In a new study, MIT climate researchers ran 400 simulations of the highly sophisticated MIT Integrated Global System Model, which is used to make probabilistic projections of climate change from 1861 to 2100. The new projections are considerably higher than 2003 projections derived from the model, with median surface warming in 2091 to 2100 of 5.2 degrees compared to 2.4 degrees C in the earlier study. Among the chances contributing to stronger warming, the researchers concluded, is taking into account the cooling in the second half of the 20th century due to volcanic eruptions and a better method for projecting GDP growth. The study also concluded that that there is a much smaller probability of warming of less than 2.4 degrees C than is implied by the lower bound of the IPCC AR4 projected range for the A1FI (high economic growth/high emissions).
The study, published in the Journal of Climate, is available in advance form at: http://dx.doi.org/10.1175/2009JCLI2863.1