IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis

2.9.1 Uncertainties in Radiative Forcing

The TAR assessed uncertainties in global mean RF by attaching an error bar to each RF term that was ‘guided by the range of published values and physical understanding’. It also quoted a level of scientific understanding (LOSU) for each RF, which was a subjective judgment of the estimate’s reliability.

The concept of LOSU has been slightly modified based on the IPCC Fourth Assessment Report (AR4) uncertainty guidelines. Error bars now represent the 5 to 95% (90%) confidence range (see Box TS.1). Only ‘well-established’ RFs are quantified. ‘Well established’ implies that there is qualitatively both sufficient evidence and sufficient consensus from published results to estimate a central RF estimate and a range. ‘Evidence’ is assessed by an A to C grade, with an A grade implying strong evidence and C insufficient evidence. Strong evidence implies that observations have verified aspects of the RF mechanism and that there is a sound physical model to explain the RF. ‘Consensus’ is assessed by assigning a number between 1 and 3, where 1 implies a good deal of consensus and 3 insufficient consensus. This ranks the number of studies, how well studies agree on quantifying the RF and especially how well observation-based studies agree with models. The product of ‘Evidence’ and ‘Consensus’ factors give the LOSU rank. These ranks are high, medium, medium-low, low or very low. Ranks of very low are not evaluated. The quoted 90% confidence range of RF quantifies the value uncertainty, as derived from the expert assessment of published values and their ranges. For most RFs, many studies have now been published, which generally makes the sampling of parameter space more complete and the value uncertainty more realistic, compared to the TAR. This is particularly true for both the direct and cloud albedo aerosol RF (see Section 2.4). Table 2.11 summarises the key certainties and uncertainties and indicates the basis for the 90% confidence range estimate. Note that the aerosol terms will have added uncertainties due to the uncertain semi-direct and cloud lifetime effects. These uncertainties in the response to the RF (efficacy) are discussed in Section 2.8.5.

Table 2.11 indicates that there is now stronger evidence for most of the RFs discussed in this chapter. Some effects are not quantified, either because they do not have enough evidence or because their quantification lacks consensus. These include certain mechanisms associated with land use, stratospheric water vapour and cosmic rays. Cloud lifetime and the semi-direct effects are also excluded from this analysis as they are deemed to be part of the climate response (see Section 7.5). The RFs from the LLGHGs have both a high degree of consensus and a very large amount of evidence and, thereby, place understanding of these effects at a considerably higher level than any other effect.

Table 2.11. Uncertainty assessment of forcing agents discussed in this chapter. Evidence for the forcing is given a grade (A to C), with A implying strong evidence and C insufficient evidence. The degree of consensus among forcing estimates is given a 1, 2 or 3 grade, where grade 1 implies a good deal of consensus and grade 3 implies an insufficient consensus. From these two factors, a level of scientific understanding is determined (LOSU). Uncertainties are in approximate order of importance with first-order uncertainties listed first.

 Evidence Consensus LOSU Certainties Uncertainties Basis of RF range  
LLGHGs  A  1  High  Past and present concentrations; spectroscopy  Pre-industrial concentrations of some species; vertical profile in stratosphere; spectroscopic strength of minor gases   Uncertainty assessment of measured trends from different observed data sets and differences between radiative transfer models 
Stratospheric ozone  A  2  Medium  Measured trends and its vertical profile since 1980; cooling of stratosphere; spectroscopy  Changes prior to 1970; trends near tropopause; effect of recent trends  Range of model results weighted to calculations employing trustworthy observed ozone trend data  
Tropospheric ozone  A  2  Medium  Present-day concentration at surface and some knowledge of vertical and spatial structure of concentrations and emissions; spectroscopy  Pre-industrial values and role of changes in lightning; vertical structure of trends near tropopause; aspects of emissions and chemistry  Range of published model results, upper bound increased to account for anthropogenic trend in lightning  
Stratospheric water vapour from CH4  A  3  Low  Global trends since 1990; CH4 contribution to trend; spectroscopy  Global trends prior to 1990; radiative transfer in climate models; CTM models of CH4 oxidation  Range based on uncertainties in CH4 contribution to trend and published RF estimates  
Direct aerosol  A  2 to 3  Medium to Low  Ground-based and satellite observations; some source regions and modelling  Emission sources and their history vertical structure of aerosol, optical properties, mixing and separation from natural background aerosol  Range of published model results with allowances made for comparisons with satellite data  
Cloud albedo effect (all aerosols)  B  3  Low  Observed in case studies – e.g., ship tracks; GCMs model an effect  Lack of direct observational evidence of a global forcing  Range of published model results and published results where models have been constrained by satellite data  
Surface albedo (land use)  A  2 to 3  Medium to Low  Some quantification of deforestation and desertification  Separation of anthropogenic changes from natural  Based on range of published estimates and published uncertainty analyses  
Surface albedo (BC aerosol on snow) B  3  Low  Estimates of BC aerosol on snow; some model studies suggest link  Separation of anthropogenic changes from natural; mixing of snow and BC aerosol; quantification of RF Estimates based on a few published model studies  
Persistent linear Contrails  A  3  Low  Cirrus radiative and microphysical properties; aviation emissions; contrail coverage in certain regions  Global contrail coverage and optical properties  Best estimate based on recent work and range from published model results  

Table 2.11 (continued)

 Evidence Consensus LOSU Certainties Uncertainties Basis of RF range  
Solar irradiance  B  3  Low  Measurements over last 25 years; proxy indicators of solar activity  Relationship between proxy data and total solar irradiance; indirect ozone effects  Range from available reconstructions of solar irradiance and their qualitative assessment 
Volcanic aerosol  A  3  Low  Observed aerosol changes from Mt. Pinatubo and El Chichón; proxy data for past eruptions; radiative effect of volcanic aerosol Stratospheric aerosol concentrations from pre-1980 eruptions; atmospheric feedbacks  Past reconstructions/estimates of explosive volcanoes and observations of Mt. Pinatubo aerosol  
Stratospheric water vapour from causes other than CH4 oxidation  C  3  Very Low  Empirical and simple model studies suggest link; spectroscopy  Other causes of water vapour trends poorly understood  Not given  
Tropospheric water vapour from irrigation C  3  Very Low  Process understood; spectroscopy; some regional information  Global injection poorly quantified  Not given  
Aviation-induced cirrus  C  3  Very Low  Cirrus radiative and microphysical properties; aviation emissions; contrail coverage in certain regions Transformation of contrails to cirrus; aviation’s effect on cirrus clouds  Not given  
Cosmic rays  C  3  Very Low  Some empirical evidence and some observations as well as microphysical models suggest link to clouds General lack/doubt regarding physical mechanism; dependence on correlation studies Not given  
Other surface effects  C  3  Very Low  Some model studies suggest link and some evidence of relevant processes  Quantification of RF and interpretation of results in forcing feedback context difficult  Not given