SH,
Thanks, sometimes it is difficult to know how much background to provide, or not.
For instance, at the following website, Aslak Grinsted, points out that in regards to sea level rise section of the Summary for Policy Makers, SPM, of Assessment Report 5, AR5:
"The explanation is that the true uncertainty is considered to be greater than the model spread.
The supplementary material to the sea level chapter says that they follow section 12.4.1.2 when they use 5-95% as the likely range.
[EDIT: I just found an explanation in an SPM footnote: "Calculated from projections as 5−95% model ranges. These ranges are then assessed to be likely ranges after accounting for additional uncertainties or different levels of confidence in models. For projections of global mean sea level rise confidence is medium for both time horizons."]"
http://www.glaciology.net/Home/Miscellaneous-Debris/whatdoeslikelymeanAlso, at the following link, Aslak Grinsted provides the accompanying figure with the following Legend
http://www.glaciology.net/Home/Miscellaneous-Debris/comparisonofsealevelprojectionsLegend:
"• Extrap: constant rate of sea level rise at present day trend from sealevel.colorado.edu. (An absolute lower limit of plausibility IMO)
• FAR: full range of SLR projections from FAR (taken from SAR table 7.
• SAR: full range of SLR projections from SAR (taken from TAR table 11.14). (SARp369: "Excluding the possibility of collapse of the West Antarctic ice sheet").
• TAR: full range of SLR projections from TAR table 11.14. (TAR p.642: "The range of projections given above makes no allowance for icedynamic instability of the WAIS".)
• AR4: SLR projection excluding scaled-up ice sheet discharge. (AR4 WG1 Table 10.7).
• AR4+: SLR projection including scaled-up ice sheet discharge. (AR4 WG1 Table 10.7). Context for "larger values cannot be excluded" can be found in the AR4 SPM.
• SEM: full range of semi-empirical projections in AR5 (from AR5 fig.13.12).
• AR5: "process based" ice sheet projections from AR5 table 13.5. These do not account for a potential collapse of Antarctic marine based sectors which may contribute up to several decimetres (indicated with thin shaded line).
• Ice sheet experts*. refers to Bamber and Aspinall (2013) table S1 5-95% plus non ice sheet contributions from AR5 table 13.5. Note BA13 does not refer to a specific scenario (hence the asterisk)
• SLR experts refers to the expert elicitation of Horton et al. 2013 (table 1). They do not provide RCP45 but only RCP85 and RCP3PD. However both SEMs and AR5 agree that the projection for RCP45 lies at about a third of the way between. So I have used this weighing. "
Furthermore, in the link at the end of this post, Aslak Grinsted goes on to say:
"…. To appreciate why 0.98 m is not an upper limit of SLR then you have to read on and understand the caveats stated in the AR5. The SPM also says:
"
The basis for higher projections of global mean sea level rise in the 21st century has been
considered and it has been concluded that there is currently insufficient evidence to evaluate
the probability of specific levels above the assessed likely range.
"
To parse this you need to understand the IPCC jargon. "Likely" means the 66% confidence interval. I.e. slightly less than a one sigma interval. So, the full uncertainties are at least twice as large but they are unwilling to say by how much exactly. They also say that there is an additional uncertainty that is unlikely to be anything but positive:
"Based on current understanding, only the collapse of marine-based sectors of the Antarctic ice sheet, if initiated, could cause global mean sea level to rise substantially above the likely range during the 21st century. However, there is medium confidence that this additional contribution would not exceed several tenths of a meter of sea level rise during the 21st century. {13.4, 13.5}"
It is unclear what they mean by "several tenths of a meter". I find it remarkable that they could not agree on a more quantitative statement considering they are only stating something with "medium confidence". In any case this excluded potential contribution is clearly positive. This uncertainty strongly affects the upper tail of the uncertainty range. It is effectively a bias. Ice sheet experts appear to judge this collapse scenario quite probable, and post-AR5 modelling indicates that Pine Island Glacier in Antarctica is already engaged in an unstable retreat (Favier et al., 2014).
The literal meaning of the AR5 likely range is that there is 17% chance of exceeding 1m SLR assuming that there is no marine instability (under RCP8.5). If there is an instability then the probability is greater."
http://www.glaciology.net/Home/Miscellaneous-Debris/ar5sealevelriseuncertaintycommunicationfailure