It will be a simple forecast in the sense that we will only consider CO2 trends. While I would caution this is an important limitation of the forecast, I would also note the hindcast had the same exact limitation. Of course, it's quite possible that in analyses of historic data, CO2 acts as a proxy of other anthropogenic forcings. The behavior of this confounding in the past may differ from its future behavior.
That said, the part of the forecast that I really can't be very confident about has to do with projecting future CO2 atmospheric concentrations. This basically amounts to attempting to predict human behavior and world-wide policy decisions. What I will do is to simply define 2 scenarios based on the Mauna Loa data, as follows.
- Scenario A: A second-order polynomial forecast of CO2 concentrations.
- Scenario B: A third-order polynomial forecast of CO2 concentrations.
Each scenario is illustrated in the following graph.
If it is true that peak oil is either looming or behind us, I would say Scenario B is considerably more likely.
To get "high" and "low" estimates I was initially planning to use the 95% confidence interval of the rate of temperature change formula. This range produces forecasts that are very similar. So instead what I did is produce new formulas for sensitivities of 3.0C (low) and 4.0C (high). For additional details on how the forecast is done, see the hindcast post.
The resulting forecasts of each scenario are illustrated in the following graphs.
Again, I consider scenario B to be more probable. We'll see how they do. Under either scenario it would seem that a global temperature anomaly of 1 degree Celsius by the early 2020s is a done deal. The model also tells us that it takes about 10 years for temperatures to level off after CO2 concentrations do. Under scenario B, we are apparently at a peak in the rate of temperature change – roughly 2C/century. This rate will begin to drop. It will be 1.5C/century by 2035.