TOPIC 2 - The Earth's Heat Budget
External Forces Affecting the Earth
A. Introduction - There are numerous sources of matter and energy that reach the Earth from Outer Space. The most important energy source is Solar Insolation or sunlight. The two most important matter sources are (1) the Solar Wind which brings ionic particles from the Sun to the Earth and (2) meteors/comets.
B. The Sun:
1. The Sun is located approximately 150 million km from the Earth. The Sun is a star that is composed primarily of Hydrogen (H) and Helium (He) and has a mass 300,000 times larger than the Earth. This enormous mass leads to extremely high temperatures and pressures in the Solar interior which cause the gases to ignite and produce fussion-thermonuclear reactions (sort of like thosands of hydrogen bombs going off in a semi-controlled continuous manner). The resulting energy is spewed outward from the Sun primarily as light. Sunlight reaches the Earth at a rate of about 1370 Watts/meter2.
2. Light can be classified according to its energy or wavelength.
2. The Earth revolves around the sun in an orbit that is elliptical in shape. The fact that the Earth's orbit is not perfectly circular means that the Earth is sometimes closer to the Sun than at other times. Perhelion is the time of Earth's closest approach and Aphelion is the time of the Earth's farthest position with respect to the Sun.
3. The Earth is in just the right orbit to permit carbon-based life and the hydrologic (water) cycle to exist. Mercury and Venus are too close to the Sun and too hot to maintain life or water in a liquid state. Mars is too far away and too cold to maintain life or water in a liquid state.
The Earth's Atmosphere
A. Physical Characteristics of the Atmosphere
1. The atmosphere and oceans are different from the rest of Earth materials in being composed of fluids
2. The atmosphere extends outward from the Earth's surface for several hundred km and some traces are noted as far away as several thousand km - at some arbitrary point (~150-200 km) we consider the gas density to be so low that we term it 'outer space'
3. Over half of the mass of the atmosphere lies below 6 km altitude (18,000 ft; less than the height of many large mountains).
B. Chemical Characteristics of the Atmosphere
1. Most of the atmosphere is composed of nitrogen (N2 - 78%) and oxygen (O2 - 21%)
2. Other important trace gases include carbon dioxide (CO2), water vapor, hydrogen (H2), helium (He), radon (R)
3. Most of the gases are the result of original degassing of the planet during chemical differentiation about 4 Ga. Some of the lightest gases were lost (reached escape velocity) at the top of the atmosphere over time.
4. Oxygen was added later after development of photosynthesis in early marine microorganisms.
C. Atmospheric Structure - The Earth's atmosphere can be described
as a series of layers at increasing distances from the surface, each with
different properties. The layers really have no firm boundaries due to the
turbulent nature of the atmosphere, but they do indicate the very different
atmosphere characteristics at different altitudes.
1. Troposphere
a. The layer closest to the Earth's surface
b. Weather that we experience originates in the Troposphere
c. Temperature and pressure decrease with altitude to the top of the Troposphere (Tropopause)
2. Stratosphere
a. Next layer above the Troposphere
b. Contains the Ozone layer
c. Contains strong Jet Streams
3. Other Layers
a. Mesosphere lies above the Stratosphere - highest layer with 'Earth-like' atmospheric composition
b. Outward of the Mesosphere is the Thermosphere or Ionosphere - Auroras
occur here
The Earth's Global Heat Budget
1. Solar Insolation - The primary source of energy to drive our global climate system (including atmospheric and, to a lesser extent, oceanic circulation) is the heat we receive from the Sun, termed solar insolation. The spectrum of light which comes from the Sun is shown above. The spectrum of light in the atmosphere and at the Earth's surface is shown below.
a. Insolation arrives at the edge of our atmosphere primarily as short wavelength radiation (179 kcal/cm2/yr).
b. The amount of insolation which reaches the Earth's surface depends
on site latitude and season. Imagine a disk in front of the Earth that is
just big enough to shield all solar radiation from the Earth. Each square
meter of the disk surface will receive the same amount of insolation. Regions
of the Earth's surface near the equator are almost parallel to the disk
and will receive about the same amount of insolation/m2 if we remove the
disk, but regions near the poles will receive much less insolation/m2 because
the surface is at a large angle to the disk. Also, the sunlight has to go
through more atmosphere to reach the poles. Both effects lead to much lower
insolation at the poles versus the equator.
2. Transfer of Heat in Atmosphere
a. Part of the incoming insolation is absorbed by the atmosphere and the rest is absorbed by the Earth's surface or re-radiated back into the atmosphere.
b. Part of re-radiated heat and other heat in atmosphere is re-absorbed by Greenhouse gases.
c. Ultimately, the amount of heat that leaves the atmosphere must match that which enters, otherwise the planet's surface would rise in temperature and cook us up. (Geologic history tells us that has not happened.)
d. The temperature of the atmosphere critically depends on how much energy
is absorbed by atmospheric gases and how long that heat is held before it
is re-radiated out into outer space.
