Has climate change already begun ?
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 19th 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 20th 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 0.6oC 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 20th 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 20th 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 20th 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
Overall, there is new and stronger evidence that most of the warming observed over the last 50
years is attributable to human activities.