Formally, in the IPCC4 report, there is a range in predicted temperatures (in the year 2100) of 1.5 to 5 degrees C.

5 degrees C (= 9 degrees F) would be relatively catastrophic.

So, what drives this range and how can one estimate what is the most probable correct model?

Qualitatively one can look at the system like this:

There are two principal activities of humans that modify the pathways shown in this diagram:

1. For the incoming end (yellow), industrial processes (and volcanic ones) produce sulfate aerosols. This changes the short wavelength scattering properties of the atmosphere in principle this could lead to global dimming (there is some evidence of this) as less optical sunlight reaches the ground and more is reflected by the atmosphere (so the 6% number increases).
   
   
2. For the outgoing end (red), fossil fuel combustion together with methane release increases the radiation absorbed by atmosphere (15%).

In the climate modeling community, these alterations are called climate forcings and they are expressed in physical flux units of watts per square meter. For example:

Summarizes what we believe the relative forcings are in the year 2000 relative to the year 1750:

1. CO₂ is 1.5; The canonical value associated with a doubling of CO₂ (from 280 to 560 ppm) is 4.0 watts per square meter.
   
   
2. Sum of all the other greenhouse gases shown is (2.54) however, since 2000 the CO₂ forcing term has risen faster than the others.
   
   
3. Black carbon (basically particulate pollution suspended in the atmosphere or falling out at the poles) produces another 0.8 watts per square meter
   
   
4. Sun energy output of the sun has increased slightly. The energy reaching the top of our atmosphere from the sun is about 1300 watts per square meter and over the last 250 years the sun's output has increased buy about 1/4 of a percent or 0.3 watts pe square meter.

Summing all this up leads to a total positive forcing of 4.14 watts per square meter.

To counter that positive forcing:

1. Aerosols combine to contribute -2.1 watts per square meter (about 1/2 of the positive forcing!).
   
   
   Oceanic dimethyl sulfide (DMS) is the major natural source of sulfur to the atmosphere.
   
   
2. Changing land use (greenspace converted to buildings/parking lots) slightly increases the overall reflectivity of the Earth adding another -0.15 watts per square meter.
   
   
3. The bet balance is then essentially 1.95 watts per square meter.

This then leads to a tug of war that represents the energy balance in the atmosphere as modified by human activities:

Where the curves represent the total probability distributions of the positive and negative forcings.

The overall rise of the net forcing is illustrated below. The downward spikes seen since 1960 are the result of known, large-scale volcanic events (El Chicon, Pinatubo) that temporarily greatly increase the sulfate aerosol content of the atmosphere.

But the most important thing to realize is that the potential positive forcings from greenhouse gas build up are pretty similar to the estimate negative forcings that occur during ice ages (due to changing albedo/reflectivity of the planet, and net loss of greenhouse gases).