Predicting future temperature rise has uncertainty because of the uncertainty in future emissions of greenhouse gases. And, even if we knew exactly the future emissions path, the changes in climate due to changes in greenhouse gas concentrations and the rate of ocean uptake of heat would introduce additional uncertainties. However, for the period 1860-2000 we know both the greenhouse gas emissions and the global atmospheric temperature response. By forcing a climate model to have the same reaction (global temperature increase) to greenhouse gas forcing under future emission scenarios as observations have shown in the past 140 years, the range of possible future temperature responses can be more accurately described. Stott et al (2002) use the observed temperature history to constrain the possible outcomes of the HadCM3 model of the Hadley Centre for Climate Prediction and Research for future climate scenarios. They find that if no measures are taken to mitigate climate change, global average temperature rise is insensitive to the differences in the emissions scenarios over the next four decades. They also find that the observations of temperature over the next 20 years will reduce uncertainty in projections of climate change out to 2100.
Reference
Stott, P. A., and J. A. Kettleborough, 2002: Origins and estimates of uncertainty in predictions of twenty-first century temperature rise. Nature, 416, 723-726.