About 10 years ago, December 20, 2002 to be exact, we published a paper titled "Revised 21st century temperature projections" in the journal Climate Research. We concluded:
Temperature projections for the 21st century made in the Third Assessment Report (TAR) of the United Nations Intergovernmental Panel on Climate Change (IPCC) indicate a rise of 1.4 to 5.8°C for 1990-2100. However, several independent lines of evidence suggest that the projections at the upper end of this range are not well supported.. The constancy of these somewhat independent results encourages us to conclude that 21st century warming will be modest and near the low end of the IPCC TAR projections.
We examined several different avenues of determining the likely amount of global warming to come over the 21st century. One was an adjustment to climate models based on (then) new research appearing in the peer-reviewed journals that related to the strength of the carbon cycle feedbacks (less than previously determined), the warming effect of black carbon aerosols (greater than previously determined), and the magnitude of the climate sensitivity (lower than previous estimates). Another was an adjustment (downward) to the rate of the future build-up of atmospheric carbon dioxide that was guided by the character of the observed atmospheric CO2 increase (which had flattened out during the previous 25 years). And our third estimate of future warming was the most comprehensive, as it used the observed character of global temperature increase-an integrator of all processes acting upon it-to guide an adjustment to the temperature projections produced by a collection of climate models. All three avenues that we pursued led to somewhat similar estimates for the end-of- the-century temperature rise. Here is how we described our findings in paper's Abstract:
Since the publication of the TAR, several findings have appeared in the scientific literature that challenge many of the assumptions that generated the TAR temperature range. Incorporating new findings on the radiative forcing of black carbon (BC) aerosols, the magnitude of the climate sensitivity, and the strength of the climate/carbon cycle feedbacks into a simple upwelling diffusion/energy balance model similar to the one that was used in the TAR, we find that the range of projected warming for the 1990-2100 period is reduced to 1.1-2.8°C. When we adjust the TAR emissions scenarios to include an atmospheric CO2 pathway that is based upon observed CO2 increases during the past 25 yr, we find a warming range of 1.5-2.6°C prior to the adjustments for the new findings. Factoring in these findings along with the adjusted CO2 pathway reduces the range to 1.0-1.6°C. And thirdly, a simple empirical adjustment to the average of a large family of models, based upon observed changes in temperature, yields a warming range of 1.3-3.0°C, with a central value of 1.9°C.
We thus concluded:
Our adjustments of the projected temperature trends for the 21st century all produce warming trends that cluster in the lower portion of the IPCC TAR range. Together, they result in a range of warming from 1990 to 2100 of 1.0 to 3.0°C, with a central value that averages 1.8°C across our analyses.