The combination of three factors: Distance to the Sun, the albedo, and the greenhouse effect, make it possible for water to stay on Earth.
N 2 and O 2 are not greenhouse gas.
Not much CO 2 in the atmosphere.
Variable amount of H 2 O 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…
The primary components of Earth’s atmosphere, N 2 and O 2 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:
H 2 O – Water vapor.
CO 2 – Carbon Dioxide
CH 4 – 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
M ars 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…
T he 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
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 CO 2 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, H 2 O, O 2 , N 2 ) escape, heavy gases (CO 2 ) stay. Why?
Without liquid water, CO 2 doesn’t have a place to go, except to stay in the atmosphere …in comparison, most of the CO 2 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.
The Long-Term Stability of Earth’s Climate − 400,000 years
The atmospheric concentration of CO 2 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 CO 2 concentration in the past?
Precise measurements of atmospheric CO 2 concentration is available only in the last few decades…
Information about atmospheric CO 2 concentration and temperatures in the past can be inferred by several different methods, such as
Deep ocean sediment
Ice core records
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.
L ocated 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.
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; however, all point to carbon dioxide levels in the past that have been significantly higher than they are at present.
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.)  
So, what’s the big deal if human CO 2 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 H 2 O vapor in turn causes enhanced greenhouse effect, raising the temperature more.
Higher atmospheric temperature will cause more evaporation of water
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…
C ontrails 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…
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.
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…