[note: The below post has several minor revisions and adjustments to the original post]
I have developed a model of the global temperature anomaly (using the NOAA NCDC time series as the benchmark series) for the 2008 to 2037 period based on three assumptions:
1. That the general temperature trend during this period will be similar to that observed in the 1880 to 1911 and 1943 to 1975 cooling periods, and;
2. That the atmospheric concentration of CO2 will increase linearly to 600 ppm by the end of the 21st century. Using a formula for radiative forcing posted by Gavin Schmidt on realclimate.org and assuming an increase in average global temperatures of 0.3 degrees Celsius for 1 W per square meter, I have calculated the average direct annual anthropogenic effect during the 2008 to 2038 period;
3. That the equilibrium sensitivity (that is, the feedback effect) from an increase in atmospheric CO2 concentration is zero.
Thus, if one assumes that the underlying temperature trend (minus the direct anthropogenic effect) in the 2008 to 2037 period will be the same as that for the 1880 to 1911 and 1943 to 1975 periods, the forecast provides a test of the equilibrium sensitivity from an increase in atmospheric CO2 concentration. Should the actual temperature anomaly trend above the forecast, then we may say that the equlibrium sensitivity is positive and the degree to which the actual temperature is above this level would provide an indication of the equilibrium sensitivity.
The figure below shows the average annual forecasted global temperature anomaly for the 2008 to 2037 period based on the above assumptions:
The temperature anomaly is given in degrees Celsius. The data point for 2007 represents the actual average global temperature anomaly according to NOAA NCDC. With the exception of a spike in the anomaly to a new record high in 2015, the forecast is for average global temperatures to decline by .19 degrees Celsius between 2007 and 2019 before recovering to a new all-time high during the 2020's. The forecast anticipates that between 2o21 and the end of the period in 2037, the global temperature anomaly will fluctuate between 0.60 and 0.75 degrees Celsius.
It is interesting to note that the Keenlyside et. al. paper recently published in Nature suggested that "global surface temperature may not increase over the next decade" before increasing beyond 2018. As a result, the forecast here can be measured against the Keenlyside et. al. general forecast to provide a good test of the actual equilibrium sensitivity level. That is, should the global temperature anomaly remain at or near the 2007 average for the following decade, it would be reasonable to consider that proof of a significantly positive equilibrium sensitivity.
Because the forecast is monthly and because the relevant forecast period actually began in October of 2006, we can compare the forecast thus far with the actual anomaly figures through March of 2008. The figure below makes this comparison:
As in the previous figure, temperatures are in degrees Celsius. The actual average anomaly over the 18 month period in the above figure is .5407 degrees while the forecast average over the same period is .5850 degrees. Since the mean temperature anomaly for the 18 months prior to October 2006 was .5640 degrees, a simple projection based on that average would have performed slightly better than the forecast thus far.
The model's global temperature anomaly forecast for April to December is:
We can use the global temperature forecast for 2008 to 2015 to compare with Tamino's "bet" that I have been highlighting every month in this blog. The table below shows Tamino's annual average threshold below which the "not-warming" side wins with the global temperature anomaly forecast for the 2008 to 2015 period:
The bold highlighted rows show those years in which the forecast average anomaly is below Tamino's threshold in which the "not warming" side "wins" a point in the bet. Since the bet is whether one side or the other wins two points in the 2008 to 2015 period and since the "still warming" side wins a point only if the global temperature anomaly is above .7350 degrees, my forecast is that the "not warming" side will win the "bet" at the end of 2013. Stay tuned!