In order to try to predict what the changes above may mean for the future of the climate on Earth, the Intergovernmental Panel on Climate Change (IPCC) has developed a range of scenarios to try to project atmospheric concentrations of greenhouse gases and aerosols and natural fluctuations of natural radiative forcing – from the period 1990 to 2100. These scenarios are based on assumptions about population and economic growth, land use, technological changes, energy availability and fuels likely to be used in this period of time.
Using these scenarios, the following projections about the future global climate have been made:

• For a mid-range emissions scenario (and using a “best –estimate” value for climate sensitivity) – models predict an increase in global mean surface air temperature (relative to 1990) of about 2oC by 2100.
  
• The lowest emission scenario (in combination with a low value for climate sensitivity) predicts an increase of about 1oC by 2100.
  
• The highest emission scenario (combined with a high value for climate sensitivity) would lead to a warming of about 3.5oC.
  
• In all cases the average rate of warming would probably be greater than any seen in the last 10 000 years, but the actual annual to decadal changes would include considerable natural variability. Regional temperature changes could differ substantially from the global mean value.
  
• Because of the thermal inertia of the oceans, only 50 – 90 % of the eventual equilibrium temperature change would have been realized by 2100 and temperature would continue to increase beyond 2100 even if concentrations of greenhouse gases were stabilized by that time.
  
• Average sea level is expected to rise as a result of thermal expansion of the oceans and melting of glaciers and ice sheets. For the IPCC mid-range emissions scenario (and assuming “best estimate” values for climate sensitivity and of ice-melt sensitivity to warming) – models predict an increase in sea level of about 50cm from the present to 2100. The lowest emissions scenario (combined with low climate and ice melt sensitivities) would give a projected sea level rise of 1 cm by 2100. The corresponding projection for the highest emission scenario (combined with high climate and ice melt sensitivities) gives a sea level rise of about 95 cm by 2100.
  
• Sea level would continue to rise at a similar rate in future centuries beyond 2100, even if concentrations of greenhouse gases were stabilized by that time – and would continue to do so even beyond the time of stabilization of global mean temperature. Regional sea changes may differ from the global mean value owing to land movement and ocean current changes.
  
• In general, scientists have more confidence in larger scale hemisphere and continental projections, than in regional projections. Confidence in the latter remains very low. They also have more confidence in temperature projections than in hydrological changes. In terms of the impact on global weather patterns….
  
• All model simulations show the following features: greater surface warming of the land than of the sea in winter; a maximum surface warming in high northern latitudes in winter; little surface warming over the Arctic in summer; an enhanced global mean hydrological cycle; increased precipitation and soil moisture in high altitudes in winter.
  
• Most simulations show a reduction in the strength of the north Atlantic thermohaline circulation and a widespread reduction in diurnal range of temperature.
  
• A general warming is expected to lead to an increase in the occurrence of extremely hot days and a decrease in the occurrence of extremely cold days.
  
• Warmer temperatures will lead to a more vigorous hydrological cycle – which translates into prospects for more severe droughts and/or floods in some places and fewer severe droughts and/or floods in other places. Several models indicate an increase in precipitation intensity and suggest the possibility of more extreme rainfall events. Knowledge is currently insufficient to say whether there will be any changes in the occurrence or geographical distribution of severe storms.
  
  
  NOTE: Aerosols – and their influence on predictions
  All of the model simulations discussed above have included projected changes in future aerosol concentrations in the atmosphere. Both the direct and indirect effects of anthropogenic aerosols have an important effect on the projections the models provide. Generally, the size of temperature and precipitation changes are smaller when aerosol effects are included in the models – especially in northern mid-latitudes. However, the cooling effect of aerosols is not a simple offset to the warming effect of greenhouse gases – but it significantly affects some continental scale patterns of climate change. The spatial and temporal distribution of aerosols greatly influences regional projections, which are therefore also uncertain.
  
  For more information on the information from climate models, see the UNFCCC Climate Change Information Kit Fact Sheet on Evidence from Climate Models