2008-08-07 News

Estimation of future increase of sea-level more complex than expected - Results of a simulation study with the climate model ECHO-G

The calculation of scenarios for the development of possible future change of sea-level is an important mission of climate models. Therefore it is important to find out which parameters or processes influence the change of sea-level. In the past it has been accepted that simple, empiric derivable contexts make it possible to statistically estimate change of sea-level from the global mean air temperature. In a simulation study with the climate model ECHO-G it was recently tested to what extent four simple estimation methods based on the global mean temperature could describe the simulated changes of sea-level during of the past 1000 years.

In the article: Hans von Storch, Eduardo Zorita and Jesús F. González-Rouco “Relationship between global mean sea-level and global mean temperature in an climate simulation of the past millennium” published in Ocean Dynamics it has been found that a simple linear relationship between mean temperature and the rate of change of sea-level does not exist. The physic can not be reduced in this simple way, even if only the thermal expansion of the water column is considered.

Proportionality between the rate of change sea-level and air temperature

With this study the millennial simulation with ECHO-G has turned out to be, again, a very useful tool to test the physical plausibility of simple ad hoc methods. The first case was related to the air temperature during the last 1000 years, which was . It was widely, but prematurely, represented in the media as the “hockey stick curve”. Also the present analyses revealed that too simplifying assumptions had been made and that reality is probably much more complex. Similar analyses, for instance regarding coastal upwelling, are currently being carried out. This work at the Institute for Coastal Research shows again that outstandingly statistical analysis and good modelling practice go hand in hand in Geestacht. It shows that science is thoroughly able to test the validity of premature scientidic claims and to falsify them if necessary, even when they were presented in the journal “Science” with big media coverage.

As an example the figure shows the proportionality between the rate of change sea-level and air temperature derived from gliding 120- year time windows (only data from about 120 years are available to derive this proportionality from real data). It shows that depending on the period, this proportionality may be positive, but sometimes also negative. A direct physical relationship between these two quantities (rate of change of sea-level and yearly averaged air temperature) cannot be made.

Contact

Prof. Dr. Hans von Storch
Max-Planck-Str. 1
21502 Geesthacht
Germany
phone: +49 (0) 4152-87-1831
e-mail: hvonstorch[at]web.de

Abstract

The possibility of using global mean near-surface temperature, its rate of change or the global mean ocean heat-flux as predictors to statistically estimate the change of global mean sea-level is explored in the context of a long climate simulation of the past millennium with the climate model ECHO-G. Such relationships have recently been proposed to by-pass the difficulty of estimating future sea-level changes based on simulations with coarse-resolution climate models. It is found that, in this simulation, a simple linear relationship between mean temperature and the rate of change of sea level does not exist. A regression parameter linking both variables, and estimated in sliding 120-year windows, varies widely along the simulation and, in some periods, even attains negative values. The ocean heat-flux and the rate-of-change of mean temperature seem to better capture the rate-of-change of sea level due to thermal expansion.

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2008-08-07 News

Estimation of future increase of sea-level more complex than expected - Results of a simulation study with the climate model ECHO-G

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