  Observations are increasingly used to detect critical slowing down (CSD) in
potentially multistable components of the Earth system in order to warn of
forthcoming critical transitions in these components. In addition, it has been
suggested to use the statistical changes in these historical observations to
extrapolate into the future and predict the tipping time. Here we argue that
this extrapolation is too sensitive to uncertainties to give robust results. In
particular, we raise concerns regarding (1) the modelling assumptions
underlying the approaches to extrapolate results obtained from analyzing
historical data into the future, (2) the representativeness of individual time
series representing the variability of the respective Earth system components,
and (3) the effect of uncertainties and preprocessing of the employed
observational datasets, with focus on non-stationary observational coverage and
the way gaps are filled. We explore these uncertainties both qualitatively and
quantitatively for the Atlantic Meridional Overturning Circulation (AMOC). We
argue that even under the assumption that these natural systems have a tipping
point that they are getting closer to, the different uncertainties are too
large to be able to estimate the time of tipping based on extrapolation from
historical data.
