[Vision2020] Goblal Warming: Evidence

[Ted Moffett]

All:

Has climate change already begun and is it due to human activity?

The forwarded info below on global warming came from this link:

http://unfccc.int/resource/iuckit/fact06.html

A wealth of other info on climate and global warming can be found at this 
link:

http://unfccc.int/resource/iuckit/index.html

Climate Change Information Sheet 6
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Has climate change already begun ?
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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), 
it 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, the 
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 variability

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



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© UNFCCC

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