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Attribution of Climate Change
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| 9.7 |
There is now stronger evidence for a human
influence on the global climate.
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9.8
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An increasing body of observations
gives a collective picture of a warming world and modeling studies indicate
that most of the observed warming at the Earth's surface over the
last 50 years is likely to have been due to human activities. Globally,
the 1990s were very likely to have been the warmest decade in the instrumental
record (i.e., since the year 1861). For the Northern Hemisphere, the magnitude
of the warming in the last 100 years is likely to be the largest of any
century during the past 1,000 years. Observations, together with model
simulations, provide stronger evidence that most of the warming observed
over the last 50 years is attributable to the increase in greenhouse gas
concentrations. The observations also provide increased confidence in
the ability of models to project future climate change. Better quantification
of the human influence depends on reducing the key uncertainties
relating to the magnitude and character of natural variability
and the magnitude of climate forcings due to natural factors and anthropogenic
aerosols (particularly indirect effects) and the relating of regional
trends to anthropogenic climate change.
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Q2.7 & Q2.10-11 |
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Future Emissions and Concentrations of
Greenhouse Gases and Aerosols
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| 9.9 |
Human activities increase the atmospheric
concentrations of greenhouse gases.
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| 9.10 |
Since the year 1750 (i.e., the beginning
of the Industrial Revolution), the atmospheric concentration of CO2
(the largest contributor to anthropogenic radiative forcing) has increased
by 31% due to human activities, and all SRES scenarios project substantial
increases in the future (Figure 9-1a).
Other greenhouse gases have also increased in concentrations since the year
1750 (e.g., CH4 by 150%, N2O by 17%). The present
CO2 concentration has not been exceeded during the past 420,000
years (the span measurable in ice cores) and likely not during the past
20 million years. The rate of increase is unprecedented relative to any
sustained global changes over at least the last 20,000 years. In projections
of greenhouse gas concentrations based on the set of SRES scenarios (see
Box 3-1), CO2 concentrations
continue to grow to the year 2100. Most SRES scenarios show reductions in
SO2 emissions (precursor for sulfate aerosols) by the year 2100
compared with the year 2000. Some greenhouse gases (e.g., CO2
, N2O, perfluorocarbons) have long lifetimes (a century or more)
for their residence in the atmosphere, while the lifetime of aerosols is
measured in days. Key uncertainties are inherent
in the assumptions that underlie the wide range of future emissions in the
SRES scenarios and therefore the quantification of future concentrations.
These uncertainties relate to population growth, technological progress,
economic growth, and governance structures, which are particularly difficult
to quantify. Further, inadequate emission scenarios have been available
of lower atmosphere ozone and aerosol precursors. Smaller uncertainties
arise from lack of understanding of all the factors inherent in modeling
the carbon cycle and including the effects of climate feedbacks. Accounting
for all these uncertainties leads to a range of CO2 concentrations
in the year 2100 between about 490 and 1,260 ppm (compared to the pre-industrial
concentration of about 280 ppm and of about 368 ppm in the year 2000).
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Q2.4, Q3.3,
Q3.5, & Q5.3 |
| 9.11 |
Fossil-fuel CO2 emissions are
virtually certain to remain the dominant influence on the trends in CO2
concentrations over the 21st century. This is implied by the range
of SRES scenarios in which projected fossil-fuel emissions exceed the foreseeable
biospheric sources and sinks for CO2 . It is estimated that,
even if all the carbon so far released by land-use changes could be restored
to the terrestrial biosphere (e.g., by reforestation), CO2 concentration
would be reduced by 40 to 70 ppm. There are key uncertainties
in the influence of changing land use and biospheric feedbacks
on the uptake, storage, and release of carbon that in turn could influence
CO2 concentrations.
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Q4.11 & Q7.4 |
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Figure 9-1a: Observations of atmospheric CO2
concentration over the years 1000 to 2000 from ice core data supplemented
with data from direct atmospheric measurements over the past few decades.
Over the period 2000 to 2100, projections are shown of CO2 concentrations
based on the six illustrative SRES scenarios and IS92a (for comparison with
the SAR).
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WGI TAR SPM Figures 2a
& 5b |
