Different climate centers and research groups have been using computer climate models of various complexities to project the global climate in the coming centuries. The projections are based on different future greenhouse gas emission scenarios which depend on global scenarios of economic and social development.

It is important to note that climate projection is very different from weather or seasonal forecasts. It aims at describing the plausible change in the future climate from a long term perspective rather than depicting the "day to day" or "year to year" variations in weather. Moreover, climate models are always subject to uncertainty in the future greenhouse gas and aerosol emission scenarios and limitations on our knowledge of how the climate system works.

Different climate models can give different projections. The global climate projections reported in the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) are summarized below. Further details of the projections are available from the IPCC AR4 published in 2007.

In order to make a projection of the future climate, it is necessary to develop scenarios regarding the emission of greenhouse gases into the atmosphere. Future greenhouse gas emissions are dependent on many factors such as population growth, socio-economical development, technological advancement, etc. In IPCC AR4, six emission scenarios for greenhouse gases are employed. The six scenarios in order of descending greenhouse gas emission are: A1FI, A2, A1B, B2, A1T and B1. Detailed descriptions of these emission scenarios are available at the IPCC website: http://www.ipcc.ch/pdf/special-reports/spm/sres-en.pdf.

According to the IPCC AR4, relative to the 1980-1999 average temperature, the likely range of the projected global average surface warming for the decade 2090-2099 given by multiple model simulations is between 1.1蚓 (low emission scenario) to 6.4蚓 (high emission scenario).

Solid lines are multi-model global averages of surface warming (relative to 1980-1999) for the scenarios A2, A1B and B1, shown as continuations of the 20th century simulations. Shading denotes the ±1 standard deviation range of individual model annual averages. The orange line is for the experiment where concentrations were held constant at year 2000 values. The grey bars at right indicate the best estimate (solid line within each bar) and the likely range assessed for the six emission scenarios.
(Source: IPCC AR4 WG I)

According to IPCC AR4, multiple model projections suggest an increase in global average annual precipitation during the 21st century. However, changes in precipitation will vary from region to region. Increases in the amount of precipitation are very likely in high latitudes, while decreases are likely in most subtropical land regions.

Relative changes in precipitation (in percent) for the period 2090-2099, relative to 1980-1999. Values are multi-model averages based on the A1B emission scenario for December to February (left) and June to August (right). White areas are where less than 66% of the models agree in the sign of the change and stippled areas are where more than 90% of the models agree in the sign of the change.
(Source: IPCC AR4 WG I)

IPCC AR4 estimates that, under a range of emission scenarios, the global mean sea level rise relative to the average from 1980 to 1999 will range from 0.18 m to 0.59 m in the late 21st century. However, IPCC AR4 cautions that the projections have not captured the full effects of changes in ice sheet flow in Greenland and Antarctic.