This chapter presents projections of future climate change based on simulations from global climate models (GCMs) and applies to spatial scales of hundreds of kilometers and larger. Chapter 10 provides results from regional models operating on smaller scales. Climate change simulations are assessed for the period 1990 to 2100, based on scenarios for greenhouse gas concentrations and sulphate aerosol loadings. Some AOGCM simulations include effects of ozone and/or indirect aerosol effects. Most simulations do not include less dominant or less well understood forcings like land-use changes, mineral dust, black carbon, etc. No AOGCM simulations include estimates of future changes in solar forcing or volcanic aerosol concentrations. AOGCM projections for future climate are more numerous than in the IPCC Second Assessment Report (SAR). The chapter focuses on IS92a and draft SRES A2 and B2 scenarios. Uncertainty in projections is assessed by comparing model responses. The range and ensemble standard deviation are used to measure uncertainty. Simulations combine a forced climate change component with internally generated natural variability. Ensembles of simulations with the same forcing but different initial conditions are averaged to preserve the forced climate change signal and reduce natural variability noise. For the IS92a scenario, the mean global average surface air temperature (SAT) change from 1961-1990 to mid-21st century is +1.3°C with a range of +0.8 to +1.7°C for greenhouse gas plus sulphates, and +1.6°C with a range of +1.0 to +2.1°C for greenhouse gas only. For SRES A2, the mean is +1.1°C with a range of +0.5 to +1.4°C, and for B2, the mean is +1.2°C with a range of +0.5 to +1.7°C. For the end of the 21st century, the mean SAT change for A2 is +3.0°C with a range of +1.3 to +4.5°C, and for B2, +2.2°C with a range of +0.9 to +3.4°C. A simple climate model is used for projections of climate change for the next century, tuned to simulate responses from AOGCMs. The forcings for the simple model are based on radiative forcing estimates from Chapter 6. The SAR gives a range of +1 to +3.5°C for global mean temperature change for 2100 relative to 1990. The estimated range for the six final illustrative SRES scenarios using updated methods is +1.4 to +5.6°C. The range for the full set of SRES scenarios is +1.4 to +5.8°C. These estimates are larger than in the SAR, partly due to increasesThis chapter presents projections of future climate change based on simulations from global climate models (GCMs) and applies to spatial scales of hundreds of kilometers and larger. Chapter 10 provides results from regional models operating on smaller scales. Climate change simulations are assessed for the period 1990 to 2100, based on scenarios for greenhouse gas concentrations and sulphate aerosol loadings. Some AOGCM simulations include effects of ozone and/or indirect aerosol effects. Most simulations do not include less dominant or less well understood forcings like land-use changes, mineral dust, black carbon, etc. No AOGCM simulations include estimates of future changes in solar forcing or volcanic aerosol concentrations. AOGCM projections for future climate are more numerous than in the IPCC Second Assessment Report (SAR). The chapter focuses on IS92a and draft SRES A2 and B2 scenarios. Uncertainty in projections is assessed by comparing model responses. The range and ensemble standard deviation are used to measure uncertainty. Simulations combine a forced climate change component with internally generated natural variability. Ensembles of simulations with the same forcing but different initial conditions are averaged to preserve the forced climate change signal and reduce natural variability noise. For the IS92a scenario, the mean global average surface air temperature (SAT) change from 1961-1990 to mid-21st century is +1.3°C with a range of +0.8 to +1.7°C for greenhouse gas plus sulphates, and +1.6°C with a range of +1.0 to +2.1°C for greenhouse gas only. For SRES A2, the mean is +1.1°C with a range of +0.5 to +1.4°C, and for B2, the mean is +1.2°C with a range of +0.5 to +1.7°C. For the end of the 21st century, the mean SAT change for A2 is +3.0°C with a range of +1.3 to +4.5°C, and for B2, +2.2°C with a range of +0.9 to +3.4°C. A simple climate model is used for projections of climate change for the next century, tuned to simulate responses from AOGCMs. The forcings for the simple model are based on radiative forcing estimates from Chapter 6. The SAR gives a range of +1 to +3.5°C for global mean temperature change for 2100 relative to 1990. The estimated range for the six final illustrative SRES scenarios using updated methods is +1.4 to +5.6°C. The range for the full set of SRES scenarios is +1.4 to +5.8°C. These estimates are larger than in the SAR, partly due to increases