I think I have a reasonable feel for the development issues, but not so much the actual science. Mr Tea, can you sum up the state of knowledge for us in one easy to digest post, and throw in a few links and that?
Oh blimey, I've probably made myself sound much more knowledgeable on this than I actually am, but here goes anyway - most of it will be just be stuff I remeber from lectures, articles, TV programmes or whatever, so I'll try and flesh it out with some links later.
Basically CO2 is transparent to visible light and what's called 'near' infra-red, i.e. IR near the visible part of the spectrum. Most of the Sun's energy output is in the form of this kind of radiation (there's also UV, but on Earth most of that is blocked by the ozone layer). This light is absorbed by the Earth's surface which then re-emits the energy in the form of much longer-wavelength radiation, i.e. far IR. This is because the hotter (cooler) an object is, the shorter (longer) the peak wavelength it emits in - so a blue flame is hotter than a yellow one, for instance.
Anyway, CO2 is opaque to far IR so this energy radiated back up into the sky is either absorbed by the atmosphere or reflected back down to the surface. Glass does much the same thing, which is how a greenhouse works and why the global analogue is called the greenhouse effect. The result is a heating of the surface, the sea and the lower atmosphere. In itself, this is no bad thing as without it the Earth would be unable to support much in the way of life - it would be too hot on the day side and too cold on the night side, like Mercury, which has no atmosphere - of course the atmosphere helps protect us from solar radiation, as well us insulate us from the cold of space. So it's due to the greenhouse that the planet is hospitable to complex life in the first place.
Now it's well established that the amout of CO2 in the atmosphere changes over time, and that this has an effect on the climate. There's a very complex feedback system and some its elements are better understood than others. So you've got carbon sinks, i.e. stored masses of carbon that are not free in the atmosphere in the form of CO2 - the one people mostly focus on is the rainforests, because these are the 'lungs of the planet' to coin a cliche, fixing carbon and releasing oxygen. But actually the biggest carbon sink is the ocean, both in the biomass of plant plankton and in the form of CO2 simply dissolved in the water. Warmer water is less good at holding dissolved CO2 than cooler water - cf. opening a tepid can of Coke vs. a can straight from the fridge - so as ocean temperatures rise less CO2 is absorbed and you have a positive feedback effect, accelerating warming. Then there are other gasses which also contribute to the greenhouse effects, such as CH4 (methane) which is much worse than CO2, kg for kg. A major source of this is farting cattle, as you may have heard. Another is organic material undergoing anaerobic decay, which happens when dead plants rot underwater; one source is paddy fields in E/SE Asia and another, annoyingly, is submerged vegetation in lakes created by hydroelectric projects which are generally seen as greener than burning fossil fuels because they don't release CO2. Even water vapour is implicated, because although it can help shield the Earth from solar radiation it also has greenhouse properties (I think), and one effect of warming is increased cloud cover due to faster evaporation from the oceans.
And melting ice dilutes the ocean's salinity, which has knock-on effects for its overall chemistry and dynamics, for example reducing mixing between surface and deep waters which further reduces its carbon capacity. And so on and so on...as you can appreciate that the complexity of the whole phenomenon is, well, phenomenal.
All I'll mention about how much of this is due to natural variation and how much is anthropogenic is to say that rate of change seems to be much faster than at any time we can accurately reconstruct, and has accelerated as global industrial development has accelerated over the past century or so.
Edit: as promised, a few links...
The fact of increasing global average temperatues in the recent past is not, itself, up for debate.
Correlation between atmospheric CO2 content and mean temperature, over the last 400,000 years. I don't think anyone contests the correlation but some sceptics of man-made climate change say that CO2 levels lag behind temperature, implying the opposite causality from that promoted by mainstream climate science. Note the big spike at the far right of the graph (the x-axis is reversed, so '0' is today and age of data increases to the right).
Sunspots are thought to be a good indicator of the Sun's power output, which changes over time. Observations of sunspot numbers indicates that although low solar activity may have been responsible for cool conditions between the end of the Middle Ages and the mid-19th century - the so-called 'little ice age' - the number of sunspots has stabilised since about 1950, the period of fastest warming within reconstructable history. So something other than solar variation must have an important climatic effect.
Methane is about 20 times more effective as a greenhouse gas than CO2, and is produced by all kinds of human activity.
Methane from cattle is often used an argument for eating less meat...
...but it would be rather unreasonable to ask people in south and east Asia to simply 'stop eating so much rice' because of its associated CH4 emissions.
Wikipedia has some nice extensive articles on most of these issues - look up 'carbon sink' especially and the section on the role of seawater in regulating atmospheric CO2.
