 F)
90 times sea level pressure on Earth
Mostly CO2
Similar N2 to Earth, but minor gas on Venus
Similar amount of C in Earth rocks
Cloudy
Cloud layer 50 km above surface
Clouds are made of H2SO4 droplets
• That’s battery acid …
Runaway greenhouse effect
Venus started similar to Earth, except
temperatures slightly higher
More water vapor in atmosphere leads
to more warming (greenhouse effect)
Warmer atmosphere causes more water
evaporation; oceans evaporate
At higher temperatures, carbon comes
out of rocks, forms CO2, sends
temperatures even higher
Past climate: rivers?
Past climate: crater lake?
Past climate:
delta?
Runaway atmosphere hypothesis
Mars’ atmosphere was once Earth-like
Rain? Rivers. Glaciers. Ocean? Seas?
Mars’ atmosphere may have escaped to
space (CO2 and esp. H2O)
Mars’ former atmosphere may be in a
subsurface reservoir
Greenhouse Enhancement
Global warming is occurring due to an
increase in greenhouse gases
Carbon dioxide
Methane
Nitrogen Oxide
Chlorofluorocarbons
Positive feedbacks continue the
warming trend.
Negative feedbacks decrease warming.
Seasons
Why?
Sun-Earth distance?
Changes in the Sun?
Where the Sun is in the sky?
How long the day is?
Canberra, Australia
in June
Some observations
Hottest days ≠ closest to Sun
June vs. December
Our summer = Australian,
Chilean winter
Days are longer in summer,
shorter in winter
Extreme: “land of the midnight
Sun”
Sun is very high in the sky in
summer, not winter
Why the Earth has seasons
Earth revolves in elliptical path around
sun every 365 days.
Earth rotates counterclockwise or eastward
every 24 hours.
Earth closest to Sun (147 million km) in
January, farthest from Sun (152 million
lm) in July.
Distance not the only factor impacting
seasons.
Orbit
Earth travels around Sun in near-circle
Sun is near center
Earth 2 C warmer in December, 2 C cooler in
June
X-ray Sun
Closer to
Solar Min
Closer to
Solar Max
Solar “constant”
“Weather”: day to day, <0.1%
Cycle: 11-year period, <0.05%
Earth Rotation
Once every 24 hours
• 23h 56m sidereal (star) day
• 24h 0m solar day
Rotational axis offset by 23.5o
• Axis is “fixed”
– Changes hemispheric orientation through
orbit
– Causes seasons
Extreme Hypothetical Axis Orientation
Seasons
Solstice
Maximum axial tilt to/from the Sun
• 23.5 degrees
• June and December
Hemispheric axes inclined toward or away
from Sun
• Causes maximum or minimum solar
radiation receipt
Note
Tilt toward Sun
Geometry
• Angled toward
Sun
Distance
• Winter
hemisphere is
only 0.0004%
farther from Sun
Seasons
Equinox
Temporally centered between solstices
• ~ March 21 and ~ Sept 21
The subsolar point = 0o
Solstices & Equinoxes
Solstices & Equinoxes
Reading
Wed.: Ch 2
Fri.: Ch 2-3 (to p. 64)
Ch. 2 study materials
Optional, NOT to be turned in
Questions for review: 1-19
Questions for thought: 6, 7, 10-12