Discussion of Boretti, A., 'Is there any support in the long term tide gauge data to the claims that parts of Sydney will be swamped by rising sea levels?'

Boretti (2012) claims that sea-level records show insufficient acceleration to support the projections of sea-level rise that are used worldwide for planning and policy-making. Unfortunately, his claim is based more on flawed qualitative reasoning than on quantitative analysis. We replicate Boretti's methodology of fitting quadratic functions to tide-gauge observations from Fremantle and Sydney, in order to estimate the sea-level acceleration. However, we also evaluate the uncertainty in these estimates (a crucial step, omitted by Boretti), and thereby show that the observed accelerations are statistically consistent with the projections of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Our finding is the same when we repeat this analysis using two data sets which have smaller uncertainties, one from satellite altimeters and the other from a sea-level reconstruction. We therefore conclude that Boretti's claim is without foundation.