A review of trend models applied to sea level data with reference to the “acceleration-deceleration debate”

Global sea levels have been rising through the past century and are projected to rise at an accelerated rate throughout the 21^st century. This has motivated a number of authors to search for already existing accelerations in observations, which would be, if present, vital for coastal protection planning purposes. However, no scientific consensus has been reached yet how to separate a possible acceleration from intrinsic climate variability in sea level records. This has led to an intensive debate on its existence and, if absent, also on the general validity of current future projections.

Here we shed light on the controversial discussion from a methodological point of view. We provide a comprehensive review of trend methods used in the community so far. This resulted in an overview of 30 trend methods, each having its individual mathematical formulation and flexibilities. We discuss various pros and cons of these methods and illustrate that varying trend approaches may lead to contradictory acceleration–deceleration inferences.

We argue that model selection techniques and good modeling practices in combination with a comprehensive understanding of sea level dynamics yield a way-out here. In particular, we conclude the following:

• several differently characterized methods should be applied and discussed simultaneously.
• uncertainties provided as confidence bounds, should be taken into account to prevent biased or wrong conclusions.
• internally generated climate variability can be removed by incorporating atmospheric or oceanographic information. This helps to uncover externally forced climate change signals.