The Greenhouse Effect on Earth
Earth’s atmosphere is slightly warmer
than what it should be due to direct
solar heating because of a mild case of
greenhouse effect…
• The ground is heated by visible
and (some) infrared light from the
Sun.
• The heated surface emits infrared
light.
• The majority of Earth’s
atmosphere (N2 and O2) are not
good greenhouse gas.
• The small amount of greenhouse
gases (H2O, CO2) traps (absorb
and re-emit) the infrared radiation,
increasing the temperature of the
atmosphere…
Click on image to start animation
Water On Earth
The condition is just right!
• The combination of three factors: Distance to the Sun, the
albedo, and the greenhouse effect, make it possible for water to
stay on Earth.
• N2 and O2 are not greenhouse gas.
• Not much CO2 in the atmosphere.
• Variable amount of H2O in the atmosphere…regulated by the
temperature.
The result is a mild
greenhouse effect…not too
hot, and not too cold, just
the right temperature for
most of the water to stay in
liquid phase, and some to
stay in gas phase in the
atmosphere on the surface
of the Earth…
Greenhouse Gases
• The primary components of Earth’s atmosphere, N2 and
O2 do not have absorption in the IR wavelength range,
therefore, do not have a significant role in setting the
surface temperature of the planet…
• Greenhouse gas are efficient in absorbing IR light…
The most important greenhouse gases are:
– H2O – Water vapor.
– CO2 – Carbon Dioxide
– CH4 – methane
The most abundant greenhouse gas in Earth’s
atmosphere is water vapor. Most of the greenhouse
heating of Earth’s atmosphere is due to Water vapor
absorption of IR radiation emitted by Earth, and then
transferring the energy to the surrounding air
molecule
Source of Water
The terrestrial planets were built from rock and planetesimals. No gases or water
can condense at the high temperature near the Sun. So, where did the water on
Earth come from?
• The water on Earth (and other terrestrial worlds) most likely was brought over
by the comets during the period of heavy bombardment about 4 billion years
ago…
• These water (and other gases) were trapped in the interior, and released by
volcanic activities…by Outgassing
•
Mt. St Helen eruption, 2004!
The Atmosphere of Earth
The atmosphere of Earth contains primarily N2
(77%) and O2 (21%).
• What happened to all the CO2?
• Where did all the O2 come from?
CO2
CO2 is a colorless gas…
• condenses into solid form (dry ice) at -78°C in atmospheric
pressure.
• condenses into liquid at -57°C at pressure above 5.1
atmospheric pressure.
Atmospheric CO2 is derived from (The sources…)
• Volcanic outgassing
• burning of organic matter
• Respiration of living organisms
• …
CO2 can be stored in (The Sinks…)
• Highly soluble in water: forms H2CO3
• Dissolved CO2 in water can interact with silicate minerals
to form carbonated minerals…
• …
Carbon Dioxide Cycle
The mechanism by which Earth self-regulates its temperature is
called the carbon dioxide cycle, or the CO2 cycle for short.
Starting with the carbon dioxide in the atmosphere:
• Volcanoes outgas CO2 into the atmosphere.
• Atmospheric carbon dioxide dissolves in the oceans.
• At the same time, rainfall erodes rocks on Earth’s continents
and rivers carry the eroded minerals to the oceans.
• In the oceans, the eroded minerals combine with dissolved
carbon dioxide and fall to the ocean floor, making carbonate
rocks such as limestone.
• Over millions of years, the conveyor belt of plate tectonics
carries the carbonate rocks to subduction zones, and
subduction carries them down into the mantle.
• As they are pushed deeper into the mantle, some of the
subducted carbonate rock melts and releases its carbon
dioxide, which then outgasses back into the atmosphere
through volcanoes.
The CO2 Cycle
The CO2 cycle acts as a thermostat that regulates the temperature of
the Earth…
If Earth warms up a bit, then
• carbonate minerals form in the oceans at a higher
rate.
• The rate at which the oceans dissolve CO2 gas
increases, pulling CO2 out of the atmosphere.
• The reduced atmospheric CO2 concentration leads
to a weakened greenhouse effect that counteracts
the initial warming and cools the planet back
down.
If Earth cools a bit,
• carbonate minerals form more slowly in the
oceans.
• The rate at which the oceans dissolve CO2 gas
decreases, allowing the CO2 released by volcanism
to build back up in the atmosphere.
• The increased CO2 concentration strengthens the
greenhouse effect and warms the planet back up
Feedback Loop
• Positive Feedback
– Mechanisms that make things worse…
– e.g., Increasing CO2 in the atmosphere
leading to the release of more CO2
• Negative Feedback
– Mechanisms that are self-correcting…
– e.g., Increasing CO2 in the atmosphere
leading to higher rate of CO2 removal, such
as our CO2 cycle.
Plate Tectonics
Plate tectonics plays an important role in the CO2 cycle in that it helps to
carry the carbonate rocks into the mantle, which are then released again by
volcanic activities.
– Earth’s lithosphere is broken into pieces (the plates).
– These plates float on top of the mantle, interacting with each other to
produce the geological features we see and feel today.
Click on image to start animation
Where Did O2 Come From?
The most important source of O2 on Earth is
Life and Photosynthesis.
•
•
•
•
Photosynthesis converts CO2 to O2, and
incorporates carbon into amino acids, proteins,
and other components of living organisms.
O2 will be depleted from the atmosphere very
rapidly without a source.
O2 is a very reactive chemical that likes to be
combined with other elements through
oxidation. For examples, CO2, H2O, FeO (rust)
 That’s how we make fire!
O2 Absorbs UV, which also transform some of
the O2 into O3, which absorbs even more UV
 O2 not only supports life, it also protect life!
UV light can break the water molecules to release
oxygen, but the contribution is small….
The Role of the Magnetic Field of Earth
Another important characteristics of the Earth is its magnetic fields, which shield
us from the bombardment of the high-energy charged particles, mostly from the
Sun.
• Without magnetic field, the high energy particles of solar wind can strip much
of the Earth’s atmosphere by breaking the bounds between the atoms in the air
molecules
– N2 → N + N
– O2 → O + O
– H2O → H + H + O
• The lighter gases then have higher probability of acquiring velocity higher
than escape velocity and escape from Earth!
Water On Earth in the Past
Was it always like this on Earth?
• Yes. Water was plentiful throughout most of Earth’s history,
for about three billion years.
• No! Geological evidences suggest that Earth used to be
covered by ice about 600-700 million years ago
 Snowball Phase.
How did Earth recover from the snowball phase?
• Once the water was frozen, CO2 can no longer be removed
from the atmosphere by dissolving in water  interruption of
the CO2 cycle.
• Increased CO2 level in the atmosphere leads to stronger
greenhouse effect, which warms the atmosphere.
• Higher temperature melt the ice  restoration of the CO2
cycle.
Comparative Planetology
Mars and Venus are very similar to Earth in
their size and location to the solar system.
However, their surface environments are
drastically different from that of the Earth
today. By understanding how Mars and
Venus end up with their current state, we
may be able to better understand our
Earth…
Mars
Mars image from
Hubble Space Telescope
Martian Season
• The tilt of Mar’s rotation axis with respect to its
ecliptic plane is 25.19°
• The eccentricity of Mar’s orbit around the Sun is
0.093
• The seasons on Mars are affected by both its
orbital distance and its axis tilt.
– Mars is closer to the Sun during the southern
hemisphere summer, and farther away from the Sun
during its winter
– Mars therefore has more extreme seasons in its
southern hemisphere—that is, shorter, hotter
summers and longer, colder winters—than in its
northern hemisphere.
Martian Weather
• Even though Mars only has a very
thin atmosphere, it still has a
weather system…
• Martian weather are due to its
extreme seasonal changes.
– Polar temperatures at the winter pole
drop so low (about –130°C) that
carbon dioxide condenses into “dry
ice” at the polar cap.
– frozen carbon dioxide at the summer
pole sublimates into carbon dioxide
gas.
– The atmospheric pressure therefore
increases at the summer pole and
decreases at the winter pole, driving
strong pole-to-pole winds.
– Storms on Mars can engulf the entire
planet.
Geology of Mars
• Martian surface is
similar to Earth’s desert
and volcanic plane
– High elevation and
numerous large impact
craters in the southern
hemisphere
– Lower elevation and few
impact craters in the
northern hemisphere
Volcanism is the most
likely mechanism
responsible for changing
the surface features of
Mars.
• Many geological
features suggest past
water flows…
Dry Ice (frozen CO2) in the north
and south poles…
Water on Mars in the Past?
Many geological features of Mars suggest that it had a lot of water about 3 billion years
ago. It may even have a pleasant, hospitable environment.
•
Dried up riverbeds…
•
Gullies?
Images from Mars Rover Spirit at
•
Lake bottom
a suspected ancient lake site
showed rock structures consistent
with those formed from sediments
in standing water
Riverbed?
Gullies
Lake Bottom?
Ancient Martian Ocean?
Mars may once have an ocean. The smoother surface in
the low lying areas in the northern hemisphere (blue
areas in the image on the right) may once hold an
ocean…
Water on Mars Today?
The gullies form when snow
accumulates on the crater walls
in winter and melts away in
spring. Because the gullies are
relatively small (note the scale
bar in Figure 7.26), they should
be gradually covered over by
blowing sand during Martian
dust storms. Thus, gullies that
are still clearly visible must be
no more than a few million
years old. Geologically
speaking, this time is short
enough to make it quite likely
that water flows are still forming
gullies today
Why doesn’t Mars have water
today?
If Mars used to hold a large amount of water, then why is Mars so different
today? What caused it to lose its water?
We don’t know exactly what happened, but one likely explanation was because of
the relatively small size of Mars:
•
•
•
The smaller size of Mars means
that it cools off faster. Once it
cools, volcanic activities stop,
halting the release of gases into the
atmosphere.
The cool interior temperature may
means that Mars does not have a
fluid metallic core to generate
magnetic fields anymore.
Without a magnetosphere, the
atmosphere is exposed to the
bombardment of high energy
charged particles of solar wind,
which break the air molecules,
making them easier to escape.
•
•
As Mars cools, the remaining CO2 gases are frozen in
the north and south pole, forming the ice cap.
The remaining oxygen are trapped on surface rock,
making it look red
Venus
We cannot
see the
rocky
surface of
Venus due
to its thick
atmosphere.
..
NASA Image of Venus
Geology of Venus
•
•
•
Venus’ surface is similar to Earth and Mars – few impact craters, volcanoes,
and evidence of tectonics activities…
– But no plate tectonics
The volcanoes of Venus is most likely still active today
– few impact craters,
– sulfuric acid cloud (the volcanoes are still outgasing)
However, there is no sign of erosion
– No liquid water?
– No wind, due to its slow rotation (243 Earth days per rotation).
• Venus dos not have a magnetic field!
This is quite surprising given that most of
the ingredients required for the dynamo are
all present…
Click on the image to see
image obtained by Venera
14 spacecraft
Why doesn’t Venus have water?
Given the similarities between Earth and Venus, why is the
atmosphere of Venus so different from Earth’s?
Venus is too hot!
•
The proximity to the Sun keep the temperature on Venus
high, even without greenhouse effect. Any water on
Venus (from out-gassing of water trapped inside the
planet) are vaporized into gaseous phases (water vapor).
•
Water vapor and CO2 are both greenhouse gas, causing
the atmosphere to warm up more  runaway greenhouse
effect  T = 740 ºK
•
At 740 ºK, the molecules of gases has much higher
average kinetic energy (recall the definition of
temperature)  higher average velocity.
•
If the velocity of the gas molecules exceed the escape
velocity, then they can escape into space…
•
Light gases (H, H2O, O2, N2) escape, heavy gases (CO2)
stay. Why?
•
Without liquid water, CO2 doesn’t have a place to go,
except to stay in the atmosphere…in comparison, most of
the CO2 on Earth are locked in rock or liquid water...
Runaway Greenhouse Effect
If we were to move the Earth closer to the Sun, like where Venus is now, then we
would suffer the runaway greenhouse effect as well, lose all the water, and become hot
like Venus.
What makes a planet habitable?
Two important factors
determine whether a
planet is habitable…
Size:
– Need substantial mass
to maintain an
atmosphere
– Small planets cool off
faster than large ones.
Without the volcanic
outgasing and a hot,
fluid metallic core to
generate magnetic
field, atmospheric gas
are easily depleted.
Distance to the Sun – the
distance to the Sun
determine the energy input
to the planet:
– Too close  too hot –
water evaporates.
– Too far  too cold –
water freeze.
•
•
•
•
Internal Structure
Surface Features
Atmosphere
What makes the Earth
hospitable to life?
• Global Warming?
Global Warming, A Quick Poll
Is global warming real?
a) Yes
b) No
How much has the average
temperature of the Earth risen in
the last 100 years?
a) ~ 0.5°C
b) ~ 1.0°C
c) ~ 5.0°C
d) ~ 10.0°C
This is the highest temperature ever
recorded in Earth’s history.
a) Yes.
b) No.
Who is to blame?
a) Human activities
b) Nature causes
c) ET
d) NoCanTell!
What is to blame?
a) Ozone
b) CO2
c) H2O
d) O2
Global Warming, A Quick Poll
Is global warming real?
a) Yes
b) No
How much has the average
temperature of the Earth risen in
the last 100 years?
a) ~ 0.5°C
b) ~ 1.0°C
c) ~ 5.0°C
d) ~ 10.0°C
As far as we know, this is the highest
temperature ever recorded in
Earth’s history.
a) Yes.
b) No.
Who is to blame?
a) Human activities
b) Nature causes
c) ET
d) NoCanTell!
What is to blame?
a) Ozone
b) CO2
c) H2O
d) O2
e) NoCanTell. It is real
complicated!
Global Warming
There is a gradual increase in
the average temperature of the
Earth’s atmosphere in the last
100 years…It has risen about
1°C since 1900…
• Are human activities
causing global warming?
• What other (non-human)
factors can cause global
warming?
• How does global warming
affect our life?
Just watch the movies…
Earth’s Temperature Variation in
the past 1,100 years
Reconstructions of (Northern Hemisphere average or global average) surface temperature variations from six
research teams (in different color shades) along with the instrumental record of global average surface
temperature (in black). Each curve illustrates a somewhat different history of temperature changes, with a range
of uncertainties that tend to increase backward in time (as indicated by the shading). Reference: NRC, 2006.
(Figure reprinted with permission from Surface Temperature Reconstructions© (2006) by the National
Academy of Sciences, Courtesy of the National Academies Press22 18, Washington, D.C.).
Reproduced from EPA Climate Change Website.
The Long-Term Stability of Earth’s
Climate−400,000 years
• The atmospheric
concentration of CO2
measured from Antarctic
ice core data implies that
Earth’s climate has being
pretty stable over the
past 400,000 years
• It also shows a rapid
increase of about 30%
in the past few
centuries…
– 270 ppm (parts per
million) to 370 ppm
Fluctuations in temperature (blue) and in the atmospheric
concentration of carbon dioxide (red) over the past 400,000 years as
inferred from Antarctic ice-core records. The vertical red bar is the
increase in atmospheric carbon dioxide levels over the past two
centuries and before 2006. From A. V. Fedorov et al. Science 312,
1485 (2006)17. 18.
Reproduced from EPA Climate Change Website.
How do we measure atmospheric
CO2 concentration in the past?
•
•
Precise measurements of atmospheric CO2
concentration is available only in the last few
decades…
Information about atmospheric CO2 concentration and
temperatures in the past can be inferred by several
different methods, such as
– Tree-ring
– Deep ocean sediment
– Ice core records
– Coral
– …
Link to NOAA Paleoclimatology Website
Paleoclimatology is the study of climate prior to the widespread availability of
records of temperature, precipitation and other instrumental data.
Antarctic Ice
Core
Located high in mountains and in
polar ice caps, ice has
accumulated from snowfall over
many millenia. Scientists drill
through the deep ice to collect ice
cores. These cores contain dust,
air bubbles, or isotopes of
oxygen, that can be used to
interpret the past climate of that
area.
From NOAA Paleoclimatology
Website.
•
•
•
•
This figures shows estimates of the changes
in carbon dioxide concentrations during the
Phanerozoic. Three estimates are based on
geochemical modeling: GEOCARB III (Berner
and Kothavala 2001), COPSE (Bergmann et
al. 2004) and Rothman (2001). These are
compared to the carbon dioxide measurement
database of Royer et al. (2004) and a 30 Myr
filtered average of those data. Error envelopes
are shown when they were available. The right
hand scale shows the ratio of these
measurements to the estimated average for
the last several million years (the Quaternary).
Customary labels for the periods of geologic
time appear at the bottom.
Direct determination of past carbon dioxide
levels relies primarily on the interpretation of
carbon isotopic ratios in fossilized soils
(paleosols) or the shells of phytoplankton and
through interpretation of stomatal density in
fossil plants. Each of these is subject to
substantial systematic uncertainty.
Estimates of carbon dioxide changes through
geochemical modeling instead rely on
quantifying the geological sources and sinks
for carbon dioxide over long time scales
particularly: volcanic inputs, erosion and
carbonate deposition. As such, these models
are largely independent of direct
measurements of carbon dioxide.
Both measurements and models show
considerable uncertainty and variation;
CO2 over
500 million
years
From:
http://en.wikipedia.org/wiki/Image:P
hanerozoic_Carbon_Dioxide.png
Which gas is keeping the Earth
warm?
A.
B.
C.
D.
N2?
O 2?
CO2?
H2O?
The major natural greenhouse gases are
• water vapor, which causes about 36-70% of the
greenhouse effect on Earth (not including clouds);
• carbon dioxide, which causes 9-26%;
• methane, which causes 4-9%, and
• ozone, which causes 3-7%.
Note that it is not really possible to assert that a certain
gas causes a certain percentage of the greenhouse
effect, because the influences of the various gases are
not additive. (The higher ends of the ranges quoted
are for the gas alone; the lower ends, for the gas
counting overlaps.)[3] [4]
From http://en.wikipedia.org/wiki/Greenhouse_gas
•
•
•
•
So, what’s the big deal if human
CO2 causes 1°C temperature
increase?
An increase in atmospheric temperature
(human or natural origin) will lead to the
increase in the water vapor content of the
troposphere.
Because water vapor is a strong
greenhouse gas, the increase in H2O vapor
in turn causes enhanced greenhouse effect,
raising the temperature more.
Higher atmospheric temperature will cause
more evaporation of water
Which leads to even higher temperature…
 Runaway Green House Effect!
How about Clouds and Ice?
Water vapor (water in gaseous phase) is one of the most
potent and abundant greenhouse gas…but
• Clouds (water in liquid form) reflect sunlight, decreasing
the solar energy input into Earth’s atmosphere during the
day, but they trap IR radiation from the Earth during the
night. It’s net effect is not well know so far…
– Albedo of clouds range from close to 0 to 70%.
– Testing climate impact of clouds after Sept. 11,
2001…
• Ice has a very high albedo, ~ 80 to 90%.
– Thus, reduction of the polar ice cap can cause more heating…
Contrails and Climate
Contrails are artificial clouds made by
the exhaust of the aircraft engines,
or the wingtip vortices
(http://en.wikipedia.org/wiki/Contrail
). Contrails produced by the heavy
air traffic over the US may have
noticeable influences on the
weather…
• Commercial air traffic were
suspended for three days after the
Sept. 11, 2001 attack. This provided
a rare chance for the climate
scientist to test their theory…
• Measurements show that without
contrails the local difference of day
and night-time temperatures was
about 1 degree Celsius higher than
immediately before the attack…
How About The CO2 Cycle?
The CO2 cycle acts as a thermostat that regulates the temperature of
the Earth…
If Earth warms up a bit, then
• carbonate minerals form in the oceans at a higher
rate.
• The rate at which the oceans dissolve CO2 gas
increases, pulling CO2 out of the atmosphere.
• The reduced atmospheric CO2 concentration leads
to a weakened greenhouse effect that counteracts
the initial warming and cools the planet back
down.
If Earth cools a bit,
• carbonate minerals form more slowly in the
oceans.
• The rate at which the oceans dissolve CO2 gas
decreases, allowing the CO2 released by volcanism
to build back up in the atmosphere.
• The increased CO2 concentration strengthens the
greenhouse effect and warms the planet back up
Feedback Loop
• Positive Feedback
– Mechanisms that make things worse…
– e.g., Increasing CO2 in the atmosphere
leading to the release of more CO2
• Negative Feedback
– Mechanisms that are self-correcting…
– e.g., Increasing CO2 in the atmosphere
leading to higher rate of CO2 removal, such
as our CO2 cycle.
My Two Cents…
• It looks like most of the scientists agree that the global warming
observed in the last century were caused by human activity.
However, as we tried to demonstrate here, the global climate is a
very complicated system. We understand the basic principle of the
climate system, but we still don’t understand how nature regulates
Earth’s climate over the long run, nor do we have the capability to
create a realistic climate model and be able to predict with any
certainty the effects of human activities on our climate system.
My advices…
•
•
•
•
Keep an open mind.
Read, and think for yourself!
Do not rush into judgment (especially after you watch the movies).
Please trust the scientific community to come up with an honest
answer…There are enough check and balance in the scientific
community to weed out the bad theories…