General Overview: The Greenhouse Effect

Introduction

The "greenhouse effect" is widely discussed in the media, and although its details are complicated, its principles are not difficult to understand. Without a greenhouse effect, radiation from the Sun (mostly in the form of visible light) would travel to Earth and be changed into heat, only to be lost to space. This scenario can be sketched as follows:

Sun’s radiation → absorbed by Earth → Re-radiated to space as heat

The greenhouse effect is a process where energy from the sun readily penetrates into the lower atmosphere and onto the surface of Earth and is converted to heat, but then cannot freely leave the planet. This can be sketched as follows:

Sun’s Radiation → absorbed by Earth → some re-radiated to space as heat → some trapped by the atmosphere

Due to the presence of certain “greenhouse gases” that trap heat, like carbon dioxide, methane, water vapor, and CFC’s, the atmosphere retains the sun’s radiation and warms up the planet. By increasing the abundance of these gases in the atmosphere, humankind is increasing the overall warming of the Earth’s surface and lower atmosphere, a process called "global warming." The figure below illustrates the radiation balance and the role of greenhouse effect.

The Radiation Balance

Illustration of the Earth’s radiative balance. (Adapted from: NOAA)

Another way to think about the greenhouse effect is to consider that according to physics the radiation we receive from the Sun must be equally balanced by the heat Earth radiates out to space. If we were to give back less energy than we receive, our planet would soon be too hot for life. Likewise, if we were to give back more energy that we receive, our planet would soon be too cold for life. This can be written as a balanced equation of radiation:

Solar radiation input to Earth = Earth’s output of re-radiated heat

If we were to measure the temperature of the Earth from space, the Earth's "surface" would show a temperature appropriate for this requirement of energy balance: a measurement of roughly -18 degrees Celsius (about 0 °F). At this temperature, our planet radiates a quantity of heat into space that is equivalent to the amount of energy received from the Sun.

At this point you may be asking how we can speak of “global warming” when we have just stated that the Earth (as seen from space) MUST stay at the same temperature? And how is it that the temperature of the Earth’s surface is only a chilly 0°F? The key to understanding this apparent contradiction is to remember that we live at the bottom of the atmosphere. As far as the radiation balance is concerned, the lower atmosphere and the surface of Earth form part of a “warm interior” of the planet.

Figure demonstrating the importance of greenhouse gases in regulating the temperature of the lower atmosphere. The top diagram shows a greenhouse Earth where the apparent temperature “surface” lies 5000m up in the atmosphere from the land surface. In the past 100 years this apparent temperature “surface” has been rising. By contrast, without a greenhouse effect, the Earth would look like the lower diagram.

The apparent temperature "surface" that we would see from space is located well above the real surface of the Earth where we live. This apparent temperature "surface" is about 5000 meters up (17,000 feet) within the atmosphere. To get a better handle on this concept consider the following: the difference in elevation between 0 meters and 5,000 meters corresponds to a difference in temperature of about 60°F. In other words, at sea level it is 60°F warmer than it would be without the atmosphere. For the last 100 years or so this apparent temperature “surface” has been moving upward in the atmosphere as a result of global warming. As the apparent "surface" rises, the bottom of the atmosphere gets warmer, a fact that can be seen in the positions of the snow line (the elevation where snow begins to form) and tree line (the elevation where it becomes to cold for trees to grow). However, despite all these changes happening in the lower atmosphere, the overall temperature of the planet as seen from space stays the same.

How is it possible that the Earth exactly balances the incoming sunlight with the outgoing heat radiation? The answer is simple: the amount of heat radiation from Earth is precisely tied to the temperature of the atmosphere. If the temperature of the apparent “surface” is too low and Earth radiates too little heat to keep the balance, Earth will warm up and radiate more heat into space. If the temperature of the apparent “surface” is too high and Earth radiates more heat than it receives, the planet will become colder and radiate less energy back to space. Overall, this “negative feedback” stabilizes the radiation balance despite all the variations of temperature from one place to another and within the vertical column of the atmosphere. It sets the temperature so that the incoming and outgoing energy is balanced.

Average Temperatures on the Moon

We can get another idea about what the temperature on Earth would be like without a greenhouse atmosphere by contemplating the Moon. The Earth’s satellite has no atmosphere because its gravitational force is not strong enough to retain gas for long. It has the same distance from the Sun as the Earth, but its temperature varies enormously: where the Sun is shining, the Moon’s temperature rises to 230°F and where it is dark falls to negative 290°F. The average surface temperature of the moon, about the same distance as the Earth from the Sun, is also near 0°F, but of course, the moon has no atmosphere. By contrast, the average surface temperature of the Earth is 60°F at sea level. On Earth, the contrast between maximum and minimum temperatures would not be as great as on the Moon, even without an atmosphere, because the Earth rotates once in a day, while the Moon only rotates once in a month. However, without an atmosphere the Earth’s contrast between day and night and the contrast between summer and winter would be very large indeed.

Not all the gases in the atmosphere are equally active in keeping Earth warm. In fact, the atmosphere’s most abundant gas, molecular nitrogen, does very little in this regard, and the same is true for the second most abundant gas, molecular oxygen. The most important ingredient of the air for producing the greenhouse effect is water vapor. However, its abundance depends on the air's temperature. The warmer the air, the more water vapor it can hold. (As air cools, the vapor condenses into rain or snow.) It is carbon dioxide that moves the air toward higher temperature, so that water vapor can take over and warm it some more. Carbon dioxide molecules intercept infrared radiation, warming the air and increasing water vapor through evaporation from the sea surface and from plants and soil moisture. Water vapor then increases the temperature even more. The process is checked by a rise in infrared radiation to space and by formation of clouds. Unfortunately, the role of clouds in the radiation balance is as yet poorly understood. Different types of clouds have different effects, and this makes the calculations complicated and the results uncertain.

Solar Variations

One last point to consider when discussing the greenhouse effect is the amount of sunlight coming in to Earth. The quantity of sunlight we receive depends on the size and the brightness of the Sun and the distance between it and the Earth. As far as we know, the size of the Sun does not change much over the time spans we are considering; we can assume it is constant. The sun’s brightness varies only a little, about one-thousandth over an eleven-year sunspot cycle, but perhaps more over longer time spans. We can take that as constant too, calling the incoming energy flux "the solar constant." One of the contentious issues in the discussions about the global warming of the last 100 years that has not been fully resolved is the question of whether a brighter Sun may have contributed to the recently observed temperature rise.

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Earth

Earth is the third planet from the Sun and the fifth largest:
orbit: 149,600,000 km (1.00 AU) from Sun
diameter: 12,756.3 km
mass: 5.972e24 kg




Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and Germanic. There are, of course, hundreds of other names for the planet in other languages. In Roman Mythology, the goddess of the Earth was Tellus - the fertile soil (Greek: Gaia, terra mater - Mother Earth).

It was not until the time of Copernicus (the sixteenth century) that it was understood that the Earth is just another planet.


Mir space station and Earth's limb
Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful.

The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km):
0- 40 Crust
40- 400 Upper mantle
400- 650 Transition region
650-2700 Lower mantle
2700-2890 D'' layer
2890-5150 Outer core
5150-6378 Inner core
The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle.

Most of the mass of the Earth is in the mantle, most of the rest in the core; the part we inhabit is a tiny fraction of the whole (values below x10^24 kilograms):
atmosphere = 0.0000051
oceans = 0.0014
crust = 0.026
mantle = 4.043
outer core = 1.835
inner core = 0.09675

The core is probably composed mostly of iron (or nickel/iron) though it is possible that some lighter elements may be present, too. Temperatures at the center of the core may be as high as 7500 K, hotter than the surface of the Sun. The lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium and aluminum. We know most of this only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes but the majority of the Earth is inaccessible. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth's chemical composition (by mass) is:
South America by Galileo

34.6% Iron
29.5% Oxygen
15.2% Silicon
12.7% Magnesium
2.4% Nickel
1.9% Sulfur
0.05% Titanium

The Earth is the densest major body in the solar system.

The other terrestrial planets probably have similar structures and compositions with some differences: the Moon has at most a small core; Mercury has an extra large core (relative to its diameter); the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth may be the only one with distinct inner and outer cores. Note, however, that our knowledge of planetary interiors is mostly theoretical even for the Earth.

Unlike the other terrestrial planets, Earth's crust is divided into several separate solid plates which float around independently on top of the hot mantle below. The theory that describes this is known as plate tectonics. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below. Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle. There is also transverse motion at some plate boundaries (i.e. the San Andreas Fault in California) and collisions between continental plates (i.e. India/Eurasia). There are (at present) eight major plates:
Earth's Plate Boundaries delineated by earthquake epicenters
North American Plate - North America, western North Atlantic and Greenland Earth's Plate Boundaries delineated by earthquake epicenters
South American Plate - South America and western South Atlantic
Antarctic Plate - Antarctica and the "Southern Ocean"
Eurasian Plate - eastern North Atlantic, Europe and Asia except for India
African Plate - Africa, eastern South Atlantic and western Indian Ocean
Indian-Australian Plate - India, Australia, New Zealand and most of Indian Ocean
Nazca Plate - eastern Pacific Ocean adjacent to South America
Pacific Plate - most of the Pacific Ocean (and the southern coast of California!)
There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates. Earthquakes are much more common at the plate boundaries. Plotting their locations makes it easy to see the plate boundaries.

The Earth's surface is very young. In the relatively short (by astronomical standards) period of 500,000,000 years or so erosion and tectonic processes destroy and recreate most of the Earth's surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters). Thus the very early history of the Earth has mostly been erased. The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years are rare. The oldest fossils of living organisms are less than 3.9 billion years old. There is no record of the critical period when life was first getting started.

Space Shuttle view of the Strait of Gibraltar
71 Percent of the Earth's surface is covered with water. Earth is the only planet on which water can exist in liquid form on the surface (though there may be liquid ethane or methane on Titan's surface and liquid water beneath the surface of Europa). Liquid water is, of course, essential for life as we know it. The heat capacity of the oceans is also very important in keeping the Earth's temperature relatively stable. Liquid water is also responsible for most of the erosion and weathering of the Earth's continents, a process unique in the solar system today (though it may have occurred on Mars in the past).

Earth's atmosphere seen at the limb
The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. There was probably a very much larger amount of carbon dioxide in the Earth's atmosphere when the Earth was first formed, but it has since been almost all incorporated into carbonate rocks and to a lesser extent dissolved into the oceans and consumed by living plants. Plate tectonics and biological processes now maintain a continual flow of carbon dioxide from the atmosphere to these various "sinks" and back again. The tiny amount of carbon dioxide resident in the atmosphere at any time is extremely important to the maintenance of the Earth's surface temperature via the greenhouse effect. The greenhouse effect raises the average surface temperature about 35 degrees C above what it would otherwise be (from a frigid -21 C to a comfortable +14 C); without it the oceans would freeze and life as we know it would be impossible. (Water vapor is also an important greenhouse gas.)

View from Apollo 11
The presence of free oxygen is quite remarkable from a chemical point of view. Oxygen is a very reactive gas and under "normal" circumstances would quickly combine with other elements. The oxygen in Earth's atmosphere is produced and maintained by biological processes. Without life there would be no free oxygen.

The interaction of the Earth and the Moon slows the Earth's rotation by about 2 milliseconds per century. Current research indicates that about 900 million years ago there were 481 18-hour days in a year.

Earth has a modest magnetic field produced by electric currents in the outer core. The interaction of the solar wind, the Earth's magnetic field and the Earth's upper atmosphere causes the auroras (see the Interplanetary Medium). Irregularities in these factors cause the magnetic poles to move and even reverse relative to the surface; the geomagnetic north pole is currently located in northern Canada. (The "geomagnetic north pole" is the position on the Earth's surface directly above the south pole of the Earth's field.)

The Earth's magnetic field and its interaction with the solar wind also produce the Van Allen radiation belts, a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around the Earth. The outer belt stretches from 19,000 km in altitude to 41,000 km; the inner belt lies between 13,000 km and 7,600 km in altitude.
Earth's Satellite
Earth has only one natural satellite, the Moon. But
thousands of small artificial satellites have also been placed in orbit around the Earth.
Asteroids 3753 Cruithne and 2002 AA29 have complicated orbital relationships with the Earth; they're not really moons, the term "companion" is being used. It is somewhat similar to the situation with Saturn's moons Janus and Epimetheus.
Lilith doesn't exist but it's an interesting story.
Distance Radius Mass
Satellite (000 km) (km) (kg)
--------- -------- ------ -------
Moon 384 1738 7.35e22

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Aerobics

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What To Think About When Choosing A Weight Reduction Cleanse By : Bob Tripmeyster
Are you currently interested in reducing weight? If you're, have you heard of weight loss cleanses, also commonly referred to as colon cleanses, just before?

 
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Aerobics are a good way to stay fit and trim off fat, but it also does wonders for your cardiovascular system and helps to keep your heart strong. Here's a few ways that it can benefit your health and fitness levels.

 
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Electrical treadmills became standard from previous few decades and prove to be quite useful for individuals of all age teams may be younger or older ones.

Aerobics: Burn Off That Fat  
One of the best things about aerobics is that you can use it to burn fat in methods that are easier on your body than dieting. Aerobic exercise such as jogging is great for your overall well-being and getting rid of that belly fat.

HOW AGING EFFECTS YOUR BODY

Before undertaking any senior exercise program it is important to understand the effects of aging. Most seniors can follow the obvious signs of aging. We watch as our hair grays (or disappears entirely) and as our skin wrinkles. But few people notice the almost imperceptible changes taking place within our bodies; changes so subtle that it's possible to miss them completely until a major health crisis forces us to sit up and take notice

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