Time series of two observational data sets of sea surface temperature anomalies (HadSST2, without correction and HadSST3, with correction) as well as reconstructed temperatures from corals in the Indian Ocean. From Pfeiffer et al., 2017).

Time series of two observational data sets of sea surface temperature anomalies (HadSST2, without correction and HadSST3, with correction) as well as reconstructed temperatures from corals in the Indian Ocean. From Pfeiffer et al., 2017).

Sampling of a coral. Photo: C. Dullo, GEOMAR.

Sampling of a coral. Photo: C. Dullo, GEOMAR.

05.12.2017

Corals reveal changes in the measurement technique

Reconstructed temperatures provide information about systematic errors in ship measurements

01.12.2017 / Aachen / Kiel. The western Indian Ocean has warmed faster than any other tropical ocean during the 20th century, making the largest contribution to the increase in global mean sea surface temperature (SST). The reconstruction of this warming is largely based on measurement data routinely determined on ship tracks. Independent coral temperature reconstructions show differences, especially during World War II, which can be attributed to a change in the measurement method for water temperature measurement. The study has now been published by an international team of scientists with substantial contribution by GEOMAR Helmholtz Centre for Ocean Research Kiel in the scientific journal Scientific Reports.

The standard method for measuring water temperatures until the Second World War was a water-drawing thermometer, in German called “Pütz”. The device was lowered overboard a ship to measure water temperature. In the 1940's the procedure changed. The measurement now takes place at the filler neck of the cooling water for the engine of the ship. Such changes can result in systematic errors in the measurements and generally provide lower values. It is striking that especially in the Indian Ocean during World War II cooler sea surface temperatures were measured.

“We have studied five different coral samples from the western Indian Ocean”, explained Dr. Miriam Pfeiffer, first author of the study from RWTH Aachen University and former PhD student at GEOMAR. “We were able to reconstruct temperatures until the middle of the 19th century”, Pfeiffer continued. These reconstructions are very accurate and show a very similar temperature behaviour which deviates significantly from instrumental datasets, especially during World War II. “This deviation was already known and is causally related to the change in the observation method”, said Prof. Dr. Christian Dullo, co-author of the study from GEOMAR. There exist already data set where this systematic error has been corrected, Professor Dullo explained. This is true for the HadSST3 observation dataset compiled by the British Hadley Centre, which is in very high agreement with the reconstructed data. According to Prof. Dullo, the quality of this data set is very good, especially in the period review in this study. Thus, this observation data set is also a highly recommended reference for testing and comparing climate models.

The reconstructions from so-called proxy data have now reached such a high quality that it is possible to identify and correct such artifacts in the observation data. “Here, observations, reconstructions and modelling interact closely with the goal of providing even better and more reliable information on natural climate variability and future predictions”, Prof. Dullo concluded.

Scientific Paper:

Pfeiffer, M., J. Zinke, W.-C. Dullo, D. Garbe-Schönberg, M. Latif, M. E. Weber, 2017: Indian Ocean corals reveal crucial role of World War II bias for twentieth century warming estimates. Scientific Reports, doi: 10.1038/s41598-017-14352-6

Contact:

Dr. Andreas Villwock (GEOMAR, Communication & Media), Phone: +49-431-600-2802, presse(at)geomar.de