Let me start by explaining what radiative forcing actually is and why it is the subject of so much debate, for those not familiar with the term. Radiative forcing is basically the change in radiation (including visible light) which the earth receives as a result of whatever gas we are talking about.
Another important term here is net irradiance which is the difference between the amount of light (I will just refer to all forms of electromagnetic radiation as light from now on, just to keep it simple) entering the earth, and the amount leaving it. Ignoring things like street lights, fires and cities (which are minimal compared to the amount of light hitting the earth), we can basically consider this net irradiance term to mean the amount of light which the earth is absorbing from the sun.
Put simply, radiative forcing is the change in Net Irradiance.
The Intergovernmental Panel on Climate Change created its own, more specific, definition of the term:
Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. In this report radiative forcing values are for changes relative to preindustrial conditions defined at 1750 and are expressed in watts per square metre (W/m2).
Carbon dioxide (CO2) has an effect on the atmosphere's ability to absorb light, because it is able to absorb radiation at frequencies not already absorbed by other gases in the atmosphere. It was actually only discovered during the 1950s, that CO2 was able to absorb frequencies not already absorbed by water. Let's take a look at the absorption bands of the most prominent gases in our atmosphere:
Absorption bands of the major gases in the Earth's atmosphere (source: Climate Change Science)
CO2 absorbs light at 2.5,4 and 20 micrometer wavelengths, giving rise to its label as a greenhouse gas. The gas in our atmosphere able to absorb the largest frequency ranges is water vapour. It should noted, that this also explains the phenomenon that more humid regions are less affected by recent temperature changes, as CO2 plays a smaller role. Cooler and drier regions have seen larger temperature increases in recent decades, which can be attributed to the larger role that CO2 plays in these areas (REFERENCE).
As a material, or gas, becomes denser, it absorbs more an more light. For example, a piece of glass 1cm thick does not appear to absorb anything, but if you look through it sideways (through the edges) it becomes very difficult to see the other side. A similar effect occurs with fluids, including CO2. However, unlike some substances, CO2 never actually experiences this saturation and continues to absorb more and more light as it gets denser and thicker. The change in the amount of light which CO2 absorbs (ie. the radiative forcing) can be represented empirically by the following equation:
source:Myhre et. al 1998
By setting the reference CO2 concentration value (C0) to 250ppmv, we can plot the resulting radiative forcing (change in net irradiation) resulting from the respective additions of CO2, shown below up to 1000ppmv:
Radiative forcing from additions of CO2 to the atmosphere
This graphical representation makes it much easier to see that the amount of potential warming which additional CO2 is by no means limited by the approach of a saturation point. This should put an end to the CO2 radiative forcing is saturated debates floating around of the global climate change forums, as it shows that radiative forcing is likely to continue to increase significantly over the next few decades.
For a more comprehensive discussions on CO2 concentrations and global temperatures, please see my blog on the relationship between CO2 and global temperature.