There is little scope for uncertainty in the predicted initial heating effect of carbon dioxide because this has been determined using highly accurate laboratory measurements. In fact, this direct heating effect is fairly small and, on its own, not large enough to be of serious concern. The uncertainties, and hence scope for debate, centre around the strength of feedback mechanisms.
The Earth’s average annual temperature can be changed by many factors: greenhouse gas levels; solar activity; volcanic eruptions; orbital cycles and many more. However, whatever primary factor causes a change in climate, this alteration will be enhanced or reduced by feedbacks in the climate system. For example, if the Earth warms a little, some ice melts in polar regions and the Earth becomes less reflective. This will enhance the initial warming as a less reflective Earth will absorb more solar heat. This is an example of a positive feedback because it increases the size of the initial effect. There are also negative feedbacks; processes that reduce the impact of an initial change in the climate system.
In KISS, feedback effects are modelled by supplying a number by which predicted temperature changes are multiplied. If the overall effect of all the feedback processes is positive then the multiplier is greater than one whilst, if the overall effect is a negative feedback, the multiplier will be less than one. Climate scientists use a slightly different measure of feedback which they call climate-sensitivity. This is usually given as the amount of warming produced by a doubling of CO2 (denoted T2x). The T2x value corresponding to a chosen feedback multiplier is shown by the KISS model to allow direct comparison. However, I prefer not to use T2x as relating feedback strength to CO2 doubling seems to imply that we’ve already assumed that CO2 is the climate-change culprit (actually, there isn’t really any such circular reasoning but some climate-change skeptics have been unnecessarily confused by this point).
KISS tests the hypothesis, that present-day climate change is driven primarily by changes in CO2, by allowing you to adjust the feedback strength until modelled global temperatures fit the observed temperatures. Once again, if you can get a good fit this supports the hypothesis whilst, if the fit is poor, the hypothesis should be rejected. So, play with the feedback strength to get the best fit you can. Does the climate system appear to have an overall positive feedback or an overall negative feedback? Is the fit close enough to convince you that rising CO2 (plus feedback) might explain the observed rise in temperatures?