Long term response of the climate system to anthropogenic forcing was investigated with an Earth System Model of intermediate complexity (MIT IGSM). An ensemble of four hundred simulations was carried out for the period 1860-2005 using versions of the model with different values of climate sensitivity, the aerosol forcing, and the strength of ocean heat uptake using historical forcing. Then, a 400-member ensemble was carried out for each of four different RCP scenarios from the year 2006 to the year 3000. By the last decade of the 21st century, the ensemble mean of surface air temperature increases, relative to 1986-2005 period, by 0.88, 2.16, 2.68 and 4.0oC for RCP26, RCP4.5, RCP6.0 and RCP8.5 respectively. In spite of the forcing being fixed beyond year 2150 for RCP4.5 and RCP6.0 and beyond 2250 for RCP8.5, surface air temperature keeps rising until the end of 30th century under these scenarios. The upper bound of the 90% probability interval increases significantly more than the median. For the RCP4.5 scenario, the median value of possible SAT change increases by 1.07oC from the last decade of the 21st century to the end of 30th century, while the value of the 95th percentile increases by 2.88 oC. Corresponding numbers for RCP6.0 and RCP8.5 are 2.21 and 8.53oC for the medians and 5.36 and 14.65oC for the 95th percentiles, respectively. Such changes in the shape of probability distributions with time, indicate an increase in the probability that surface warming will exceed a given value. For example, the probability of exceeding 3oC warming under the RCP4.5 scenario increases from 4% at the end of 21st century to 37% and 57% at the end of 23d and 30th centuries, respectively. For the RCP2.6 scenario, in which radiative forcing peaks in the year 2070 before decreasing back to the 1990s level by the year 2300, the ensemble mean surface air temperature is still about 0.5oC above present at the end of the simulation.