Chapter 2
Introduction to the Atmosphere
2. How is today’s atmosphere similar to and different from early earth’s atmosphere?
Answer: The early earth’s atmosphere consisted of light and noble gases, such as helium, neon and argon.
These gases are largely devoid of today’s atmosphere as the gases were swept away by the solar wind.
The early earth’s atmosphere apparently contained far more CO2 than today’s and consisted of little or no
O2. Today, the dry atmosphere consists primarily of diatomic nitrogen (N2) and diatomic oxygen (O2).
These two gases comprise 99 percent of the present-day atmosphere. Argon and carbon dioxide are the
third and fourth most abundant gases in the present-day dry atmosphere.
4. Given that solar output had increased over the past 4.6 billion years, how have earth temperatures
remained fairly constant over that same time?
Answer: The earth-atmosphere-ocean system must have some type of internal regular that has kept the
earth’s temperature fairly constant over time even though solar output has increased. This regulator must
have been present in the early atmosphere as the sun grew in strength. The energy levels and temperatures
of the earth’s earlier atmosphere would have remained fairly constant as CO2 concentrations were
decreasing as a result of plant evolution and proliferation. More recent explanations for the fairly constant
temperatures despite increases in solar output are that ammonia and methanogens caused the early
greenhouse effect. It is hypothesized that the strong greenhouse effect would have warmed the earth even
though the sun emitted less radiation at the time.
6. What is the carbon cycle and how does it operate?
Answer: The carbon cycle is the continuous movement of carbon through the earth-ocean-atmosphere
system. Carbon can exist in various reservoirs including the atmosphere, the biosphere and the oceans.
Over time carbon cycles among these various reservoirs across various time scales. The largest carbon
sink is sedimentary rock layers followed by the oceans. The atmosphere represents the smallest carbon
sink although the quantity of CO2 has substantially increased since 1800. Today many people are
concerned that the natural carbon cycle is being disrupted by human activities.
8. What causes the thermal characteristics associated with each thermal layer of the atmosphere?
Answer: The troposphere is heated primarily through the absorption of terrestrial radiation. This explains
why temperatures in the troposphere decrease as you move away from the heat source at the surface.
Temperatures remain fairly constant in the isothermal layer of the stratosphere and increase with height
through the rest of the stratosphere. The temperature inversion observed in the stratosphere is due to the
absorption of UV radiation by the ozone layer. Similar to the troposphere, temperatures in the mesosphere
decrease with height. The coldest temperatures are found in the upper mesosphere because of a lack of a
heat source. The lower mesosphere is warmer than the upper mesosphere due to increased density and the
proximity to the stratospheric heat source below. The thermosphere, like the stratosphere, is characterized
by increasing temperatures with height. The temperature inversion exists because the N2 and O2
molecules in this layer have the first opportunity to absorb insolation, allowing the molecules to attain
extremely high temperatures.
10. Why is there no defined top to the atmosphere?
Answer: There is no defined top of the atmosphere; instead the atmosphere slowly merges into
interplanetary space. This is because individual gas molecules are gravitationally attracted to the earth for
great distances into space. Most agree that the atmosphere extends no higher than about 1000 km (600 mi)
above the earth’s surface.