Causes of
change
Carbon cycle
121
Plant growth
and
Soil-atmosphere
decomposition
exchange
0.5
60
Biosphere 60
540 to 610
Fires
Changes
in soil use
1.5
There is no longer much doubt that most of the ob-

served increase in global average temperatures since
the mid-20th century is due to the observed increase
Soil and organic matter
1 580
in greenhouse gases (GHGs) emanating from human
activities. It is likely that there has been significant
human-caused warming over the past 50 years, av-
eraged over each continent except Antarctica. Over
that period the combined effect of natural variations
in solar radiation and volcanic eruptions would have
produced cooler temperatures, not warmer. There has
in fact been a cooling influence on the atmosphere,
caused by aerosols, some of them caused naturally,
for instance by volcanic eruptions, and some by hu-
Sources: Philippe Rekacewicz and Emmanuelle Bournay, GRID-Arendal,
man activities, principally emissions of sulphate, or- 2005, based on and updated with: Centre for Climatic Research, Institute
ganic and black carbon, nitrate and dust. These aero-
for Environmental Studies, University of Wisconsin at Madison (USA);
Okanagan University College (Canada), Geography Department; World
sols reflect some of the Sun’s rays back to space or
Watch, November-December 1998; Nature; Intergovernmental Panel on
Climate Change, 2001 and 2007.
absorb some of them, in either case preventing them
from reaching the surface of the Earth.
14 Planet in Peril
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