My short answer: Make your guess.
Climate change forcing elements — which are measured in Watts per square meter — are the first step in creating a model. Forcing is estimated for each greenhouse gas, clouds, land use, etc. and negative forcing from particulates and radiation to space. The next step is to analyze the forcing from feedbacks: for example, how much will clouds change, how much will the ocean change, how much additional water vapor will be evaporated.
Once the forcing is estimated, one must apply a climate sensitivity to convert Watts per square meter to temperature degrees. The amount that temperature will change, the climate sensitivity, usually is estimated from the past decades, but sometimes from the distant past based upon paleontological studies. A wide range of values has been proposed.
All of the forcing factors are plugged into climate general circulation models with often millions of lines of code. There are at least 20 general circulation models1. In principle the consistency of the model can be tested by running it backwards to 1850. However, with thousands of variables, thousands of possible models can duplicate this history. Despite each model groups best efforts, the range of outcomes between models is large. Can the creators of these models now understand what is in them? Are there programing errors? Are there biases? Since 1º C is 1/3 of a percent change in absolute temperature and all the input data has error ranges, can any of the data be accurate enough to make a model prediction within one degree?
Climate warming predictions traditionally are for the time of CO2 doubling from the preindustrial level. As discussed on the page about CO2, that presently appears to be about 1970 or 1980. A few years ago different models predicted a warming of 1.5C to 4.5C. Now, with new science, their prediction for average world temperature is a degree higher, 2.5ºC (4.5º F) up to 5.6ºC (10º F)2. (These temperatures do not include the urban heat island effect.) How much warmer will the world temperature be in fifty years? What will be the effects?
Your guess will be as good as mine!