Dear Thaleia,
A good question, what is a proper reference case ("null case") to compare your results with. Is zero sea level rise a good null case? Probably not. Is a zero growth for population and for economy and no land use changes a good "null" case for the socio-economic inputs? Probably not. It all depends on your definition of what kind of a case you would like to refer to / compare with. In policy analysis usually the reference case is the one with autonomous developments caused by external changes (external with regard to the model scope). That would mean that there is population growth and there is economic growth and there will be land use changes and there will be sea level changes.
In DIVA the input data for scenario's are highly interdependant. So what we have done is to take the six SRES storylines for future development as the basis. This provides a consistent dataset for population, economic growth and land use changes. On top of this you specify a choice for the sea level scenario. The sea level scenario's are again consistent with the selected SRES scenario as the calculation of the sea level rise (by the CLIMBER model from PIK in Potsdam Germany) uses the SRES storyline data as input. So the scenarios we created are internally consistent.
The easiest for you to do is to select one of SRES storylines as your "reference" and then select a sea level rise scenario (medium uniform?). Then you can simply use the GUI directly.
If you really want to make your own null case, you will have to make your own scenario dataset (i.e population growth data, economic growth data, land use change data, sea level rise data) and then select that in the GUI. In the frequently asked questions there is a section dealing with creating you own scenario, I suggest you study that and if have any questions after that please let me know. See this post
Editing the DCC file manually is not what should be done to prepare a new scenario ;) It is overwritten based on the internal data whenever a calculation is made. By making the selected scenario's blank in the file you create an invalid case specification. There always must be a selected SRES scenario and a selected sea level rise scenario.
Kind regards,
Poul Grashoff