The earth's climate is already adjusting to past greenhouse gas emissions. The climate system must adjust to changing greenhouse gas concentrations in order to keep the global energy budget balanced. This means that the climate is changing and will continue to change as long as greenhouse gas levels keep rising. Scientists are now convinced that a growing body of evidence gives a collective picture of a warming world and other changes in the climate system.

Measurement records indicate a warming an increase of 0.6±0.2°C in global average temperature since the late 19^th century. These observation are in line with model projections of the size of warming to date, particularly when the cooling effect of sulphur emissions aerosols is included. Most of the warming occurred from 1910 to 1940 and from 1976 to the present. In the Northern Hemisphere (where there are sufficient data to make such analyses), iIt is likely that the rate and duration of 20^th century warming has been greater than any other time during the last 1,000 years. In addition, tThe 1990s are likely to have been the warmest decade of the millennium in the Northern Hemisphere, and 1998 is likely to have been the warmest year.

Mean sea level has risen by 10 to 20 cm. As the upper layers of the oceans warm, water expands and sea level rises. Models suggest that a 0.6^oC warming should indeed result in the sea-level rise to date. But other, harder-to-predict, changes also affect the real and apparent sea level, notably snowfall and ice-melt in Greenland and Antarctica and the slow "rebound" of northern continents freed from the weight of ice age glaciers.

Snow cover has declined by some 10% since the late 1960s in the mid- and high latitudes of the Northern Hemisphere. It is also very likely that the annual duration of lake and river ice cover has shortened by about two weeks over the course of the 20^th century. Almost all recorded mountain glaciers in non-polar regions have retreated during this time as well. In recent decades, the extent of Arctic sea-ice in the spring and summer has decreased by about 10 – 15%, and the ice Arctic sea-ice has likely thinned by 40% during late summer and early autumn.

There is more precipitation in many regions of the world. An increase of 0.5 – 1% per decade has been measured over most mid- and high latitude areas of the Northern Hemisphere continents, accompanied by a 2% expansion in cloud cover. Precipitation over the tropical land areas (10°N – 10°S) seems to have increased by 0.2 – 0.3% per decade. On the other hand, declines have been observed over Northern Hemisphere sub-tropical land areas (10 –30°N) during the 20^th century, by of about 0.3% per decade. In parts of Africa and Asia the frequency and intensity of droughts seem to have worsened.

The way climate has changed over the 20^th century is consistent with what we would expect as a result of increases in greenhouse gases and aerosols. Observed spatial patterns of global warming is are consistent with model predictions. For example, surface, balloon and satellite measurements show that while the earth’s surface has been warming, the stratosphere has cooled. In addition, the earth is warming more slowly over the oceans than over the land, particular in those ocean regions where surface water mixes down, distributing any warming to the ocean depths. Yet another example is reduced warming in areas affected by sulphate aerosols. Together, this evidence suggests that recent climate changes are unlikely to be entirely due to known sources of natural variability

Overall, there is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.