## I am disappointed in how the sea level rise projection uncertainties are presented in the IPCC AR5. The way the numbers are presented makes people believe 98 cm by 2100 is a worst-case scenario which it clearly isn't. The AR5 does have caveats which explains why it could be more, but unfortunately this is buried in language that clearly goes over the heads of most people.[ UPDATE: The AR5 sea level chapter authors have written a letter to Science where they emphasize/clarify that "The upper boundary of the AR5 “likely” range should not be misconstrued as a worst-case upper limit.". This is my point exactly with this page.UPDATE2: We have published an estimate of worst case sea level rise.]This is how it is presented in the summary for policy makers:
The numbers of 0.26 to 0.98 m is what people pick up. 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:
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:
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. ## What is the worst case? 1.7 m?[ Update: We have constructed an estimate of the worst-case SLR.]
## Further information:Here's our estimate of what worst case sea level rise (we try to estimate the full community uncertainty). On this companion page I argue that the AR5 ice sheet projections are both optimistic and over-confident. Go take a look. On this page I compare AR5 with other sea level projections. It shows the evolution of IPCC sea level projections. ## Comments |

Aslak Grinsted > Misc. Debris >