The document summarizes key aspects of the greenhouse effect on Earth. It notes that Earth's atmosphere is slightly warmer than expected due to greenhouse gases like water vapor and carbon dioxide trapping infrared radiation emitted from the planet's surface. These greenhouse gases absorb and re-emit the infrared light, increasing the atmosphere's temperature. This mild greenhouse effect creates just the right conditions for liquid water to exist on the Earth's surface.

1. 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

2. 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

3. 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

4. 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!

5. 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?

6. 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…
• …

7. 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.

8. 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

9. 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.

10. 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

11. 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….

12. 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
– H2 O → H + H + O
• The lighter gases then have higher probability of acquiring velocity higher
than escape velocity and escape from Earth!

13. 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.