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Three kinds of things can happen when light strikes something, depending on frequency and the material:
Earth's atmosphere also transmits, reflects, and absorbs some of the light from the sun.
Carbon dioxide, water vapor, methane, CFC's and others are 'greenhouse' gases. They
This is why both the atmosphere, and your car on a sunny day are hotter than their surroundings.
Without some greenhouse effect the surface of earth would be much colder, perhaps inhospitable to life.
The Intergovernmental Panel on Climate Change (ipcc.ch)
Their measured consensus is that
Compare to Al Gore:
"There is overwhelming and incontrovertible evidence of global warming".
The concern is not so much because of the 0.5 C ($1{}^o$F) temperature change *so far*, but rather the possibility of a further 4-6 C rise in the next 100 years.
Greenhouse warming is due to a handful of trace gases - water vapor, carbon dioxide, methane--not nitrogen nor oxygen.
[The gray bars indicate natural ranges in last 20,000 years.]
[Units of radiative 'forcing': Compare to average solar power in N Indiana of about 150 W / m^2.]
Water vapor is a GHG--GreenHouse Gas. But there's so much of it, and it comes and goes from the atmosphere very quickly.
Carbon dioxide does not "rain" out of the atmosphere, once added it stays and stays. See: the carbon bathtub.
Methane (natural gas) has a much higher greenhouse effect per molecule (24 X $CO_2$), but there is much less of it in the atmosphere than carbon dioxide.
Carbon dioxide has the greatest effect on the climate.
Other GHGs include nitrous-oxide, & CFCs.
These data come from the IPCC - Intergovernmental Panel on Climate Change.
What happened around 1800?
One way that carbon dioxide leaves the atmosphere (phew!) is it dissolves in water.
But, when carbon-dioxide dissolves in water, the water becomes more acidic.
Coral reefs are largely made of calcium carbonate skeletons of corals. But acidity increases the amount of calcium carbonate that can dissolve in water.
How do we know atmosphere composition and temperature in the distant past?
Tree rings (thickness and density) depend on temperature and go back ~11,000 yr. (Temperatures)
Ice cores preserve air samples back about 650,000 yr. (CO2 levels)
Temperature affects the ratio of O16 to O18. (Temperatures)
There seems to be a correlation between higher temps and higher carbon-dioxide levels. Usually temperature rises came first and later carbon-dioxide levels rose.
Models of increasing sophistication attempt to project into the future
It's not always obvious which way global warming effects should go.
Are reports of massive glaciers breaking off of Antarctica (60 miles long in photo) evidence of global warming?
The models of a decade ago would have predicted that increased temperatures
$\Rightarrow$ greater precipitation, and more snow in Antarctica $\Rightarrow$
growth of glaciers.
Disappearing sea ice in the Arctic (animation) [NASA images 1979-2003] is one of the more visually convincing images.
Adding carbon-dioxide to the earth's atmosphere causes many different kinds of changes to the Earth. Some of the changes might make the situation worse--this is positive feedback. Other changes might act against the rise of carbon dioxide--this is negative feedback. Modellers need to consider many of these mechanisms and try to calculate or better yet, measure whether these are large or small effects.
For these mechanisms, decide what the final effect is going to be on temperatures or GHG levels, and say whether it is positive feedback or negative feedback:
Uncertainty about the effect of clouds is one of the big reasons behind the IPCCs hedging.
Required reading: Muller (Climate Change) Solutions (7-page pdf)
Jason Swaby, IPCC, Andreas Christen, Manuel Scheikl, David MacKay