Synthesis Report - Question 9

Climate Change 2001: Synthesis Report

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Attribution of Climate Change


There is now stronger evidence for a human influence on the global climate.


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.

Q2.7 & Q2.10-11
  Future Emissions and Concentrations of Greenhouse Gases and Aerosols

9.9 Human activities increase the atmospheric concentrations of greenhouse gases.

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).

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.

Q4.11 & Q7.4
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).
WGI TAR SPM Figures 2a & 5b

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