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What does the frequency distribution associated with the sea level projections actually mean?

The marine report reports on frequency distributions. These should not be referred to in terms of probability.

In detail

For mean sea-level rise, although the models contain known physical relationships and have been tested against observations during their development, their results are not formally constrained by observations. Therefore, it would not be correct to refer to these in terms of probability. Instead we present frequency distributions based on these current models that can be interpreted with statements such as “50% of the models available project sea level rise to be greater than X cm”

The 5th–95th percentile range is reported on which should not be confused with the 10th and 90th percentiles of probability referred to in the climate projections report. When the 5th–95th percentile range is presented it should be interpreted as 90% of the modelled results lying between these bounds.