• Global mean surface temperature has increased by between about 0.3 and 0.6°C since the late 19th century, a change that is unlikely to be entirely natural in origin.
• Global sea level has risen by between 10 and 25 cm over the past 100 years and much of the rise may be related to the increase in global mean temperature.

The balance of evidence, from changes in global mean surface air temperature and from changes in geographical, seasonal and vertical patterns of atmospheric temperature, suggests a discernible human influence on global climate. There are uncertainties in key factors, including the magnitude and patterns of long-term natural variability.

Future projections concerning climate evolution are based on computer simulations. Confidence is higher in the hemispheric-to-continental scale projections of the models than in the regional projections, where confidence remains low. There is also more confidence in temperature projections than hydrological changes. The models project

- an increase in global mean surface temperature relative to 1990 of between 1 and 3.5°C (with a "best estimate" of 2°C

depending on the development of future emission and on how sensitive the temperature responds to the CO2 increase. 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.

• Average sea level is expected to rise between 15 and 95 cm (with the best estimate of 50 cm) as a result of thermal expansion of the oceans and melting of glaciers and ice-sheets.

All model simulations show the following features which are associated with identifiable physical mechanisms:

• 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 hydrological cycle, and increased precipitation and soil moisture in high latitudes in winter.

When rapidly forced, non-linear systems like the climate system are especially subject to unexpected behaviour. Examples of such non-linear behaviour include rapid circulation changes in the North Atlantic and feedbacks associated with terrestrial ecosystem changes.