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activities were substantially increasing the GHG concentrations, which in turn
would result in an additional global warming of the surface air. Third, some
GHGs are more effective than others in changing the climate; thus, carbon
dioxide (CO2) has been the most responsible (by more than 50%) for increasing
the greenhouse effect in the past, and it will continue to be so in the future. It is
reported that the increasing rate of GHG emissions would increase their
concentrations
for
centuries,
forcing
further
reductions
to
stabilize
concentrations at a certain level. For example, a reduction above 60% of CO2
emission is needed to return to the 1990 emission levels and about 15-20% for
methane. Fourth, under the “Business-as-usual” scenario, air temperatures
could rise by 0.3ºC (0.2 - 0.5ºC) per decade, the highest rate that could occur in
the last 10,000 years. Thus, the global mean air temperature of the planet could
increase by 1ºC by 2025 and 3ºC by the end of the 21st century. For scenarios
where emissions have had some control, projected global warming would
fluctuate between 0.1 and 0.2ºC per decade, depending on the degree of GHG
reduction. On average, an increase in mean sea level of 3 to 10 cm per decade
was estimated, mainly due to the thermal expansion of the ocean and melting of
some continental ice.
The Second Report (IPCC, 1995) indicates that CO2, methane, and nitrous
oxide had increased by 30%, 145%, and 15%, respectively (1992 values) and
that the increase in GHGs is attributable to human activities, and that the 500
ppmv of CO2 would be reached at the end of the 21st century. In addition,
according to carbon cycle models, a stabilization of the CO2 concentration at
450 ppmv could be achieved in 40 years if GHG emission levels were reduced
to at least the values observed in 1990. The global average temperature on the
surface recorded from the end of the 19th century to the beginning of the 1990s
had increased between 0.3 and 0.6ºC. Rainfall showed an increase in land areas
in the high latitudes of the Northern Hemisphere.
Evidence from the observed environmental changes suggested that human
influence was already discernible in climate change.
Climate projection according to the intermediate scenario of future climate
simulation indicated an average global temperature increase of 2ºC for the year
2100, with respect to the year 1990. In the same period, a rise in the mean sea
level of 15 to 95 cm was estimated, depending on the ice melting rate.
The Third Report (IPCC, 2001) resolved that the global average temperature
increase was 0.6 ± 0.2ºC during the 20th century. The mean sea level would
have risen 10 – 20 cm in the same period. Snow cover and sea ice extent have