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IPCC WGI Fourth Assessment Report |
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Written by Joachim Willms [Managing Director]
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[...] INTRODUCTION: The Working Group I contribution to the IPCC Fourth Assessment Report describes progress in understanding of the human and natural drivers of climate change1, observed climate change, climate processes and attribution, and estimates of projected future climate change. It builds upon past IPCC assessments and incorporates new findings from the past six years of research. Scientific progress since the TAR is based upon large amounts of new and more comprehensive data, more sophisticated analyses of data, improvements in understanding of processes and their simulation in models, and more extensive exploration of uncertainty ranges. The basis for substantive paragraphs in this Summary for Policymakers can be found in the chapter sections specified in curly brackets.
HUMAN AND NATURAL DRIVERS OF CLIMATE CHANGE
Changes in the atmospheric abundance of greenhouse gases and aerosols, in solar radiation and in land surface properties alter the energy balance of the climate system. These changes are expressed in terms of radiative forcing, which is used to compare how a range of human and natural factors drive warming or cooling influences on global climate. Since the Third Assessment Report (TAR), new observations and related modelling of greenhouse gases, solar activity, land surface properties and some aspects of aerosols have led to improvements in the quantitative estimates of radiative forcing.
Global atmospheric concentrations of carbon dioxide, methane and
nitrous oxide have increased markedly as a result of human activities
since 1750 and now far exceed pre-industrial values determined from ice
cores spanning many thousands of years (see Figure SPM-1). The global
increases in carbon dioxide concentration are due primarily to fossil
fuel use and land-use change,
while those of methane and nitrous oxide are primarily due to agriculture. {2.3, 6.4, 7.3 [...]
The
understanding of anthropogenic warming and cooling influences on
climate has improved since the Third Assessment Report (TAR), leading
to very high confidence that the globally averaged net effect of human
activities since 1750 has been one of warming, with a radiative forcing
of +1.6 [+0.6 to +2.4] W m-2. (see Figure SPM-2). {2.3. 6.5, 2.9} [...]
Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level (see Figure SPM-3). {3.2, 4.2, 5.5}
• Eleven of the last twelve years (1995 -2006) rank among the 12 warmest years in the instrumental record of global surface temperature9 (since 1850). The updated 100-year linear trend (1906–2005) of 0.74 [0.56 to 0.92]°C is therefore larger than the corresponding trend for 1901-2000 given in the TAR of 0.6 [0.4 to 0.8]°C. The linear warming trend over the last 50 years (0.13 [0.10 to 0.16]°C per decade) is nearly twice that for the last 100 years. The total temperature increase from 1850 – 1899 to 2001 – 2005 is 0.76 [0.57 to 0.95]°C. Urban heat island effects are real but local, and have a negligible influence (less than 0.006°C per decade over land and zero over the oceans) on these values. {3.2}
• New analyses of balloon-borne and satellite measurements of lower- and mid-tropospheric temperature show warming rates that are similar to those of the surface temperature record and are consistent within their respective uncertainties, largely reconciling a discrepancy noted in the TAR. {3.2, 3.4}
• The average atmospheric water vapour content has increased since at least the 1980s over land and ocean as well as in the upper troposphere. The increase is broadly consistent with the extra water vapour that warmer air can hold. {3.4}
• Observations since 1961 show that the average temperature of the global ocean has increased to depths of at least 3000 m and that the ocean has been absorbing more than 80% of the heat added to the climate system. Such warming causes seawater to expand, contributing to sea level rise (Table SPM-0).{5.2, 5.5} [...]
Source: IPCC Fourth Assessment Report |
TF: Hazards & Risks 
