• The carbon dioxide greenhouse effect provides a scientifically valid mechanism. CO2 produces a warming effect not by soaking up the radiation in its immediate vicinity, like a sponge, but by raising the effective radiating level of the atmosphere to a colder layer.

• The atmosphere doesn't act as a single unit, but is made up of hundred of different layers. Some of the energy radiated from Earth is stopped in each of these layers. The energy is then re-radiated in a random direction, but on average the energy is moving either "up" or "down." The atmosphere gets thinner as altitude increases, so eventually the radiation will reach a layer high enough and thin enough to escape to space.

• If you increase the concentrations of a greenhouse gas in the atmosphere, the thin upper layers will become more opaque and thus absorb more of the IR; therefore the place where the majority of the energy finally escapes moves to a higher level. These higher levels are much colder (until you reach the stratosphere, but as most infrared photons escaping to space are emitted by the troposphere, this can be ignored), and so they do not radiate heat very well. Thus the rate that radiation escapes to space is lower, and the planet will take in more than it radiates. As the higher levels emit some of the excess downwards, the lower levels will warm all the way down to the surface.

• The imbalance will remain until the higher levels get hot enough to radiate as much energy back out as the planet is receiving.

• 19th century physicist John Tyndall described the effect neatly like so:

"As a dam built across a river causes a local deepening of the stream, so our atmosphere, thrown as a barrier across the terrestrial rays, produces a local heightening of the temperature at Earth's surface."

• The Earth's climate is a complex and dynamic system that is subject to a multitude of different forcings on many different timescales. The fact that the climate can change and has changed independently of carbon dioxide does not mean that carbon dioxide cannot change the climate. It simply means that some other forcing, be it an increase in solar output, a change in the planet's albedo, etc., has overwhelmed the forcing signal from carbon dioxide.

• In 1991, for example, after the eruption of Mount Pinatubo, there was a sharp drop in global temperature following the massive outpouring of ash from the volcano. This sudden increase in air born particulate matter briefly overwhelmed many other forcing factors and drove the temperature down by several degrees until the ash settled out of the atmosphere. This is just one of many examples of a short term forcing managing to overwhelm a longer term forcing, such as an enhanced greenhouse effect.

There are probably other factors which are contributing to the warming effect, both natural and man-made. However, calculations also show that the rise in temperature can be easily explained by the increased levels of CO2 in the atmosphere. Until another compelling explanation is put forward, this would seem to tbe the most likely cause.

Also, just because something else could be causing global warming doesn't mean that it is.

• All of the energy flowing from Earth to space is necessarily in the form of electromagnetic radiation. Earth is floating in a vacuum, making it physically impossible for it to lose energy through conduction or convection. So while much of the energy moving across the planet's surface is not in the form of radiation, it is the only form of energy important in determining the planet's temperature–the planet's temperature being determined by the ratio of the flux of outgoing terrestrial radiation to the flux of incoming solar radiation.

• Also, carbon dioxide is an effective absorber in the infrared spectrum. Due to its temperature, Earth radiates energy primarily in the infrared spectrum. So while carbon dioxide may fill a small absorptive window, it fills a very important one.

Oceans do absorb atmospheric CO2. And under normal conditions the oceans and the biosphere will generally keep the amount of CO2 in the atmosphere constant. However, human activities are producing CO2 at a much higher rate than the planet's oceans and biosphere are able to absorb it. This imbalance is responsible for virtually the whole of the 20th century rise in CO2.

• Surface monitoring stations are not the only evidence that the planet is warming. satellite data, radiosondes, borehole analysis, glacial melt observations, sea ice melt, sea level rise, proxy reconstructions, and permafrost melt all conclusively show that the planet has undergone a pronounced and rapid warming over the past century and a half. So even if the entire surface temperature record were contaminated beyond use, there would still be no doubt whatsoever that the planet is warming.

• Aside from this, it has been demonstrated in several independent studies (Peterson, 2003) that the urban heat island effect does not significantly affect the surface temperature record. That is, the 20th century warming trend is not simply an artefact of urban development.

• It is also important to note that the warming trend is not an output of a computer model, but a conclusion based upon observing real-world evidence such as that listed above.