Wednesday, June 9, 2010

Mediterranean Sea Warming 0.2°C per Century

According to an article published in the April 2010 issue of the Journal of Geophysical Research-Oceans, the western Mediterranean Sea has warmed at a rate of only 0.002°C/yr or 0.2°C/century. Furthermore, the authors state that this very slight long term trend can not be statistically distinguished from decadal variability, and thus there may be no statistically significant long term trend at all over the period 1943-2000. These findings contrast with those of the recent controversial paper on ocean warming which studied a shorter period from 1993-2008.

How much is the western Mediterranean really warming and salting?

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, C04001, 12 PP., 2010 doi:10.1029/2009JC005816

Abstract: Instrumental biases and data processing methods can modify temperature trend estimations and enhance decadal variability in the upper ocean. These questions have not been specifically addressed in the western Mediterranean (WMED), a region where warming and salting trends have been detected during the second half of the twentieth century. In this work we test the sensitivity of these trends and decadal variability in the WMED to the use of bathythermograph data and data processing methods. We analyze different subbasins in order to detect distinct local responses. Our results show that deep waters in the WMED are increasing their temperature and salinity at a rate of 0.002°C/yr and 9.2 × 10−4 yr−1, respectively, from 1943. These trends are spatially homogeneous, not affected by instrumental biases, data processing methods, or changes in the period of time analyzed. The heat absorbed by the whole WMED is equivalent to a surface heat flux of 0.29 ± 0.19 W/m2. This figure encloses recent estimates of 0.36 ± 0.06 W/m2 for the global ocean. The intermediate layer increased its salinity at a rate of 1.3 × 10−4 yr−1 and this result can also be considered as robust. The intermediate layer temperature and the upper layer salinity show strong time variability and in our opinion long-term changes cannot be statistically distinguished from decadal variability during the period 1943–2000. The upper layer shows a temperature and heat increase for the overall period, but it is caused by a steep warming trend initiated in the early 1980s.

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