A researcher from the KlimaCampus was able to contribute to a significantly better estimation of global oceanic temperatures. This has been shown using a new method published in the scientific journal Deep Sea Research.
The instrumental errors affecting estimates of the global ocean warming can now be reduced on average to about 0.05° Celsius. This is possible through the method published in Deep Sea Research I by Viktor Gouretski from the KlimaCampus of the University of Hamburg and Franco Reseghetti (ENEA, Italy). The scientists examined more than six million temperature profiles of the World Ocean Database for the period from 1967 to 2008.
The average global warming since 1990 in the water column down to 400 m is estimated to be around 0.2° Celsius. The margin of error, however, is extremely high – just as high as the warming itself.
Why is this? Temperature profiles have been measured for decades using various types of instruments. Particularly the data from so-called XBTs, expendable bathythermographs,which dominate in the global database are subject to error. XBTs are cast into the sea from a cruising research vessel or from a so-called ship of opportunity and sink to the sea bottom, during which they constantly transmit the in situ temperature to the vessel. Temperature data are collected all over the world in this manner, and the probes remain on the sea bottom after the measurement.
Correction formula a systematic source of uncertainty ?
The problem: XBTs do not measure the depth. The temperatures are related to corresponding depths by means of the fall rate equation, which then can be optimized using a correction factor. Through comparisons with higher-accuracy data, Gouretski could now show that the XBT data were – paradoxically - made more inaccurate by the generally accepted correction factor. He found that the factor should not be constant with depth.
The team of researchers identified a further source of uncertainty in the regional particularities of the measurement site. The colder the water the slower the fall rate of the probe due to viscosity differences. The team optimized the global data set on the basis of these results. The data will shortly be available from a KlimaCampus platform.
„Cooling“ around 2004 didn’t happen that way
The researchers confirm that such sources of bias have had an effect on assessments made up to now. Gouretski says that published reports of phenomena such as a short-term cooling of the world ocean from 2003 to 2005 and a strong warming in the 1970s could be attributed to these and other systematic sources of error and could be corrected. Goretski was co-author of a further important publication in the journal Nature, in which the optimized calculations confirm global ocean warming over the last 15 years.
The research work was carried out in part at the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven.
Gouretski, V., Reseghetti, F.: On depth and temperature biases in bathythemograph data: Development of a new correction scheme based on analysis of a global database. Deep Sea Research I (2010).
Lyman, J., Good, S., Gouretski et al.: Robust warming of the global upper ocean. Nature (2010).
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